Behavioral Ecology - Advance Access

Social and life-history correlates of hormonal partner compatibility in greylag geese (Anser anser)

Weiss, B. M., Kotrschal, K., Mostl, E., Hirschenhauser, K. — Thu, 19 Nov 2009 06:54:58 PST

In long-term monogamous birds reproductive success varies considerably among pairs. Determinants of reproductive success may be individual as well as pair-specific parameters, including the degree of compatibility and coordination among pair partners. However, little is known about the consistency of partner compatibility with regards to social contexts and life-history changes. In the long-term monogamous, biparental greylag geese, reproductive success was previously found to correlate positively with the degree of hormonal compatibility within pairs. In the present study, we analyzed the degree of within-pair testosterone covariation (TC) in relation to individual and pair-specific life history and social instability. We found that greylag goose pairs facing active social challenge had lower degrees of TC than those in unchallenged pair-bonds, whereas the permanent attachment of a third individual to an existing pair or the number of previous partners did not correspond with changed TC. Furthermore, TC decreased with increasing pair-bond duration and increased with female age but was not related with age of the male partner or other life-history parameters. Hence, our data suggest that hormonal partner compatibility in greylag geese is not a stable trait, but rather reflects a pair's status quo, which may be particularly affected by the stability of the social environment.

Migrant and resident birds adjust antipredator behavior in response to social information accuracy

Nocera, J. J., Ratcliffe, L. M. — Thu, 19 Nov 2009 06:54:57 PST

Animals can reduce their uncertainty of predation risk by attuning to antipredator behavior of others or assessing the risk for themselves. Although it has never been empirically examined in the context of predation, we predicted that animals combine information gleaned from others with their own sampling experience to estimate risk. To test this prediction, we assessed the state-dependent mobbing responses of migrant and resident songbirds at a fall migration stopover site in eastern Canada to stimuli simulating a range of predation risk situations. We presented individuals with social cues in the form of playbacks of black-capped chickadee (Poecile atricapillus) mob-calls conveying graded information about predator size in combination with a predator model (one of two owl species) that rendered the social information either correct or incorrect. The response did not differ based on migratory state; both migrant and resident birds stayed longer at experimental trials when presented with erroneous social information. In particular, response duration of birds presented with a low-threat chickadee mob-call and a high-threat model (understating the risk) was substantially longer than the response to other low-threat call trials, suggesting that individuals were capable of Bayesian updating by devaluing the social cue and acting on their own assessment.

Sex allocation strategies in response to conspecifics' offspring sex ratio in solitary parasitoids

Lebreton, S., Chevrier, C., Darrouzet, E. — Thu, 19 Nov 2009 06:54:57 PST

Parasitoid females adjust their offspring sex ratio in order to maximize their fitness. The optimal sex ratio they produce varies with several factors but especially with competition level. In solitary species, only one adult can emerge from a given host, whatever the number of eggs laid. In some species, the mortality of supernumerary individuals could be due to larval combats. This ability to fight could vary from one sex to another within species. In this way, when females explore an already parasitized host patch, the sex ratio of previous eggs can influence their fitness. These 2 factors could thus strongly influence females’ sex allocation strategies. However, this prediction assumes that parasitoid females can assess the sex of eggs previously laid by conspecifics. We used host acceptance and sex ratio behavior to test this capacity, and our experimental data provide the first evidence for this capacity in a parasitoid species. Females of the solitary ectoparasitoid Anisopteromalus calandrae discriminated the sex of eggs already laid by a conspecific but only when these eggs had reached a certain developmental stage. They adapted their offspring sex ratio as predicted by Hamilton's "sex ratio games" model, allocating the sex of their eggs differentially according to the sex of eggs already on the hosts on which they oviposited. In this way they prevented a lethal larval fight between their sons and the females they could potentially mate after their own emergence, increasing their own fitness and their sons’ reproductive success.

Olfactory recognition of predators by nocturnal lizards: safety outweighs thermal benefits

Webb, J. K., Pike, D. A., Shine, R. — Thu, 19 Nov 2009 06:54:56 PST

Many prey species are faced with multiple predators that differ in the degree of danger posed. The threat-sensitive predator avoidance hypothesis predicts that prey should assess the degree of threat posed by different predators and match their behavior according to current levels of risk. To test this prediction, we compared the behavioral responses of nocturnal velvet geckos, Oedura lesueurii, to chemicals from 2 snakes that pose different threats: the dangerous broad-headed snake Hoplocephalus bungaroides that eats geckos and the less dangerous small-eyed snake Cryptophis nigrescens that eats skinks (i.e., does not consume geckos). We also tested whether predator avoidance by prey was modulated by thermal costs associated with retreat-site selection. In both the presence and absence of thermal costs, velvet geckos avoided crevices scented by both dangerous and less dangerous snake species. When given the choice between a crevice scented by a broad-headed snake and a crevice scented by a small-eyed snake, most geckos avoided either retreat site. These results suggest that velvet geckos treat both snake predators as equally dangerous. To further explore these results, we quantified patterns of retreat-site selection by free-living velvet geckos on 2 sandstone plateaux. As in the laboratory, velvet geckos avoided thermally suitable rocks previously used by both snake species. Hence, a gecko's choice of retreat site is influenced by the presence of snake chemicals but is independent of thermal costs or the level of danger posed by the predator. To minimize their risk of predation, geckos may use a simple rule of thumb: "all snakes are dangerous."

Predator lethality, optimal escape behavior, and autotomy

Cooper, W. E., Frederick, W. G. — Thu, 19 Nov 2009 06:54:55 PST

Flight initiation distance is the distance separating predator and prey when escape begins. The optimal flight initiation distance occurs where expected postencounter fitness is maximized, which depends on the prey's initial fitness, benefits obtainable by not fleeing, energetic escape costs, and expected fitness loss due to predation risk. In current optimal escape theory, prey die when contacted by a predator. We explore effects of variable lethality, L, the probability of being killed on contact. Optimal flight initiation distance increases as lethality increases, matching expectations that prey should not flee when contact entails no fitness loss but should be increasingly wary as expected fitness loss on contact increases. Addition of lethality improves the ability of optimal escape theory to predict effects of factors affecting escape ability. Autotomy, the voluntary shedding of tails or other expendable parts as a last-ditch defense to permit escape, provides an example. After autotomy, running speed decreases in many prey, lethality increases because autotomy cannot be used again until the lost part has regenerated, and ability to obtain benefits may decrease due to reduced social status and foraging ability. These changes favor longer flight initiation distance but lowered initial fitness after autotomy has the opposite effect. Optimal escape theory including a lethality term clarifies how autotomy may lead to increase or decrease in flight initiation distance depending on the balance of its multiple effects. Effects of additional factors that may alter multiple parameters of the model, including age, sex, reproductive condition, injury, disease, and parasitism are discussed.

Evolutionary determinants of modular societies in colobines

Grueter, C. C., van Schaik, C. P. — Tue, 17 Nov 2009 05:06:19 PST

Modular societies are structurally characterized by nuclear one-male units (OMUs, or harems) embedded within larger relatively coherent social bands. Within the order Primates, modular societies are uncommon, found in only a few species, including humans. Asian colobines (Presbytini) principally form either unimale groups that forage independently and are often territorial, or modular associations, which range from tight bands composed of OMUs to loose neighborhoods of OMUs. A phylogenetic reconstruction of modularity in the Presbytini revealed that the single OMU pattern is probably the ancestral state while the modular pattern is derived. The selective forces favoring the evolution of modular societies have thus far been virtually unexplored. Although some ecological explanations cannot be ruled out at the moment due to lack of comparative and quantitative data, preliminary circumstantial evidence does not seem to support them. Instead, a social factor, bachelor threat, is consistent with many observations. This hypothesis argues that where the pressure from nonreproductive bachelor males is unusually high, OMUs aggregate as a means of decreasing the amount of harassment and the risk of takeovers and infanticide. A comparative test found an association between modular societies and bachelor threat, as proxied by sex ratio within social units. The concentration of modular systems in colobines may be due to their unusual ecology, which leads to unusually low intensity of scramble competition. Modular colobines rely more on nonlimiting ubiquitous resources than nonmodular ones and thus can afford to gather in bands. Moreover, by comparing the slopes of regressions between group size and daily travel distance for several groups of one modular and one nonmodular colobine, we found slopes in the nonmodular to be steeper by a factor 30, indicating that ecological constraints associated with scramble competition prevent higher level groupings in nonmodulars. Thus, modular sociality in Asian colobines may have arisen because both social benefits are substantial and ecological costs are relatively low.

Boldness and intermittent locomotion in the bluegill sunfish, Lepomis macrochirus

Wilson, A. D. M., Godin, J.-G. J. — Mon, 16 Nov 2009 05:29:42 PST

Intermittent locomotion, characterized by moves interspersed with pauses, is a common pattern of locomotion in animals, but its ecological and evolutionary significance relative to continuous locomotion remains poorly understood. Although many studies have examined individual differences in both intermittent locomotion and boldness separately, to our knowledge, no study to date has investigated the relationship between these 2 traits. Characterizing and understanding this relationship is important, as both locomotion and boldness are associated with several ecologically relevant behaviors such as foraging, mating, predator evasion, exploration, and dispersal. Here, we report on individual differences in boldness (risk-taking behavior) and intermittent locomotion in a novel laboratory environment in field-caught juvenile bluegill sunfish (Lepomis macrochirus). Our results show that juvenile bluegill sunfish exhibited individual-level variation in 2 modes of intermittent locomotion (undulatory and labriform swimming) and that this variation was correlated with differences in their boldness behavior. Generally, bolder individuals spent more time moving fast for longer durations and with shorter pauses than more timid individuals. Neither boldness nor locomotion was correlated with body size or body condition. This study provides the first empirical evidence for a link between an animal "personality" trait and intermittent locomotion.

Mate choice copying and mate quality bias: are they different processes?

Witte, K., Godin, J.-G. J. — Mon, 16 Nov 2009 05:29:41 PST

Small folivorous primate groups exhibit behavioral and physiological effects of food scarcity

Harris, T. R., Chapman, C. A., Monfort, S. L. — Fri, 13 Nov 2009 07:09:53 PST

The influence of diet and food distribution on the socioecology of group-living species has long been debated, particularly for primates. It has typically been assumed that folivorous primates experience relatively little feeding competition due to the abundant, widespread nature of their food, freeing them to form large groups in response to predation, to disperse with relative ease, and to have egalitarian female social relationships. Recent studies, however, have come to different conclusions about the extent to which folivorous primates are limited by food and experience food competition and how these factors affect folivore socioecology. To better understand the selective pressures that diet places on folivores, we investigated how 2 small highly folivorous groups of colobus monkeys (Colobus guereza) in Kibale National Park, Uganda, responded behaviorally and physiologically to a steep reduction in availability of their most important foods. The monkeys decreased their reliance on their 2 most frequently eaten food species and increased their daily path length, number of feeding patches visited/day, size of individual feeding areas, percentage of time spent feeding, and dietary diversity. They also showed evidence of physiological costs, in that lactating females’ urinary C-peptide levels (i.e., insulin production) declined as top foods became scarce, and parasite loads slightly, but significantly, increased in 2 of 3 adult females examined. These results suggest that highly folivorous primates, even in very small groups, may experience behavioral and physiological effects of food limitation, within-group scramble competition for food, and possibly substantial selective pressures during periods of food scarcity.

How and why the winner effect forms: influences of contest environment and species differences

Fuxjager, M. J., Marler, C. A. — Fri, 13 Nov 2009 07:09:50 PST

Winning aggressive social encounters can enhance the probability of future victories. This so-called winner effect occurs in diverse species and is thought to be an intrinsic phenomenon mediated by postencounter hormone release. Yet, recent evidence suggests the possibility that certain extrinsic factors also influence the winner effect's formation within an individual, possibly by affecting the expression of hormone titers that follow a fight. We first investigated in the monogamous and territorial male California mouse (Peromyscus californicus) whether the effect of residency, an extrinsic factor, influences the winner effect's formation and the endogenous secretion of testosterone and/or progesterone after a dispute. We found that California mice that acquire winning experience in unfamiliar physical locations do not form a full winner effect. Furthermore, this species does not experience a testosterone pulse after a fight in an unfamiliar cage. These findings indicate that environmental context can mediate the winner effect's formation, possibly by affecting the expression of postencounter testosterone pulses. Second, we compared the winner effect of the California mouse to that of the white-footed mouse (Peromyscus leucopus), a close relative that is promiscuous and less-territorial. We found that, compared with California mice, white-footed mice exhibit neither a full winner effect, despite similar past winning experiences, nor a postencounter surge in testosterone. This finding suggests that these behavioral and physiological phenomena vary among even closely related species and are possibly linked to aspects of social biology, including the degree to which individuals of each species are territorial.

Context-dependent fitness effects of behavioral manipulation by a parasitoid

Wed, 11 Nov 2009 03:54:07 PST

Many true parasites and parasitoids modify the behavior of their host, and most of these changes are thought to benefit the parasites. However, field tests of this hypothesis are scarce. We previously showed that braconid parasitoids (Glyptapanteles sp.) induce their caterpillar host (Thyrinteina leucocerae) to behave as a bodyguard of the parasitoid's pupae; they stand bent over the pupae and violently lash out at predators approaching them, resulting in reduced predation of parasitoid pupae on guava trees in the field. In contrast, we show here that this behavioral manipulation does not result in increased parasitoid survival on eucalyptus trees, an introduced host plant species. Hence, the effects of behavioral manipulation of the host depend on the ecological context. We hypothesize that this is due to a different species composition of the community of predators and hyperparasitoids on the 2 host plant species. Our results show that fitness effects of behavioral manipulation should be evaluated in a setting that includes all relevant components of the natural food web.

Long tails matter in sugarbirds--positively for extrapair but negatively for within-pair fertilization success

Thu, 05 Nov 2009 07:53:13 PST

Extrapair mating is known to occur in many animals and potentially has a significant influence on reproductive success. Female extrapair mate choice may explain the occurrence of exaggerated ornaments in socially monogamous species, but the influence of ornamentation on extrapair mating success has rarely been investigated experimentally. Cape sugarbirds (Promerops cafer) are socially monogamous, almost always lay 2 egg clutches and have long-term pair bonds often lasting several seasons. However, they also display sexually dimorphic extravagant ornamentation in the form of a long, graduated tail and have one of the highest rates (65% of young) of extrapair paternity recorded in birds. We provide a test of the hypothesis that the ornamented tail is used in mate choice for extrapair partners by conducting an experiment in which tail length was manipulated after social mating but before copulation. This experiment therefore allowed females to respond to the manipulation of male tail length when making a choice of copulation partner but not social mate. We show that the tail length of male sugarbirds has a significant effect on the success of males in gaining extrapair paternity, with long-tailed males obtaining significantly more extrapair young than short-tailed males. However, males with short tails sire a significantly greater proportion of the nestlings in their own nests than long-tailed individuals. This result suggests that males may adopt alternative strategies dependent on their ornamentation.

Predator escape tactics in birds: linking ecology and aerodynamics

van den Hout, P. J., Mathot, K. J., Maas, L. R. M., Piersma, T. — Thu, 05 Nov 2009 07:53:12 PST

In most birds, flight is the most important means of escape from predators. Impaired flight abilities due to increased wing loading may increase vulnerability to predation. To compensate for an increase in wing loading, birds are able to independently decrease body mass (BM) or increase pectoral muscle mass (PMM). Comparing nearshore and farshore foraging shorebird species, we develop a theory as to which of these responses should be the most appropriate. We hypothesize that nearshore foragers should respond to increased predation by increasing their PMM in order to promote speed-based escape. Instead, farshore foragers should decrease BM in order to improve agility for maneuvering escape. Experiments on 2 shorebird species are consistent with these predictions, but on the basis of the theoretical framework for evaluating effect size and biological significance developed here, more experiments are clearly needed.

Reduced extrapair paternity in response to experimental stimulation of earlier incubation onset in blue tits

Fri, 30 Oct 2009 06:15:13 PDT

Although the causes and consequences of extrapair paternity (EPP) have been studied extensively in birds, little is known about the regulation of extrapair copulation (EPC) behavior and how it may tie in with other aspects of avian reproduction. In birds, the presence of eggs stimulates incubation and, subsequently, the cessation of egg production. We propose that the same mechanism also regulates female motivation to engage in EPCs. To test this idea, we simulated the earlier onset of laying in blue tits (Cyanistes caeruleus), by adding model eggs to nests before natural laying commenced. Most females accepted these eggs, covering them with nest material in the natural way. As predicted, these broods hatched more asynchronously than control broods, revealing an earlier onset of incubation, and were less likely to contain extrapair offspring (EPO) suggesting that stimulation from eggs also inhibits motivation to seek EPCs. Egg stimulation is thought to cause cessation of laying a fixed number of days before clutch completion, after a certain hormonal threshold is exceeded. Similarly, a lower threshold may inhibit engagement in EPCs relative to clutch completion, explaining the proportional increase in EPO toward the extremes of clutch size, which we also observed, and would be consistent with a fertility or compatibility insurance function for EPCs. Our findings may represent the best experimental evidence for female-mediated effects on the timing of EPCs and suggest how EPP may be integrated within the regulatory mechanism of avian reproduction.

Sex matters: a social context to boldness in guppies (Poecilia reticulata)

Tue, 27 Oct 2009 11:15:16 PDT

Boldness is a key element of behavioral variation in animals. Many studies have shown variation between individuals in their propensity to take risks across a wide range of taxa, yet surprisingly few studies have investigated the importance of social context in influencing an animal's boldness. Here, we focus on the role that the sex composition of a social group plays in individual boldness in a sexually dimorphic species, the Trinidadian guppy (Poecilia reticulata). We predict that after exposure to a simulated aerial predator, male guppies should be bolder in the presence of females compared with males to maximize their mating opportunities. Furthermore, we predict that female guppies will adopt riskier behavior when shoaling with males in an effort to avoid sexual harassment. Using a model avian predator, we tested these hypotheses and found evidence to support our second prediction but not our first. Specifically, we found that male guppies returned to movement more rapidly after a fright response when shoaling with males than with females. Female focal fish in contrast returned to movement significantly quicker when shoaling with males than females. Also, we found a significant correlation in boldness across social contexts (a behavioral syndrome) in male but not female fish. This study highlights the importance of social context for individual boldness and suggests that for risk-averse behavior in social, sexually dimorphic species, sex matters.