Behavioral Ecology - recent issues

Migration cues and timing in leatherback sea turtles

Sherrill-Mix, S. A., James, M. C., Myers, R. A. — 2008-03-13

Atlantic leatherback sea turtles migrate annually from foraging grounds off eastern Canada and the northeastern United States to southern foraging and breeding areas. Using Cox's proportional hazards model, we investigated the individual timing of the southward migrations of 27 turtles equipped with satellite-linked transmitters off Nova Scotia compared with turtle characteristics and satellite-measured ocean variables. Latitude, longitude, 1-week lagged average sea surface temperature, and 1-week lagged average chlorophyll-a concentration appear to influence the probability of departure. Higher temperature and, in the northern range of the study, higher chlorophyll concentration increased departure rates, perhaps due to the acceleration of the life cycle of the leatherback's gelatinous prey and/or increased feeding efficiency in these areas. This study highlights the opportunity to use satellite telemetry and environmental data to examine the cues for and timing of animal migrations and expands the study of migration timing to include a new species and environment.

Deceptive color signaling in the night: a nocturnal predator attracts prey with visual lures

Chuang, C.-Y., Yang, E.-C., Tso, I-M. — 2008-03-13

The role color signaling plays in the nocturnal condition of the terrestrial ecosystem is currently poorly understood. In general, arthropods active in the night are inconspicuously colored. However, in addition to inconspicuously colored dorsum, several genera of nocturnal orb spiders also have conspicuous ventrum spots. In this study, we tested whether the inconspicuously colored dorsum functioned to reduce spiders‘ visibility to diurnal predators while the spiders were perching on bark with ventrum spots well concealed. We also evaluated when spiders sit on webs with conspicuous ventrum spots fully exposed, would they serve as deceptive color signals to lure visually orientated nocturnal prey. We first quantified how diurnal hymenopteran predators viewed the dorsum and nocturnal lepidopteran prey viewed the ventrum spots of nocturnal orb spiders Neoscona punctigera by calculating color contrasts. The diurnal color contrast of spiders’ dorsum when viewed by hymenopteran insects against bark was lower than the discrimination threshold. However, the nocturnal color contrasts of spiders‘ ventrum spots when viewed by moths were high. In the field, webs with N. punctigera intercepted significantly more insects than those without. When the color signal of ventrum spots was altered by paint, webs’ prey interception rates decreased significantly. These results demonstrate that even in the nocturnal condition certain terrestrial organisms exhibit visual lures to attract prey.

What best explains vigilance in elk: characteristics of prey, predators, or the environment?

Liley, S., Creel, S. — 2008-03-13

We quantified the vigilance levels of elk (Cervus elaphus) preyed on by wolves (Canis lupus) in Yellowstone National Park, between January and May in 2005 and 2006, and used Akaike's information criterion to compare a set of 38 regression models for vigilance levels. These models combined up to 9 predictor variables of 3 types: characteristics of the prey group (herd size and composition), characteristics of the predator (wolf pack size, distance away, and the presence/absence of a kill), and characteristics of the local environment (distance to woodland edges, snow depth, and snow cover). The set of models spanned a range of complexity from simple univariate models to complex combinations with up to 3 variables of each type. Complex models incorporating the characteristics of the wolf pack, the structure of the elk herd, and the environmental conditions had higher information content than simple models. Although univariate models of vigilance detect significant relationships, they have low information content relative to multivariate models. These results show that elk assesses factors of several types when assessing risk and deciding how much time to allocate to vigilance. In particular, we found that all well-supported models of vigilance included several "prey" variables and several "predator" variables. This result highlights the need to consider information about predators when trying to explain variation in vigilance levels in prey.

Foraging in honeybees--when does it pay to dance?

Beekman, M., Lew, J. B. — 2008-03-13

Honeybees are unique in that they are the only social insects that are known to recruit nest mates using the waggle dance. This waggle dance is used by successful foragers to convey information about both the direction and distance to food sources. Nest mates can use this spatial information, increasing their chances of locating the food source. But how effective is the bees' dance communication? Previous work has shown that dancing does not benefit a honeybee colony under all foraging conditions and that the benefits of dancing are small. We used an individual-based simulation model to investigate under which foraging conditions it pays to dance. We compared the net nectar intake of 3 types of colonies: 1) colonies that use dance communication; 2) colonies that did dance but could not use the dance's spatial information; and 3) colonies that did not dance. Our results show that dancing is beneficial when the probability of independent discovery of food sources is low. Low independent discovery rates occur when patches are very small or very far away. Under these conditions, dancing is beneficial as only a single individual needs to find a patch for the whole colony to benefit. The main benefit of the honeybee's dance communication, however, seems to be that it enables the colony to forage at the most profitable patches only, ignoring forage patches that are of low quality. Thus, dancing allows the colony to rapidly exploit high-quality patches, thereby preventing both intra- and interspecific competitors from using that same patch.

Survival benefits and divergence of predator-induced behavior between pumpkinseed sunfish ecomorphs

Robinson, B. W., Januszkiewicz, A. J., Koblitz, J. C. — 2008-03-13

Resource use is widely thought to influence adaptive phenotypic divergence, whereas other ecological factors, such as predation, are frequently overlooked, particularly in studies of polyphenism in fishes. Juvenile pumpkinseed sunfish (Lepomis gibbosus) reared with predatory walleye (Sander vitreus) increase body depth and dorsal spine length, indicating that developmental responses to predation can shape phenotype. Body form responses to the same predator cues though have also evolutionarily diverged between sunfish ecomorphs that coexist in single lake populations by inhabiting either littoral or pelagic habitats, suggesting that predation risk varies between habitats. Here, we test if prior exposure to predator cues influences the development of behavior in juvenile pumpkinseed sunfish, if behavioral responses to the same predator cues vary between ecomorphs, and if induced phenotypic variation affects survival under predation. Behavior depended strongly on prior exposure to predator cues, but this effect varied between sunfish ecomorphs, indicating that ecomorphs have different responses to the same predator cues. Predator-induced phenotypes had higher survival than control phenotypes under simulated littoral but not pelagic conditions. Predator-induced phenotypic responses are candidate-inducible defenses, and divergent responses between ecomorphs suggest that they can evolve in response to selection imposed by differences in habitat-specific predation risk.

Worker policing in the German wasp Vespula germanica

2008-03-13

In some ants, bees, and wasps, workers kill or "police" male eggs laid by other workers in order to maintain the reproductive primacy of the queen. Kin selection theory predicts that multiple mating by the queen is one factor that can selectively favor worker policing. This is because when the queen is mated to multiple males, workers are more closely related to the queen's sons than to the sons of other workers. Earlier work has suggested that reproductive patterns in the German wasp Vespula germanica may contradict this theory, because in some colonies a large fraction of the adult males were inferred to be the workers' sons, despite the effective queen mating frequency being greater than 2 (2.4). In the present study, we reexamine the V. germanica case and show that it does support the theory. First, genetic analysis confirms that the effective queen mating frequency is high, 2.9, resulting in workers being more related to the queen's sons than to other workers' sons. Second, behavioral assays show that worker-laid eggs are effectively killed by other workers, despite worker-laid eggs having the same intrinsic viability as queen-laid ones. Finally, we estimate that approximately 58.4% of the male eggs but only 0.44% of the adult males are worker derived in queenright colonies, consistent with worker reproduction being effectively policed.

Parent-absent begging: evidence for sibling honesty and cooperation in the spotless starling (Sturnus unicolor)

Bulmer, E., Celis, P., Gil, D. — 2008-03-13

Begging in avian nestlings is a highly conspicuous behavior with important implications for the study of parent–offspring conflict. In some species, nestlings also call for long bouts in the absence of parents, and it has been proposed that this behavior is used by nestlings as a means of negotiating access to food. We studied this phenomenon in the spotless starling (Sturnus unicolor). We found that parent-absent calls were acoustically distinct from parent-present calls. Observations showed that the probability of parent-absent begging increased with nestling age and brood size, whereas it decreased with increasing body condition. This result was confirmed by an experiment that showed that nestlings produced higher parent-absent begging rates when food deprived than when satiated. Finally, we carried out a playback experiment to test the reaction of nestlings to parent-absent begging by fellow nestlings. Principle components analyses yielded 2 independent components of begging: 1) a general begging component and 2) a second factor that measures the relative contribution of communicative begging over competitive begging. Nestlings exposed to playback decreased their general begging levels and simultaneously increased the relative contribution of communicative over competitive begging. This behavior may favor needy nestlings to obtain impending feedings while keeping high levels of food solicitation from parents and is consistent with a cooperative strategy among nestlings. Future research should consider the actual response of parents to these signals.

Male insemination decisions and sperm quality influence paternity in the golden orb-weaving spider

Jones, T. M., Elgar, M. A. — 2008-03-13

In polyandrous species, paternity may be influenced by the timing and frequency of mating. Female spiders possess 2 genital openings that lead to separate sperm-storage structures. Thus, even when mating with a previously mated female, a male may reduce direct sperm competition by inseminating the opposite opening to her first mate. Such morphology may provide females with greater control over paternity. We examined simultaneously whether males avoided already inseminated female genital openings and whether this behavior varied with the time between successive matings. To explore these questions, we mated female golden orb weaver spiders, Nephila edulis, each to 2 males and manipulated the timing of their second mating. We documented male insemination patterns and explored the influence of male mating decisions on paternity success using the irradiated male technique. We found that 60% of males avoided sperm competition by discriminating against inseminated genital openings. Moreover, male mating behavior had a dramatic impact on the paternity success of irradiated males. When males inseminated the same genital opening, the competitive ability of the irradiated male's sperm was dramatically reduced resulting in lower paternity success. In contrast, when the 2 males inseminated opposite genital openings both males sired equal proportions of offspring regardless of their radiation status. There was no evidence that the timing of the second mating affected patterns of paternity. Our data suggest that differences in sperm quality may influence paternity success of N. edulis males under a sperm-competitive scenario. In contrast, females appear to have limited postmating control over paternity.

Parasite levels in blue-black grassquits correlate with male displays but not female mate preference

Aguilar, T. M., Maia, R., Santos, E. S.A., Macedo, R. H. — 2008-03-13

The handicap principle proposes that male sexual ornaments and displays provide honest indicators of quality. Female preference for high-quality males, however, may be driven not only by genetic benefits but also by indirect benefits. We investigated the impact of parasitism on morphological, ornamental, and behavioral characteristics of male and female blue-black grassquits (Volatinia jacarina) in captivity. First, we tested whether male displays and morphology were influenced by parasitism. Second, we assessed if females were attentive to variation in male morphology and displays linked to parasitism. Third, we tested whether parasitism in females influenced health and mate preferences. We maintained 2 groups of birds in captivity: nonmedicated birds developed high levels of coccidian parasitism, whereas medicated birds were free of parasitism. Parasitized males developed, relative to nonparasitized males, lower weight/tarsus indices and mass. They also showed relative deficiencies in their displays, with less persistence and lower rates. Despite the negative effects of parasitism on males, females did not prefer nonparasitized males. This held for both parasitized and nonparasitized females. Our data suggest that coccidian parasitism has adverse effects on morphological condition and expression of displays. These effects, however, appear not to be attended to by females; moreover, female mate preferences appear not to be impacted by the threat of parasitism. It thus seems that female mate preferences may not depend only on sexual characters affected by parasitism in this species.

Sexual conflict over breeding substrate causes female expulsion and offspring loss in a cichlid fish

Maan, M. E., Taborsky, M. — 2008-03-13

Females of the Lake Tanganyika cichlid Lamprologus callipterus exclusively breed in empty snail shells that males collect in their territories. Male–male competition for shells is severe, leading to frequent shell stealing and territory takeover. As a consequence, males have breeding females in their shells that spawned with competitors. In this field study, we investigated both naturally occurring and experimentally induced encounters of territorial males with females that had spawned with other males. We found that the breeding success of females that were taken over by a different male was significantly reduced. Behavioral observations after experimental shell relocation further showed that males recognized females that they had not spawned with: males directed more exploration and manipulation behavior toward such shells compared with controls. Reoccupation rate of emptied experimental shells was significantly higher than that of unmanipulated empty shells. This indicates that shell stealing and nest takeover, followed by female expulsion, contribute to the reproductive success of L. callipterus males. We also found that female mate choice reduces expulsion risk: females preferred to mate with large males, and male size correlated with dominance. We conclude that the limited availability of breeding substrate is a key determinant of both intrasexual competition and intersexual conflict in this species.

Mating system, sexual dimorphism, and the opportunity for sexual selection in a territorial ungulate

2008-03-13

In mammals, species with high sexual size dimorphism tend to have highly polygynous mating systems associated with high variance in male lifetime reproductive success (LRS), leading to a high opportunity for sexual selection. However, little information is available for species with weak sexual size dimorphism. In a long-term study population, we used parentage analysis based on 21 microsatellite markers to describe, for the first time, variance in male lifetime breeding success (LBS) of roe deer, a territorial ungulate where males weigh less than 10% more than females. LBS ranged from 0 to 14 (mean = 4.54, variance = 15.5), and its distribution was highly skewed, with only a few males obtaining high LBS and many males failing to breed or siring only one fawn. As predicted for polygynous species with low sexual size dimorphism, the standardized variance in male LBS was low (I_m = 0.75) and was only slightly higher than the standardized variance in female LRS (I_f = 0.53), suggesting a low opportunity for sexual selection. The I_m value reported here for roe deer is much lower than values reported for highly dimorphic ungulates such as red deer (I_m > 3). We suggest that, along a continuum of opportunity for sexual selection, roe deer occupy a position closer to monogamous and monomorphic territorial ungulates than to highly polygynous, sexually dimorphic ungulates with dominance rank–based mating systems such as harems or roving mating systems.

Social constraints limit dispersal and settlement decisions in a group-living bird species

Griesser, M., Nystrand, M., Eggers, S., Ekman, J. — 2008-03-13

Dispersal is a fundamental process affecting the genetic structure of populations, speciation, and extinction. Nevertheless, our understanding of the evolution of dispersal is limited by our paucity of knowledge on dispersal decisions at the individual level. We investigated the effect of interactions between residents and juvenile dispersers on individual dispersal and settlement decisions in Siberian jays (Perisoreus infaustus). In this group-living bird species, some offspring remain on the parental territory for up to 3 years (retained juveniles) whereas other offspring disperse within 2 months of fledging (dispersers). We found that retained juveniles constrained settlement decisions of dispersers by aggressively chasing dispersers off their territory, resulting in dispersers continuing to disperse and settling in groups without retained juveniles. Experimental removal of male breeders during the dispersal period also demonstrated that dispersers were unable to settle in high-quality breeding openings, which were instead filled by older nonbreeding residents. Rather, dispersers immigrated into groups without retained offspring where they became subordinate group members, queuing for a breeding opening. Also, they preferably settled in groups with short queues where no same-sex juveniles were present. Dispersal did not inflict a cost to dispersers through increased mortality. However, the presence of immigrants was costly for breeders because it increased the rate of conflicts during the breeding season which negatively affected nestling condition. These results demonstrate that resident individuals constrain both dispersal and settlement decisions of dispersers. Social interactions between residents and dispersers can thus be a key factor to understand the evolution of dispersal.

Low light reflectance may explain the attraction of birds to defoliated trees

Mantyla, E., Klemola, T., Sirkia, P., Laaksonen, T. — 2008-03-13

Plants use volatile organic compounds to attract invertebrate predators and parasitoids of their herbivore pests. Recently, it has been suggested that plants, either through visual or olfactory cues, may also "cry for help" from vertebrate predators such as birds. We show that in a laboratory choice test, passerine birds (Parus major and Cyanistes caeruleus) were attracted to the intact branches of trees (Betula pendula) suffering from foliar damage caused by herbivore larvae (Epirrita autumnata) in nontest branches. Species, age, or sex of the experimental bird or lighting (ultraviolet [UV] or non-UV) did not affect the preference. However, the birds made a clear choice between the treatments when the trees came from a forest patch receiving more sunlight, whereas no obvious choice was observed when the trees came from a shadier forest patch. Results of the choice test were supported by the spectral reflectance of tree leaves. In the sunnier forest patch, control trees reflected more visible light than the herbivore trees, whereas no such difference was found in the shadier forest patch trees. We suggest that avian predators use their vision within visible wavelengths to find insect-rich plants even when they do not see the prey items or damaged leaves.

Male morph predicts investment in larval immune function in the dung beetle, Onthophagus taurus

Cotter, S. C., Beveridge, M., Simmons, L. W. — 2008-03-13

Investment in immunity is costly, so that resource-based trade-offs between immunity and sexually selected ornaments might be expected. The amount of resources that an individual can invest in each trait will be limited by the total resources available to them. It would therefore be informative to investigate how investment in immune function changes during growth or production of the sexual trait as resources are diverted to it. Using the dung beetle, Onthophagus taurus, which displays both sexual and male dimorphism in horn size, we examined changes in one measure of immune function, phenoloxidase (PO) activity, in the hemolymph of larvae prior to and during horn growth. We found that PO levels differed between small- and large-horned males throughout the final instar prior to the point where investment in horn growth was taking place. PO levels in females were intermediate to the 2 male morphs. These differences could not be accounted for by differences in condition, measured as hemolymph protein levels and weight. We suggest that the observed differences might be associated with sex- and morph-specific variation in juvenile hormone levels.

Optimal group positioning after a predator attack: the influence of speed, sex, and satiation within mobile whirligig swarms

Romey, W. L., Galbraith, E. — 2008-03-13

The position of animals within fish shoals, bird flocks, and insect swarms is related to individual differences in hunger, body size, and defenses. These differences relate to the way that animals balance multiple selection pressures including food-distribution and predator-attack patterns. However, the role of drafting/slipstreaming (taking advantage of the vortices of those in front of you) and sex on the position of individuals within a polarized group has not been well studied. For example, although hungry fish have been found to prefer positions toward the front of a shoal on average, the mitigating factors of sex, recent predator exposure, and drafting have not been factored into this response. We conducted a controlled laboratory experiment with individually marked whirligig beetles (Coleoptera: Gyrinidae) where sex and feeding level were controlled and the position of beetles in a polarized group (in a flow tank) was analyzed at 2 different water speeds after exposure to a simulated predator. It was predicted that males and females would balance foraging and predator avoidance needs differently, as suggested by sexual segregation theory and that males might be likely to occupy front positions because of greater energetic needs. We found that in slow water males were more likely to occupy front positions, whereas in fast water females did, suggesting a different trade-off between the sexes in the need to forage versus save energy (draft). Additionally, we found that in slow water it was the hungry males that came to the group's front, whereas hungry females were more likely to move back. These are some of the first observations of the positional complexity with which individuals in congregations display, and several adaptive and nonadaptive explanations for the observed patterns are suggested.

Evolution of parental favoritism among different-aged offspring

Jeon, J. — 2008-03-13

The theories of intrafamilial conflict and parental investment have yet to examine how parents' decisions about resource allocation are influenced by the fact that their offspring may be different in age. Two counteracting effects of offspring growth on parental allocation of resources have deterred the development of a formal model: A parent may favor older offspring due to their greater reproductive value or favor younger offspring due to their higher marginal returns from extra resources. Using an evolutionary invasion analysis in class-structured populations, I present a formal model that explores how a parent should allocate its resources among different-aged offspring from the viewpoint of the parent. The parent's evolutionarily stable strategy is to allocate its resources such that the marginal benefit to each offspring's survival, weighted by the survival probability to the reproductive age, is equal to the marginal cost to the parent's residual survival. Two general situations are considered in which younger offspring obtain higher marginal returns than older offspring. In nearly all circumstances, a parent is expected to bias its resources toward older offspring. The results may account for the widespread yet puzzling phenomenon of parental bias toward older offspring in view of previous theories of intrafamilial conflict.

Monogamy when there is potential for polygyny: tests of multiple hypotheses in a group-living fish

Wong, M. Y. L., Munday, P. L., Buston, P. M., Jones, G. P. — 2008-03-13

Monogamy within social groups where there exists a high potential for polygyny poses a challenge to our understanding of mating system evolution. Specifically, the traditional explanation that monogamy evolves due to wide female dispersion, affording males little opportunity to defend multiple females, cannot apply. Instead, monogamy in groups potentially arises because females compete for breeding resources such as breeding sites, food, and paternal care. We conducted manipulative experiments to determine whether females compete over limiting resources within groups of the obligate coral-dwelling goby, Paragobiodon xanthosomus (Gobiidae). Breeding females behaved aggressively toward individuals of their own sex and evicted subordinate females that were large and mature from the group. Experimental removal of nest sites caused breeding partners to breed in alternative nest sites, demonstrating that nest site limitation was not the cause of female competition. Supplemental feeding resulted in an increase in the fecundity of breeding females but no maturation of subordinate females, demonstrating that food-limited female fecundity was a likely cause of female competition. Finally, supplemental feeding of breeding pairs demonstrated that the difference in eggs hatched by fed versus unfed males was less than the difference in eggs laid by fed versus unfed females, suggesting that paternal care limitation might also drive female competition. These results suggest that competition over food and possibly paternal care selects for dominant, breeding females to suppress the maturation of subordinate females to minimize competition. Monogamy in association with group living is therefore likely to have evolved because female competition prevents males from utilizing the potential for polygyny.

Can experienced birds select for Mullerian mimicry?

Ihalainen, E., Lindstrom, L., Mappes, J., Puolakkainen, S. — 2008-03-13

Field experiments have shown that avian predators in the wild can select for similarity of warning signals in aposematic prey (Müllerian mimicry) because a common signal is better protected than a signal that is novel and rare. The original theory of Müllerian mimicry assumes that the mechanism promoting mimicry is predator learning; by sharing a signal, the comimic species share the mortality that is due to sampling by inexperienced predators. Predation events have not been observed in the wild, and learning experiments with naive bird predators in a laboratory have not unambiguously shown a benefit of a uniform signal compared with different signals. As predators in the field experiments are likely to be more experienced compared with previous laboratory experiments, we studied selection by experienced predators on a novel imperfect mimic. We trained great tits Parus major to avoid artificial aposematic models and subsequently introduced perfect and imperfect mimics at different frequencies. Birds with prior experience on the models selected against the imperfect mimics that were at a disadvantage also in a memory test conducted a week after their introduction. Selection against the imperfect mimics was antiapostatic. However, the imperfect mimics also benefited from some signal generalization to the models and possibly gained protection because the birds were familiar with the alternative cryptic prey that was also present. Our results suggest that experienced predators might be more important to the evolution of mimicry than the learning-based theory assumes.

Ultraviolet reflectance of plumage for parent-offspring communication in the great tit (Parus major)

Tanner, M., Richner, H. — 2008-03-13

Ultraviolet (UV) reflectance has been implicated in mate selection. Yet, in some bird species the plumage of young varies in UV reflectance already in the nest and long before mate choice and sexual selection come into play. Most birds molt the juvenile body plumage before reaching sexual maturity, and thus, some conspicuous traits of the juvenile body plumage may rather have evolved by natural selection, possibly via predation or parental preference. This second hypothesis is largely untested and predicts a differential allocation of food between fledging and total independence, which is a time period of 2–3 weeks where offspring mortality is also highest. Here, we test the prediction that parents use the individual variation in UV reflectance among fledglings for differential food allocation. We manipulated UV reflectance of the plumage of fledgling great tits Parus major by treating chest and cheek feathers with a lotion that either did or did not contain UV blockers and then recorded food allocation by parents in an outdoor design simulating postfledging conditions. The visible spectrum was minimally affected by this treatment. Females were found to feed UV-reflecting offspring preferentially, whereas males had no preference. It is the first evidence showing that the UV reflectance of the feathers of young birds has a signaling function in parent–offspring communication and suggests that the UV traits evolved via parental preference.

Colorful male guppies do not provide females with fecundity benefits

Pilastro, A., Gasparini, C., Boschetto, C., Evans, J. P. — 2008-03-13

The phenotype-linked fertility hypothesis (PLFH) predicts that males with elaborated sexual ornaments signal their high fertilizing efficiency to females and that female preferences for ornamented males are driven by direct fecundity benefits. Although some studies have demonstrated that attractive males produce more or higher quality sperm, there is limited experimental evidence that females derive fecundity benefits by mating with attractive males. Some of the best indirect evidence for the PLFH comes from work on guppies (Poecilia reticulata), an internally fertilizing species of freshwater fish in which phenotypically attractive males produce larger and relatively higher quality ejaculates than their less attractive counterparts. We used artificial insemination to impregnate female guppies using known numbers of sperm from a range of males with different phenotypes and related female fecundity (brood success, time from insemination to parturition, and brood size) to sperm numbers and male phenotype (body size and the relative area of color spots). We found no evidence that male phenotype or experimentally adjusted "ejaculate" size influenced any of our measures of female fecundity. These results highlight the importance of experimentally investigating potential fecundity benefits associated with female mating preferences before concluding that the maintenance of these preferences is driven by the pursuit of such benefits.

Split sex ratios in the social Hymenoptera: a meta-analysis

Meunier, J., West, S. A., Chapuisat, M. — 2008-03-13

The study of sex allocation in social Hymenoptera (ants, bees, and wasps) provides an excellent opportunity for testing kin-selection theory and studying conflict resolution. A queen–worker conflict over sex allocation is expected because workers are more related to sisters than to brothers, whereas queens are equally related to daughters and sons. If workers fully control sex allocation, split sex ratio theory predicts that colonies with relatively high or low relatedness asymmetry (the relatedness of workers to females divided by the relatedness of workers to males) should specialize in females or males, respectively. We performed a meta-analysis to assess the magnitude of adaptive sex allocation biasing by workers and degree of support for split sex ratio theory in the social Hymenoptera. Overall, variation in relatedness asymmetry (due to mate number or queen replacement) and variation in queen number (which also affects relatedness asymmetry in some conditions) explained 20.9% and 5% of the variance in sex allocation among colonies, respectively. These results show that workers often bias colony sex allocation in their favor as predicted by split sex ratio theory, even if their control is incomplete and a large part of the variation among colonies has other causes. The explanatory power of split sex ratio theory was close to that of local mate competition and local resource competition in the few species of social Hymenoptera where these factors apply. Hence, three of the most successful theories explaining quantitative variation in sex allocation are based on kin selection.

Winter male plumage coloration correlates with breeding status in a cooperative breeding species

Solis, E., Aviles, J. M., De La Cruz, C., Valencia, J., Sorci, G. — 2008-03-13

The function of colored ornaments is usually related to the signaling of individual quality in intra- and intersexual interactions. In cooperative breeding species, where only a fraction of the male population access the breeding status and the other fraction has the option to help breeding pairs, colored traits might provide the females with a reliable information on the quality of potential mate. Males of the cooperative breeding azure-winged magpies (Cyanopica cyanus) display conspicuous blue plumage coloration. Here we explored the role played by structural blue coloration of males and the probability of becoming a breeder or a helper. Birds were trapped during 4 consecutive years, and feather coloration was measured with a spectrometer. Males that became breeders had a more brilliant and saturated blue coloration and showed a more violet hue in the nonbreeding period compared with birds that became helpers. Breeding males also showed a seasonal decline in blueness, whereas the color properties of helpers were constant throughout the year. Blueness of individuals trapped in the nonbreeding period was positively correlated with body size and condition. These findings are consistent with a scenario in which nonbreeding blue plumage coloration may function as a signal of individual quality in the azure-winged magpie at the pair formation time and add to growing evidence suggesting that the nonbreeding season appears particularly important in impacting breeding roles in cooperative breeding birds.

Egg rejection behavior in a population exposed to parasitism: Village Weavers on Hispaniola

Cruz, A., Prather, J. W., Wiley, J. W., Weaver, P. F. — 2008-03-13

In contrast to African Village Weavers (Ploceus cucullatus) that are parasitized by Diederik Cuckoos (Chrysococcyx caprius), introduced weavers on Hispaniola existed without parasitism for at least 2 centuries until the arrival of the Shiny Cowbird (Molothrus bonariensis) in the 1970s. Cruz and Wiley (1989) found that Hispaniolan weavers had a lower rejection rate of foreign eggs than African populations. Subsequently, Robert and Sorci (1999) and Lahti (2005, 2006) found that acceptance of dissimilar eggs is not characteristic of the species throughout its Hispaniolan range. In 1999–2002, we studied egg rejection in Hispaniolan weavers on a broad regional scale. Rejection increased as experimental eggs became increasingly different from the host eggs. Rejection rates for mimetic eggs, different color eggs, different-spotting eggs, and cowbird eggs was 23.2%, 33.3%, 61.5%, and 85.3%, respectively, with higher rejection of cowbird eggs in areas where cowbirds were observed. Although rejection is likely to have a genetic component, the differences could be due to phenotypic plasticity. Plasticity in egg rejection may be expected, given the potential cost of rejection and the spatiotemporal distribution of cowbirds. Thus, egg rejection has not necessarily decreased in Hispaniolan weavers, but it may act in a plastic manner, increasing where cowbirds are present.

Male barn swallows use different resource allocation rules to produce ornamental tail feathers

Munoz, A., Aparicio, J. M., Bonal, R. — 2008-03-13

Sexual ornaments compete for resources with other functional traits. Such resource allocation trade-offs should ensure the honesty of sexual ornaments according to the Zahavi's handicap principle. However, the existence of costly signals could not be enough to guarantee honesty if different individuals invest different proportions of their limited resources in ornaments. Then, a certain level of sexual signaling would correspond to several levels of individual condition. Here, we explore whether there are different resource allocation rules in tail feather ornaments between males within a barn swallow (Hirundo rustica) population and whether these different rules confer different viability to males. We assessed the proportion of resources invested in ornamental feathers compared with other functional feathers moulted and growing during the same period at expenses of the same resources. We found that 1) different males allocate a different proportion of resources to ornamental feathers in relation to functional feathers and this proportion is repeatable between years and 2) male survival likelihood decreased as the proportion of resources allocated to ornamental feathers increased. Survival costs associated with increased investments in ornaments can maintain the sexual signaling system honest at population level but do not preclude the existence of an array of different allocation rules between males. Thus, males with different viability can produce ornamental feathers of the same length. These results show that the relationship between male viability and ornament expression can be less straightforward than considered previously.

Ultraviolet plumage does not signal social status in free-living blue tits; an experimental test

Vedder, O., Korsten, P., Magrath, M. J. L., Komdeur, J. — 2008-03-13

Ultraviolet (UV) signals are suggested to be sexually selected in a wide range of taxa. Most research, however, has focused on the role of UV signals in mate choice, whereas possible functions in intraspecific competition remain largely untested. Studies on other colors indicate that ornaments preferred by females can also function as signals of social status in competitive interactions between individuals. Whereas these colors are mainly pigment based, UV reflectance is generally caused by selective reflectance of light from surface structures. Here we test experimentally whether the structurally based UV-reflective crown plumage in the blue tit (Cyanistes caeruleus) serves as a signal of status in interindividual competition. We reduced the crown UV reflectance of free-living blue tits in winter and compared their probability of winning conflicts over food at a feeding table with control-treated and untreated individuals. Although we controlled for effects of sex, age, and distance from territory, we found no effect of reduced UV reflectance on the probability of winning nor were conflicts involving UV-reduced individuals more likely to escalate. Therefore, we conclude that the UV reflectance of the blue tit's crown does not serve as a signal of status in competition over food in winter. We suggest that the observed site-dependent dominance structure may constrain the opportunity for a status signal to evolve and that enhancing attractiveness in mate choice may be the sole function of the crown's UV reflectance.

Strong but variable associations between social dominance and clutch sex ratio in a colonial corvid

Salomons, H.M., Dijkstra, C., Verhulst, S. — 2008-03-13

We studied primary sex ratio of clutches in relation to social dominance for 6 years in a colony of free-living jackdaws, a small corvid. Social dominance was strongly associated with clutch sex ratio, with the difference in clutch sex ratio between the most and least dominant pairs being 30–40%. To our knowledge, this is the first demonstration of an association between social dominance and sex allocation in birds. However, the direction of this effect varied between years. Dominant jackdaws produced more sons during the first years of the study but fewer sons during the last years. Offspring sex was not related to laying order within a clutch, and the effect of social dominance on sex ratio was similar on eggs laid first, middle, or last. We investigated the effect of 2 factors (laying date and parental condition) that could have mediated the shift in the effect of social dominance on sex allocation in the course of the study. Laying date was positively associated with the proportion of males, but this effect was independent of social dominance. Maternal condition (residual mass over tarsus and egg volume) was related to social dominance but not to clutch sex ratio. Paternal condition (residual mass over tarsus) was not related to clutch sex ratio. We discuss how spatial or temporal variation in effects of variables such as social dominance on sex allocation can contribute to our understanding of the evolution of sex allocation in species with complex life histories.

Better to be bimodal: the interaction of color and odor on learning and memory

Siddall, E. C., Marples, N. M. — 2008-03-13

Defended prey frequently advertise to potential predators using multimodal warning displays. Signaling through more than one sensory pathway may enhance the rate of avoidance learning and the memorability of these learned avoidances. If this is so, then mimetic insects would gain more protection from mimicking a multimodal rather than a monomodal model. Day-old domestic chicks (Gallus gallus domesticus) were used to examine whether a common insect warning odor (pyrazine) enhanced learning and memorability of yellow prey, a common warning color. Pyrazine increased the rate at which the chicks learned to avoid unpalatable yellow prey, and how well this learned avoidance was remembered after a 96-h interval. After 96 h, mimics of the multimodal prey were avoided, whereas mimics of the monomodal prey were not. In the absence of pyrazine, chicks generalized their learned avoidance of the unpalatable yellow prey to palatable green prey; however, the presence of pyrazine reduced this color generalization. These results suggest that much is to be gained from signaling multimodally, for both models and mimetic prey species. The presence of multimodal prey in the habitat may also advantage the predators as it allows it them to distinguish more easily between palatable and unpalatable prey.

Paternity costs from polyandry compensated by increased fecundity in the hide beetle

McNamara, K. B., Brown, R. L., Elgar, M. A., Jones, T. M. — 2008-03-13

Polyandry-induced sperm competition is assumed to impose costs on males through reduced per capita paternity success. In contrast, studies focusing on the consequences of polyandry for females report increased oviposition rates and fertility. For these species, there is potential for the increased female fecundity associated with polyandry to offset the costs to males of shared paternity. We tested this hypothesis by comparing the proportion and number of offspring sired by males mated with monandrous and polyandrous females in the hide beetle, Dermestes maculates, both for males mating with different females and for males remating with the same female. In 4 mating treatments, monandrous females mated either once or twice with the same male and polyandrous females mated either twice with 2 different males or thrice with 2 males (where 1 male mated twice). Polyandrous and twice-mating monandrous females displayed greater fecundity and fertility than singly mating monandrous females. Moreover, males remated to the same female had greater paternity regardless of whether that female mated with another male. In both polyandrous treatments, male mating order did not affect paternity success. Finally, although the proportion of eggs sired decreased if a male mated with a polyandrous female, multiply mating females or females that remated with a previous mate laid significantly more eggs and thus the actual number of eggs sired was comparable. Thus, males do not necessarily accrue a net fitness loss when mating with polyandrous females. This may explain the absence of any obvious defensive paternity-protection traits in hide beetles and other species.

Food resources, chemical signaling, and nest mate recognition in the ant Formica aquilonia

Sorvari, J., Theodora, P., Turillazzi, S., Hakkarainen, H., Sundstrom, L. — 2008-03-13

Animals such as social insects that live in colonies can recognize intruders from other colonies of the same or different species using colony-specific odors. Such colony odors usually have both a genetic and an environmental origin. When within-colony relatedness is high (i.e., one or very few reproductive queens), colonies comprise genetically distinct entities, and recognition based on genetic cues is reliable. However, when nests contain multiple queens and colonies comprise multiple nests (polydomy), the use of purely genetically determined recognition labels may become impractical. This is due to high within-colony genetic heterogeneity and low between-colony genetic heterogeneity. This may favor the use of environmentally determined recognition labels. However, because nests within polydomous colonies may differ in their microenvironment, the use of environmental labels may also be impractical unless they are actively mixed among the nests. Using a laboratory experiment, we found that both isolation per se and diet composition influenced the cuticular chemical profiles in workers of Formica aquilonia. In addition, the level of aggression increased when both the proportions of dietary ingredients and the availability of food were altered. This suggests that increased aggression was mediated by changes in the chemical profile and that environmental cues can mediate recognition between colonies. These results also suggest that the underlying recognition cues are mutable in response to extrinsic factors such as the amount and the composition of food.

Fitness consequences of personality: a meta-analysis

Smith, B. R., Blumstein, D. T. — 2008-03-13

The study of nonhuman personality capitalizes on the fact that individuals of many species behave in predictable, variable, and quantifiable ways. Although a few empirical studies have examined the ultimate consequences of personality differences, there has been no synthesis of results. We conducted a formal meta-analysis of published studies reporting fitness consequences of single personality dimensions to identify general trends across species. We found bolder individuals had increased reproductive success, particularly in males, but incurred a survival cost, thus, supporting the hypothesis that variation in boldness was maintained due to a "trade-off" in fitness consequences across contexts. Potential mechanisms maintaining variation in exploration and aggression are not as clear. Exploration had a positive effect only on survival, whereas aggression had a positive effect on both reproductive success and, not significantly, on survival. Such results would suggest that selection is driving populations to become more explorative and aggressive. However, limitations in meta-analytic techniques preclude us from testing for the effects of fluctuating environmental conditions or other forms of selection on these dimensions. Results do, however, provide evidence for general relationships between personality and fitness, and we provide a framework for future studies to follow in the hopes of spurring more in-depth, long-term research into the evolutionary mechanisms maintaining variation in personality dimensions and overall behavioral syndromes. We conclude with a discussion on how understanding and managing personality traits may play a key role in the captive breeding and recovery programs of endangered species.

Testing the sexy son hypothesis--a research framework for empirical approaches

Huk, T., Winkel, W. — 2008-03-13

Female mating biases for bright ultraviolet iridescence in the butterfly Eurema hecabe (Pieridae)

Kemp, D. J. — 2008-01-10

Exaggerated male-limited coloration is widespread among butterflies, yet convincing demonstrations of intraspecific mating preferences for signal brightness and/or chromaticity are relatively rare in this group. Here, I couple behavioral experiments involving manipulations of ambient light environments and male reflectance patterns with observation of wild mating patterns to investigate visual mating biases in the large grass yellow (Eurema hecabe). Males in this species possess exaggerated, limited-view ultraviolet (UV) iridescence across most of their dorsal wing surface that has putative sexual signaling function. In the first experiment, conducted in small (0.7–m³) cages, individuals were significantly less likely to copulate when the UV portion of natural ambient illumination (i.e., 300–400 nm) was strongly reduced. In 2 subsequent experiments, conducted under full-spectrum sunlight in small and large (5 x 6 x 4 m) cages, males with their UV signal artificially dulled by 25% consistently copulated with fewer, and smaller, females than sham-control individuals. Importantly, the manipulated levels of UV brightness in these experiments fall well within the naturally occurring bounds of variation in male UV reflectance. These findings therefore unanimously support the presence of a UV signal–based female bias. In apparent contrast, comparison of 161 in-copula and 188 free-flying males from a high-density field assemblage revealed that copulating males were significantly older and henceforth actually possessed (subtly) less UV bright wings. Copulating male UV brightness was, however, positively related to the size of their mate, which echoes the experimental findings and may represent a signature of mutual mate choice. I discuss these results in light of the full complexities of the butterfly mating system and the potential signaling value of iridescent coloration in butterflies and animals generally.

Age- and tactic-related paternity success in male African elephants

2008-01-10

Information on age- and tactic-related paternity success is essential for understanding the lifetime reproductive strategy of males and constitutes an important component of the fitness trade-offs that shape the life-history traits of a species. The degree of reproductive skew impacts the genetic structure of a population and should be considered when developing conservation strategies for threatened species. The behavior and genetic structure of species with large reproductive skew may be disproportionately impacted by anthropogenic actions affecting reproductively dominant individuals. Our results on age- and tactic-specific paternity success in male African elephants are the first from a free-ranging population and demonstrate that paternity success increases dramatically with age, with the small number of older bulls in the competitive state of musth being the most successful sires. However, nonmusth males sired 20% of genotyped calves, and 60% of mature bulls (>20 years old) were estimated to have sired offspring during the 5-year study period. The 3 most successful males sired less than 20% of the genotyped offspring. Hence, contrary to prediction from behavior and life-history traits, reproduction was not heavily skewed compared with many other mammalian systems with a similar breeding system. Nevertheless, these results indicate that trophy hunting and ivory poaching, both of which target older bulls, may have substantial behavioral and genetic effects on elephant populations. In addition, these results are critical to the current debate on methods for managing and controlling increasing populations of this species.

Digit ratios, secondary sexual characters and condition in barn swallows Hirundo rustica

Dreiss, A. N., Navarro, C., de Lope, F., Moller, A. P. — 2008-01-10

The second and fourth digit length ratio (2D:4D) is sexually dimorphic in many vertebrates. This ratio has been suggested to provide an estimate of steroid levels encountered during prenatal development, which may have organizational consequences for physiology and behavior of adults. However, recent studies showed that the relation between digit ratio and steroids seems inconsistent and may vary among species and populations. We tested the hypothesis that digit ratios would be correlated with the expression of secondary sexual characters, using the barn swallow (Hirundo rustica) as a model system. This was done by testing whether variation in 2D:4D ratio was correlated with tail length and features of song, which are important secondary sexual characters positively correlated with circulating steroid concentration in adult birds. Furthermore, we examined the prediction that male and female digit ratios would correlate with body mass in an antagonistic way. There was no significant sexual dimorphism in digit ratio, which may be due to low levels of sexual selection in this population. Adult right 2D:4D ratio was negatively linked to tail length but not to male song output. Moreover, right 2D:4D ratio was negatively correlated with body mass in male and positively in females. These results are consistent with high digit ratios reflecting low levels of testosterone in this bird species.

Host life-history strategies and the evolution of chick-killing by brood parasitic offspring

Broom, M., Ruxton, G. D., Kilner, R. M. — 2008-01-10

Nestling brood parasites vary in the harm that they do to their companions in the nest. Here we use a game-theoretical model to attempt to account for this variation. Our model considers hosts that might routinely abandon single nestlings, regardless of whether they are host young or brood parasites and choose instead to reallocate their reproductive effort to future breeding. The nestling brood parasite must decide whether or not to kill all host young by balancing the benefits it stands to gain from reduced competition in the nest against the risk of desertion by host parents. The model predicts that 3 different types of evolutionarily stable strategies can exist. 1) When hosts routinely rear depleted broods, the brood parasite always kills host young, and the host never then abandons the nest. 2) Conversely, when adult survival after deserting single offspring is very high, hosts always abandon broods of one young, and the parasite never kills host offspring. 3) Intermediate strategies can also be evolutionarily stable, in which parasites sometimes kill their nest mates, and host parents sometimes desert nests that contain only a single chick. We provide quantitative descriptions of how the values given to ecological and behavioral parameters of the host-parasite system influence the probability of each strategy and compare our results with host–brood parasite associations seen in nature.

Does predator swamping promote synchronous emergence of turtle hatchlings among nests?

Tucker, J. K., Paukstis, G. L., Janzen, F. J. — 2008-01-10

The spatial and temporal synchrony observed for many behaviors is often viewed as deriving from selection to swamp predators, ensuring that at least some individual prey survive. However, this adaptive explanation has rarely been put to experimental test. We conducted 2 field experiments to evaluate the importance of alternative mechanisms (predator swamping and prey switching) to explain synchronous emergence among nests by red-eared slider turtle hatchlings (Trachemys scripta elegans) that initiates the critical migration from nests to water. In the 1998 experiment, we released 1400 hatchlings simultaneously and allowed another 1400 hatchlings to emerge independently from artificial nests. Hatchlings emerging from nests were just as likely to be recaptured as those in the mass release. Moreover, survivorship was highest among nests whose hatchlings began emerging early compared with hatchlings that emerged from nests later during migration. In the 2006 experiment, we released 2 groups of 100 hatchlings 8 days apart and a third group of 300 hatchlings 13 days later. We recaptured more than twice as many hatchlings from the first group as from the similar sized second group and from the larger third group. If the predator swamping mechanism had been important, we should have seen improved survivorship for the hatchlings in the mass release in the first experiment and for hatchlings in the third group in the second experiment. Overall, the results reject predator swamping as an explanation. Our findings instead suggest that synchronous emergence of turtle hatchlings from nests across a population maximizes individual survival by minimizing exposure to prey-switching predators.

Risk taking by singing males

Moller, A.P., Nielsen, J.T., Garamzegi, L.Z. — 2008-01-10

The distance at which an individual flees from a potential predator represents a measure of risk taking. If individuals are engaged in another activity that might affect fitness, trade-offs between the fitness benefits of flight and the other activity should determine the nearest distance of approach by a predator. In a comparative analysis of birds, flight distance represented a reliable measure of risk of predation by the sparrowhawk Accipiter nisus that increased with decreasing flight distance across species. To test the hypothesis that singing males adjusted their risk taking to the costs and benefits of early flight, we compared the flight distance of singing and nonsinging birds to an approaching human observing with a binocular. Singing birds on average fled at a greater distance than nonsinging birds, implying that singing birds took small risks. We used a standardized measure of difference in flight distance between singing and nonsinging individuals to investigate factors affecting interspecific variation in risk taking. Species that used more exposed song posts (sites used for singing) took smaller risks than species with less exposed song posts. Species that suffered from higher levels of parasitism as reflected by the prevalence of Plasmodium, but not by 3 other genera of blood parasites, took greater risks during singing compared with nonsinging activities. Likewise, species with high circulating levels of natural antibodies, and hence a history of natural selection caused by bacteria took relatively greater risks during singing than species with few natural antibodies. These findings suggest that risks taken by singing birds have been molded by natural and sexual selection, and that risk taking represents a compromise between the costs and benefits of flight from a potential predator.

Do male ornaments signal immunity in the common yellowthroat?

Garvin, J. C., Dunn, P. O., Whittingham, L. A., Steeber, D. A., Hasselquist, D. — 2008-01-10

Male ornaments may advertise genetic benefits to females choosing mates. These benefits may come in the form of genes for resistance to parasites and disease. Thus, females that prefer more ornamented males as mates may receive genes for enhanced immune system function for their offspring. The common yellowthroat (Geothlypis trichas) is the only species to date in which a male plumage ornament (size of the black facial mask) is known to be related to extrapair mating, and extrapair young are more immunocompetent than their within-pair half-sibs (at least in colder years). To investigate whether male mask size signals superior immune function, we examined male ornaments (mask and bib size and color) in relation to measures of overall health (hematocrit) and immune system function (plasma immunoglobulin G [IgG] concentration and cutaneous immune activity). We also investigated the role that testosterone may play in mediating the relationship between ornaments and immunity. Male mask size was correlated positively with IgG level, suggesting that male mask size may indicate humoral immunity. However, mask size was correlated negatively with hematocrit and cutaneous immune activity (our measure of nestling immunocompetence). Bib size and color were not related to these indices of immune function or health. Plasma testosterone level was neither related to immune function or health nor to the expression of male ornaments. These results suggest that there might be a trade-off between immune system components, as well as between immunity and the production or advertisement of male ornaments.

Effect of UV-reflecting markings on female mate-choice decisions in Cosmophasis umbratica, a jumping spider from Singapore

Lim, M. L.M., Li, J., Li, D. — 2008-01-10

Earlier studies have shown that Cosmophasis umbratica, a jumping spider from Singapore, is sexually dimorphic in the reflectance of ultraviolet (UV) (males, but not females, have UV-reflecting markings). Here we present the first experimental evidence that the male's UV markings influence mate choice of C. umbratica females. When presented with males whose appearance was manipulated by the use of a UV-blocking filter, females spent more time watching UV+ males (i.e., males with UV present) and less time watching UV– males (UV absent). We also manipulated the levels of male brightness by using 2 UV-transmitting neutral density filters and showed that UV reflectance was used specifically for hue discrimination instead of being used for detecting differences in brightness alone. This is not only the first strong evidence of UV influence on female mate-choice decisions for a spider but also the best experimental demonstration of color vision, whatever the wavelength.

Spatial relatedness and brood parasitism in a female-philopatric bird population

Waldeck, P., Andersson, M., Kilpi, M., Ost, M. — 2008-01-10

The spatial structure of relatedness between individuals in a population can be crucial for social selection and evolution. Here we analyze a female alternative reproductive tactic, conspecific brood parasitism, in relation to spatial relatedness among females in a Baltic Sea population of the common eider Somateria mollissima. The role of relatedness in brood parasitism is debated: some models predict parasite avoidance of related hosts, others predict host–parasite relatedness. We estimate pairwise relatedness from protein fingerprinting of egg albumen in 156 nests, with pairwise nest distances ranging from 1 to 6 km. Relatedness increases significantly from the longest distances to an average of r 0.09 below 20 m. Brood parasitism is common, and average pairwise relatedness between host and parasite is estimated at 0.18–0.21. Parasites thus do not avoid relatives, and combined with the findings of a similar study in another eider population, the results show that mean host–parasite relatedness is higher than that among close neighbors. High host–parasite relatedness is therefore not an effect of natal philopatry alone; some other form of kin bias is also involved. Recognition and association between birth nest mates is a candidate mechanism for further study.

Do 3-D predators attack the margins of 2-D selfish herds?

Romey, W.L., Walston, A.R., Watt, P.J. — 2008-01-10

To explain the evolution of grouping, Hamilton's selfish herd theory assumes that predators attack the nearest prey and that both are acting on a 2-dimensional (2-D) plane. This proximity assumption in his theory is one explanation for marginal predation, the phenomenon whereby predators attack peripheral members of a prey group. However, in some ecological circumstances, predators move in 3-dimensional (3-D) space and prey in 2 dimensions. Because a predator coming from above or below the group may have relatively equal access to all members, marginal predation cannot be assumed. In this paper, we test whether marginal predation occurs in such a 3-D/2-D geometry. We carried out 3 controlled laboratory experiments in which fish attack prey grouped at the water's surface. Predators were bass (Micropterous salmoides) or goldfish (Carassius auratus), and prey groups were either free-swimming whirligig beetles (Dineutes discolor) or a constrained group of tadpoles (Bufo bufo). In all 3 experiments, predators were significantly more likely to attack the periphery of prey groups. Our experiments also show that marginal predation is robust to differences in overall density within a prey group and that the fish are not reacting to observable state or behavioral correlates to position within a prey group. Furthermore, our results showed that predators will attack group margins even when there is no variation, due to position, in nearest neighbor distance.

Shared parental care is costly for nestlings of common cuckoos and their great reed warbler hosts

Hauber, M. E., Moskat, C. — 2008-01-10

Obligate avian brood parasitism typically involves one of 2 strategies: parasite chicks are either 1) virulent and evict all other eggs and nest mates to be raised alone or 2) more tolerant and share foster parental care with host chicks for some or the entirety of the nestling period. We studied the consequences of experimentally forced mixed broods of age-matched one common cuckoo (Cuculus canorus) and 2 great reed warbler (Acrocephalus arundinaceus) chicks. In these broods, both cuckoo and host chicks grew slower than did either individual cuckoos or great reed warblers in broods of 1 parasite or 3 host chicks, respectively. Video records showed that in mixed broods, cuckoo chicks received feedings less frequently than the 33% predicted by chance at 4 days of age but parental food allocations increased to chance levels at 8 days of age. The consistent patterns of lower growth rates arose even though chicks in broods of 1 parasite and 2 hosts received the largest prey items per feeding. In addition, several other measures of parental provisioning also did not predict species and brood-specific differences in nestling growth rates across the different treatments. However, variation in begging displays and its specific costs on host and parasite chicks in the different nest treatments were not quantified in this study. We conclude that young of nest mate–evictor common cuckoos benefit from the sole occupancy of host nests in part owing to an initial competitive disadvantage for parental care in broods with age-matched great reed warbler chicks.

Early interactions with adults mediate the development of predator defenses in guppies

Chapman, B. B., Morrell, L. J., Benton, T. G., Krause, J. — 2008-01-10

Antipredator defenses in many species have been shown to exhibit phenotypic plasticity in response to variable predation risk. Some evidence suggests that in certain species adults act as proxy predators, triggering the development of adaptive defenses in juveniles where interaction with a predator is unlikely to occur. However, almost nothing is known about how adult/juvenile interactions mediate plasticity. Here, we examine the nature of the antipredator defenses that develop in Trinidadian guppies as a function of early social experience and investigate the importance of different types of cue (physical, visual, and olfactory) by rearing juveniles under 3 different social conditions. In the first, only juveniles are present; in the second, only visual and olfactory interaction occurs between adults and juveniles; and in the third, adults physically interact with juveniles. Our analyses show that juveniles reared in the physical presence of adults spend significantly less time shoaling with adults than fish from other treatments in an adult versus juvenile shoal-choice trial. Further, we show that juveniles with experience of adult aggression have a decreased response latency to a simulated avian predation attempt and travel a greater distance in the first 5 frames of movement after the simulated strike. Finally, juveniles reared with physical experience of adults developed relatively deeper bodies and were significantly shorter in standard length than guppies reared without physical experience of adults.

Age-related decrease in male reproductive success and song quality in Drosophila montana

Hoikkala, A., Saarikettu, M., Kotiaho, J. S., Liimatainen, J. O. — 2008-01-10

In the present paper, we have studied the effects of aging on male reproductive success and song quality in Drosophila montana. We analyzed the reproductive success of wild-caught males at their normal breeding age during the mating season and after maintaining the males in laboratory from 1 to 5 months. In line with the mutation accumulation theory of aging, none of the factors affecting the reproductive success of wild-caught males during the mating season were related to male longevity. However, mating activity and progeny production of the males decreased with male age. Interestingly, there was no significant variation in progeny production between males at their normal breeding age, whereas at older age, the variation between males becomes significant. The quality of sexually selected song traits deteriorated with male age in concert with the decrease in male reproductive success. The size of the males did not have an effect on male reproductive success at the normal breeding age, but at older age, larger males were able to maintain the sexually selected carrier frequency of the song at higher level than the smaller males. We conclude that by experimentally extending the reproductive age of the males beyond that which they normally experience in nature, it is possible to expose relationships between reproductive success, sexually selected characters, and body size that are not apparent when analyzed for the males at their normal breeding age.

Getting rid of the cuckoo Cuculus canorus egg: why do hosts delay rejection?

Antonov, A., Stokke, B. G., Moksnes, A., Roskaft, E. — 2008-01-10

Egg discrimination is well documented in many hosts of avian brood parasites, but the proximate mechanisms of egg recognition and rejection decisions are poorly understood. Relevant in this respect is the observation that rejectors of parasite eggs often delay their response. This delay has implications for understanding mechanisms important for egg recognition and is the main focus of the present study. We investigated experimentally the relative effects of egg mimicry and eggshell strength of common cuckoo Cuculus canorus eggs on the delay in rejection in marsh warblers Acrocephalus palustris. In addition, by video recording host responses, we elucidate the proximate mechanisms behind the delayed rejections. Host nests were experimentally parasitized with 3 types of real eggs differing in mimicry and/or eggshell strength. Both egg mimicry and eggshell strength significantly affected the time to rejection, but the effect of mimicry was dominant. The delayed rejection of mimetic eggs was explained by the existence of latency to the release of rejection behavior because of recognition problems. Second, when rejection response towards mimetic eggs was initiated, it was less intense compared with hosts experiencing nonmimetic eggs. Our results are consistent with the hypothesis that host motivation when confronted with mimetic eggs needs to increase above a certain threshold before rejection behavior is released, which likely minimizes the risk of recognition errors. An additional component of the delay in rejection as shown by hosts facing nonmimetic eggs was the seemingly inefficient host rejection behavior, probably reflecting lack of previous experience.

Nestling begging intensity and parental effort in relation to prelaying carotenoid availability

Helfenstein, F., Berthouly, A., Tanner, M., Karadas, F., Richner, H. — 2008-01-10

Carotenoids are antioxidants playing major roles in physiological functions at various stages of an animal's life. Female birds deposit large amounts of carotenoids into their eggs. Carotenoids are, however, a limiting resource, and females are expected to balance carotenoid deposition into the eggs with their utilization for themselves. Carotenoid availability is thus likely to determine both the levels of yolk carotenoids and maternal care during rearing. Carotenoids have been shown to benefit the embryo and the growing nestling, and it can be hypothesized that an increase in carotenoid availability during laying leads to higher nestling condition and competitive ability. We manipulated carotenoid availability to great tit pairs prior to and during egg laying and later partially cross-fostered chicks at hatching. During the rearing period, we measured how carotenoid availability affected nestlings begging behavior and male and female feeding effort. We also manipulated the ectoparasite load, predicting that carotenoid supplementation would help adults and nestling to cope with parasites. Nestlings hatched from eggs laid by carotenoid-supplemented females and raised in small broods begged more intensely. Nestlings in small deparasitized broods also begged more actively. The feeding effort of control females increased with brood size, whereas the feeding effort of carotenoid-supplemented females was high whatever the brood size. Male feeding effort was unaffected by our treatment. Our results support the hypothesis that maternally derived carotenoids increase nestling begging behavior and hence competitive ability. They further suggest that carotenoid availability determines the level of parental investment and can mediate trade-offs between life-history traits.

Can hungry nestlings be trained to reduce their begging?

Grodzinski, U., Erev, I., Lotem, A. — 2008-01-10

Nestling begging behavior is usually characterized by a behavioral response of increasing begging levels with an increase in nestling need or hunger. Recent evidence for the possible effect of learning on begging intensity raises the question of how learning can shape this response rule. In particular, it is not clear whether hungry nestlings can learn to reduce their begging when it is not successful or, rather, whether they must first acquire positive experiences with low begging levels in order to do so. To explore this question, we conducted 3 hand-feeding experiments with pairs of house sparrow (Passer domesticus) nestlings. In the first 2 experiments, the nestlings targeted to lower their begging were rewarded mainly or only for low begging postures. However, despite the high expected reward for low begging, these nestlings did not lower their begging. Controlled by their behavioral response function, hungry nestlings were "stuck" at high postures without being able to experience the potential success of low postures. In the third experiment, nestlings targeted to lower their begging levels were rewarded for any begging posture, ensuring that satiation would provide their initial "positive experience" with low begging postures. Begging postures were reduced by this treatment. In light of these results, we suggest that parents are unlikely to reduce offspring begging levels by simply ignoring them. However, they might be able to do so by attending to the begging as soon as possible, thereby allowing their offspring to explore low begging and learn that it is sufficiently effective.

The confusion effect from neural networks to reduced predation risk

Ioannou, C.C., Tosh, C.R., Neville, L., Krause, J. — 2008-01-10

The confusion effect is often cited as an antipredatory benefit of group living and has been demonstrated by numerous studies across a range of taxa. However, there have been relatively few studies examining the mechanisms behind the effect and no experimental test of its supposed theoretical basis (information degradation in neural networks) using a natural predator–prey pairing. In agreement with other studies, we demonstrate that attack success of the three-spined stickleback (Gasterosteus aculeatus L.) is reduced by an increase in Daphnia magna group size. Neural network models attempt to explain this trend with multiple prey inducing poor neural mapping of target prey, thus leading to an increase in the spatial error of each attack. We explicitly tested this prediction and demonstrate that the decrease in attack success by sticklebacks does correspond to an increase in spatial targeting error with larger prey group size. Finally, we show that the number of targets, rather than the density or area occupied by the group, has the greatest effect on reducing the rate of attack. These results are discussed in the context of the information processing constraints of predators, the ultimate cause of the confusion effect.

Increasing vulnerability to predation increases preference for the scrounger foraging tactic

Mathot, K. J., Giraldeau, L.-A. — 2008-01-10

When animals forage in groups, individuals can search for food themselves (producer tactic) or they can search for and join other individuals that have located food (scrounger tactic). The scrounger tactic may provide greater antipredator benefits than the producer tactic because "scroungers" hop with their heads up and tend to occupy central positions in a group, whereas "producers" hop with their heads down and tend to occupy edge positions. We tested whether increasing an individual's vulnerability to predation (using wing-loading manipulations) causes an increased preference for the scrounger tactic in zebra finches (Taeniopygia guttata). Wing-loading manipulations were effective at increasing focal individuals' perception of vulnerability to predation; treatment individuals increased their total time allocated to vigilance, whereas control individuals did not. Treatment individuals also increased their use of the scrounger tactic (proportion of hops with head up) and scrounged a greater proportion of patches, whereas control individuals exhibited no changes. Our results are consistent with the hypothesis that the scrounger tactic confers greater antipredator benefits than the producer tactic, although whether antipredator benefits are achieved through differences in head orientation, spatial position, or both, remains unclear. Our finding that individuals adjust their use of the scrounger tactic according to changes in their phenotype provides evidence for phenotype-limited allocation strategies in producer–scrounger games.