Behavioral Ecology Vol. 13 No. 6: 750-756
© 2002 International Society for Behavioral Ecology
Male tactics and reproductive success in the harem polygynous bat Saccopteryx bilineata
Institut für Zoologie II, Universität Erlangen-Nürnberg, Staudtstrasse 5, D-91058 Erlangen, Germany
Address correspondence to G. Heckel, who is now at Computational and Molecular Population Genetics (CMPG), Zoologisches Institut, Baltzerstrasse 6, CH-3012 Bern, Switzerland. E-mail: gerald.heckel{at}zoo.unibe.ch.
Received 20 June 2001; revised 7 February 2002; accepted 14 February 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Alternative tactics in reproductive behavior enable individuals to maximize their fitness in relation to competitors in the same population. In many taxa, territoriality is a common tactic of males to increase their reproductive success. In the bat Saccopteryx bilineata, territorial males defend roosting areas for females against other males and court females throughout the year. Peripheral males in the same colonies do not defend territories but compete with territorial males for reproduction with females. In this study, we monitored the behavior of the males in a natural colony over three reproductive seasons. We compared morphological and age data and measured the reproductive output of males adopting the territorial or peripheral tactic. No differences in body size or weight were detected between male types, but the probability of adopting a tactic seemed to be age dependent. Peripherals were often young males and replaced territorials in several cases, whereas the opposite case was not observed. Peripherals were not excluded from reproduction, but territorials were more likely to reproduce. Variation in reproductive success was high within both male tactics, and the reproductive success of some peripherals was comparable to territorials, but, on average, the reproductive success of territorials was more than twice as high. Therefore, behavioral tactics do not seem to be equally profitable in general but may represent different phases in the reproductive life of many S. bilineata males.
Key words: age dependence, alternative mating tactics, bats, Chiroptera, reproductive success, paternity testing, Saccopteryx bilineata.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Alternative reproductive behaviors allow individuals to maximize their fitness in relation to competitors. Reproductive behaviors may be classified according to their genetic backgrounds (Gross, 1996
). Alternative
reproductive strategies are based on a genetic polymorphism and have been
demonstrated for only few systems (marine isopod, Paracerceis
sculpta: Shuster and Wade,
1991; Shuster and Sassaman,
1997; swordtail, Xiphiphorus nigrensis:
Ryan et al., 1992; ruff,
Philomachus pugnax: Lank et al.,
1995). Much more frequently, variation in reproductive behavior
has been attributed to alternative tactics within a conditional,
evolutionarily stable strategy. The use of alternative tactics involves a
decision of the individual based on its current status in comparison to other
individuals in the population. The chosen behavioral tactic should result in
higher reproductive success for the individual compared to the alternative
(Gross, 1996). Whereas the
parameters that influence an individual's decision to exhibit a particular
tactic received much attention earlier (e.g., age dependence:
Brockmann and Penn, 1992;
Coltman et al., 1999;
Eadie and Fryxell, 1992;
Koprowski, 1993; body size
dependence: Danforth, 1991;
Emlen, 1994;
Forsyth and Alcock, 1990;
Gross, 1982), the inequality
of fitness in terms of realized reproductive success was not investigated
until recently (Asa, 1999;
Brockmann et al., 1994;
Hogg and Forbes, 1997;
Martin and Taborsky, 1997;
Stockley et al., 1994).
In mammals, reproduction of females can be determined by observation of
parturition, and maternal care is in many species a good indicator of
motherhood. Therefore, a female's reproductive success can often be determined
by behavioral observations. In contrast, a male's reproductive success is much
more difficult to determine. Even detailed observations of male mating success
may result in inaccurate estimates for reproductive success (e.g.,
Pemberton et al., 1992), and
complex social systems with many competing males challenge the quantification
of mating behavior through observations. This is especially true for
small-bodied, nocturnal, and highly mobile animals like bats, for which
observations of behavior are particularly difficult in most species.
The roosting ecology of the white-lined bat (Saccopteryx
bilineata) is exceptional because it allows detailed observations of
behavior and social structuring (e.g.,
Bradbury and Emmons, 1974;
Bradbury and Vehrencamp, 1976;
Tannenbaum, 1975). In S.
bilineata "territorial" or "harem" males defend
in the daytime roost territories with groups of one to eight females against
other males. These associations are relatively stable and were characterized
as harems by Bradbury and Emmons
(1974) and Tannenbaum
(1975). However, in contrast
to harems sensu Emlen and Oring
(1977; e.g.,
Ambs et al., 1999;
Amos et al., 1993;
Asa, 1999;
Pemberton et al., 1992;
Storz et al., 2001), males
dominating territories were shown to sire on average only 30% of the juveniles
born to the females in their harems
(Heckel et al., 1999; Heckel
and von Helversen, in preparation). Larger colonies of S. bilineata
typically contain several harem groups in the same roost in close proximity.
Colonies may comprise up to 60 individuals, and, typically, additional to
territorial males, so-called "peripheral" males, are present in
the same roost. Peripheral males roost either directly at the boundary of
harem territories or within a short distance. Territorials maintain their
territories throughout the year and spend a lot of time and presumably
considerable energy on activities that serve to maintain a harem territory.
Both the numbers of flight maneuvers and courtship displays of males increase
with the number of females in a harem
(Voigt and von Helversen,
1999). In contrast, peripherals show no territorial defense and
court females only occasionally.
In this study we investigated the occurrence and fitness relevance of alternative reproductive tactics in male S. bilineata. We analyzed morphological and life-history data of males adopting the territorial or peripheral tactic over several reproductive seasons to identify possible causes for differences in behavior. We hypothesized that differences in reproductive effort between male tactics should result in differences in the probability of reproduction. We used molecular genetic methods to determine the reproductive output of the territorial and peripheral tactic in a natural colony and to assess the extent of variation in reproductive success between males within and among tactics.
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MATERIALS AND METHODS |
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Study population
We conducted observations and collected genetic samples in a natural population of Saccopteryx bilineata at the research station La Selva of the Organization for Tropical Studies in Costa Rica (10°20' N, 84°10' W). Behavioral observations were performed in a S. bilineata colony that used an abandoned house as a daytime roost, which was surrounded by primary and secondary lowland rainforest. Our field data were collected over three consecutive reproductive seasons. Initial observations were conducted from September to October 1995, and individual marking of bats was complete at the end of this period. We performed observations in greater detail from June to August 1996, January to February 1997, June to August 1997, and June to August 1998. Observations in June—August covered the parturition period until weaning. The periods between September—October and January—February correspond to the time before and during the mating period, respectively. We caught the bats with mist nets when they emerged from the colony at dusk. Capture took place at least 5 m from the roost to avoid adverse effects on behavioral observations. Upon capture, we weighed all bats (Pesola Scale, accuracy 0.1 g), measured the forearm, assessed the reproductive status of new individuals, took a small piece of wing tissue from the plagiopatagium for genetic analysis, and attached colored plastic bands to the forearm for individual marking. For details on the study colony and marking and sampling techniques, see Heckel et al. (1999
).
We assessed the age and reproductive status of males by the condition of
the odor sacs in the wing membrane. In adult males, the interior of the sacs
is of light color, and the sacs emit a strong odor, whereas juvenile and
subadult males have dark, nonsmelling sacs
(Tannenbaum, 1975;
Voigt and von Helversen,
1999). First-year males can be identified by the dark appearance
of the odor sacs up to 9 months after their birth in June (Heckel, unpublished
data). New adult males caught during the mating period had been born no later
than June 1.5 years earlier, and new adult males caught during the parturition
periods had been born in the previous June or earlier. As a consequence, our
age estimates may underestimate the variation in age among the proportion of
males that immigrated or that were adults before the beginning of individual
marking in 1994 (see Voigt and von
Helversen, 1999). Tannenbaum
(1975) extrapolated from a
2-year study that some males may reach 7 years of age or more.
Observations of social behavior
In advance of the study, the animals were habituated to human observers in
the colony. We surveyed the presence of individual bats and social structuring
of the colony by census observations as described in Heckel et al.
(1999). Censuses were
conducted once a week during September and October 1995 and at least twice a
week during the next observation periods. For males, we noted all territorial
and courtship behavior during censuses and used this information to determine
the social status of a male (see Bradbury
and Emmons, 1974; Heckel et
al., 1999). We classified males as territorials if they roosted in
close proximity to females, courted them repeatedly, and defended the area
against other males. All other males were classified as peripherals. The term
"peripheral" is used here for all males without females, as well
those that roost adjacent to a territory with females as others that use more
distant sites in the roost (see
Tannenbaum, 1975). When the
entire study period was considered in our analyses, males were classified as
adopting the territorial tactic if they defended a territory at some point
during the study. This classification may underestimate the number of males
that become territorial because some peripherals could change their status
after the end of our observations. However, under the assumption that switches
of tactics occur unidirectional from peripheral to territorial, potential
differences between tactics would only be diminished by a misclassification of
later territorials.
Paternity analyses
We obtained tissue samples (n = 176) from all observed bats of the
colony with the exception of two juveniles born in the colony in 1996 that
disappeared before they could be sampled and one immigrant female in 1998 that
left the colony with her juvenile before being sampled. These nonsampled
individuals were not considered in further analyses.
Paternity analyses were performed for each juvenile cohort separately. To
obtain maximal reliability of paternity assignments, we determined the
maternal descent of juveniles and used this information for final
determination of paternal descent. First, we assigned juveniles to their
mothers according to the observation of mother—offspring-specific
behavior as described elsewhere (Heckel et
al., 1999; Heckel and von Helversen, in preparation). Second,
genetic maternity assignments (see below) were used to verify behavior
observations. Third, in the cases where behavioral observations of juveniles
and their associated mothers were insufficient, we used genetic data alone to
assign juveniles to females. Sires were always determined by genetic
assessment only.
We used a set of 11 microsatellite loci for genetic parentage determination
(Sb60—Sb111, see Heckel et al.,
2000). Details on laboratory procedures and the exclusionary power
for each locus in the population are described in Heckel et al.
(2000). The minimal number of
analyzed loci per individual was 10, and, on average, 99.8% of the individuals
in the colony could be typed at each microsatellite locus. The total
exclusionary power in the analyzed data set was 99.999% for assignment of the
father (calculated using Cervus 1.0 B;
Marshall et al., 1998).
Exclusion probabilities were also calculated using the formula provided by
Jamieson (1994). This method
uses only allelic data of adult individuals and assumes non-related males and
females. The average combined exclusion probability for paternity according to
Jamieson (1994) was 99.9999%
for the study population.
We used traditional parentage exclusion as well as a likelihood method
(Marshall et al., 1998) to
obtain maximal reliability for assignment of fatherhood. In parentage
exclusions, we allowed a maximum of one nonmatching allele among all 11
microsatellite loci per parent—offspring pair to account for possible
allelic mutations (see Pemberton et al.,
1995). When mother and juvenile genotypes were not identical or
when juveniles were homozygous, the allele could be identified that had been
transmitted to the juvenile by the father. This allowed the exclusion of those
males as fathers, which did not share the paternal allele with the
juvenile.
In a second approach, we determined the most likely sire of a juvenile
among the males in the colony with the program Cervus 1.0 B
(Marshall et al., 1998).
Cervus 1.0 B uses simulations to attach confidence levels to comparisons of
the likelihood ratios of putative fathers. We deducted realistic, yet
conservative, estimates for the parameters used in simulations from the
maternity exclusions. The typing error for simulations was set to 0.01. A
proportion of 89% sampled candidate parents was used in simulations with
Cervus, although maternity exclusions provided no evidence for missing
mothers, and all observed males were sampled at the end of each observation
period. Calculations were performed for two confidence levels (relaxed: 80%;
strict: 95%) to assess the extent of uncertainty in assignments.
Statistical analyses
Statistical analyses were performed using the program SPSS 6.0 (SPSS Inc.).
We used parametric tests for comparisons of morphological characters. Data on
reproductive success were not normally distributed. Therefore, we used
nonparametric tests for all comparisons of reproductive success. Corrections
for tied ranks were applied to statistical tests whenever necessary. All tests
were performed two-sided.
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RESULTS |
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Occurrence of behavioral tactics
We observed 32 different males in the colony over the course of the study (Table 1). The number of males varied between observation periods from 16 to 21, and on average 86 ± 7% of the males in the colony were observed in the next observation period. The number of males was stable within the periods. All males present in the colony within an observation period were observed during each census. In the 3 years altogether, 18 different males were seen to defend territories and court females in their territory. Fourteen males were never observed to defend a territory with or without females and were therefore classified as peripherals. Consistently more territorial than peripheral males were present in the study colony in each observation period. Within a single observation period, up to 12 males showed territorial behavior in the colony, and a maximum of nine males were peripheral. The proportion of peripherals in the colony was not significantly different between single periods, with a minimum of 30% and a maximum of 44% of all males (Fisher's Exact test; all p > .5)
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Temporal patterns and morphological characteristics of male
tactics
Social behavior of some males changed over the course of the study
(Figure 1). Seven males became
territorial during the study period. Three of these males were known
descendants of the colony. Initially, the males showed peripheral behavior
after sexual maturity and then replaced territorial males. In seven additional
cases, males that had been born in the colony remained there as peripherals
after adulthood, and one male obtained territorial status within his first
year. A switch from the territorial to the peripheral tactic was not observed.
Switches between tactics occurred only between the observation periods. When
territories had been taken over by new males, the former owners of the
territories were neither seen in the colony afterward nor recorded in any
other roost in the La Selva area, nor were they caught during mist
netting.
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The average age of the males differed significantly between male groups. Territorials were typically older than peripheral males (territorials: n = 18, mean = 37 ± 13 months; peripherals: n = 14, mean = 23 ± 16 months; Mann-Whitney U, U = 55.5; p = .006). Because the age of some territorial males could only be estimated as a minimum age since the beginning of the study, actual differences might be larger. Morphological measures revealed no significant differences between the males of the two behavioral groups. The average forearm length and standard deviation was 44.9 ± 1.9 mm for territorials and 45.3 ± 0.6 mm for peripherals (t test, unequal variances, t = -1.05; df = 25.03; p = .3). The mean weight of territorials was also not significantly different from that of peripherals (territorials: 7.6 ± 0.4 g; peripherals: 7.6 ± 0.4 g; t test, t = 0.07; df = 30; p = .5)
Determination of male reproduction
We were able to determine the fathers of 77 of 93 juveniles born in the
colony (83%). After paternity exclusions, only one nonexcluded male each
remained as putative father for 70 juveniles (75%). For 16 juveniles (17%),
all analyzed males were excluded by at least two genotypic mismatches. For
seven juveniles (8%), more than one male was either not excluded as father or
excluded at only one microsatellite locus. For these seven juveniles, final
assignments of fathers were based on significant differences between the
likelihood ratios of the most likely males. Likelihood calculations for six
juveniles resulted in a significant difference between the likelihood ratios
of the two most likely males at the 95% confidence level. Paternity for one
juvenile with ambiguous paternity after exclusions of nonfathers was assigned
at the 80% confidence level using the likelihood method.
Reproduction of territorial and peripheral males
Reproduction was not evenly distributed among male groups
(Figure 2). Proportionally more
territorial than peripheral males reproduced in the colony over the course of
the study: 15 of 18 territorials (83%) and 6 of 14 peripherals (43%; Fisher's
Exact test, p = .027). Thus, territorial males were almost twice as
likely to reproduce within the three reproductive seasons than peripheral
males. The proportion of reproductive males within single years was
significantly different between tactics for 1996, but not for the other years.
In 1996, 9 of 12 territorials sired offspring in the colony and only 2 of 9
peripherals (Fisher's Exact test, p = .03). In 1997, 10 of 12
territorials reproduced and 3 of 6 peripherals (Fisher's Exact test,
p = .27); in 1998, 9 of 12 territorials reproduced and 5 of 10
peripherals (Fisher's Exact test, p = .38).
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The number of juveniles sired by territorial and peripheral males differed
for single years as well as for total reproduction between 1996 and 1998
(Figure 3). In general,
territorials fathered a significantly higher number of juveniles in each
breeding season (chi-square test; 1996: 
2 = 14.7; p
< .001, 1997: 2 = 7.2; p = .007, 1998:
2 = 4.8; p = .028). Overall, territorials sired 61
(65.6%) of the analyzed juveniles, whereas peripherals fathered 16 juveniles
(17.2%) within the same period in the investigated colony (2 =
24; p < .001).
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The difference in the reproductive output of the tactics was also caused by the higher number of territorial males in the colony. Therefore, the average number of juveniles sired per male and season was calculated for both male groups. Territorial males sired, on average, significantly more offspring per reproductive season than peripherals. The average for territorials was 1.7 juveniles per reproductive season, whereas peripherals had on average 0.6 offspring per season (Mann-Whitney U; U = 59.5; p = .009). The distribution of annual reproductive success of territorial and peripheral males is presented in Figure 4. In both groups, some males failed to reproduce in single years, others in two, or some males even in all years. Variation in reproductive success was greater in territorial males. Territorials had zero to six offspring per year in the colony (median 1), whereas peripherals fathered a maximum of four juveniles per year (median 0). Thus, males adopting territorial behavior had, on average, higher reproductive success in the colony compared to peripherals.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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S. bilineata males show differences in reproductive behavior within the same colony. Territorial males defend roosting territories within the daytime roost and court associated females. Peripheral males show no territorial defense and engage only in courtship if approached by a female. In a natural population at La Selva, Costa Rica, morphological traits were similar among males using these two alternative behavioral tactics. Which tactic was adopted depended on the age of the individual. Territorial males were typically older males. Some peripheral males changed their behavior over the course of the study and became territorial, whereas territorials maintained their status until they disappeared from the colony. Males of both tactics reproduced in the colony, with a higher proportion of reproduction in territorials. Variation in reproductive success was similar in both groups of males. However, on average, territorial males produced more offspring per reproductive season than peripherals.
Mating or reproductive success has often been investigated in regard to
dominance status of males (e.g., Amos et
al., 1993; Coltman et al.,
1999; Feh, 1999;
Hogg and Forbes, 1997;
Koprowski, 1993;
Pemberton et al., 1992;
Worthington Wilmer et al.,
1999) or territorial behavior
(Moore et al., 1995;
Wirtz, 1982). However, the
role of subordinate males in a population has received little attention, and
reproductive success of these males has rarely been measured. In mammals, only
few studies have investigated the reproductive outcome of alternative male
tactics in natural populations (e.g., common shrews, Sorex araneus:
Stockley et al., 1994; bighorn
sheep, Ovis canadensis canadensis:
Hogg and Forbes, 1997). The
inability to clearly define the entire extent of variation in reproductive
success between males in natural populations has often been caused by the
transient nature of social associations and limitations in sampling or
paternity testing (e.g., Amos et al.,
1993; Burland et al.,
2001; Coltman et al.,
1998; Petri et al.,
1997; Rossiter et al.,
2000; Worthington Wilmer et
al., 1999).
The high stability of S. bilineata groups enabled us to determine
the variation in reproductive success within and among male behavioral
tactics. The observed turnover in males was low, but temporary switches in
territory ownership during the mating season could have remained unnoticed.
However, such temporary switches are unlikely in a mating system with
year-round defense of territories. Takeovers of territories are typically
accompanied by fierce contests among S. bilineata males, and former
harem holders disappear from the colony thereafter
(Tannenbaum, 1975; Heckel,
unpublished data; Meister S, personal communication). Moreover, if such
temporary switches occurred, they obviously had few consequences because they
resulted at most in the 16 paternities for which the peripheral males
accounted. Attributing such potential paternities of peripherals to the
territorial tactic would only further increase the detected differences in
reproductive success.
Further consideration is required for the possibility that the studied
males reproduced additionally with females in other colonies. Particularly,
high reproductive success of peripherals with females from other colonies
could diminish the detected differences to territorials. However, within a
colony there is no evidence that peripherals reproduce more successfully than
territorials with females in other territories
(Heckel, 2000). Preliminary
data from a larger survey of paternity showed that several territorial males
sired offspring with females in a neighboring colony, whereas this was not
documented for peripherals (Heckel et al., in preparation). This indicates
that the detected differences in reproductive success between territorials and
peripherals are unlikely to be reversed by additional reproduction outside the
colony.
The fitness consequences of alternative male tactics were tested under
seminatural conditions in the cichlid Pelvicachromis pulcher
(Martin and Taborsky, 1997).
The analysis of cost and benefit of reproduction revealed higher reproductive
success for territorial males compared to nonterritorials. Harem males of
P. pulcher sired significantly more offspring than monogamous or
satellite males. Calculations of costs per sired offspring indicated that
satellite males had higher costs for parental defense than the alternative
monogamous and harem tactics. Martin and Taborsky
(1997) attributed the low
reproductive cost for territorial harem males in P. pulcher to the
fact that these males cooperate with subordinate satellite males, which do
most of the territorial defense and sire only minor proportions of the harems
offspring.
Similar types of cooperation between males and skewed reproduction in
coalitions have been demonstrated for some mammals (lions, Panthera
leo: Packer et al., 1991;
feral horses, Equus caballus: Asa,
1999; Feh, 1999).
In those systems, the formation of coalitions depends strongly on the
intensity of competition for female mates in the population. In S.
bilineata, paternity data proved that many males competed successfully
for females, and fierce contests were observed among males
(Tannenbaum, 1975; Meister S,
personal communication). Cooperation in territorial defense among males,
however, or tolerance of other adult males in a territory has not been
observed. Harem males defend territory boundaries throughout the year and try
to expel any male intruder immediately. This suggests high risk and energetic
demand for territorial males in comparison to peripherals.
The higher investment of territorials is paid off by higher reproductive
success compared to peripherals. Energy measurements showed that the variation
in energy expenditure of males was large, but unexpectedly, peripheral males
had metabolic rates similar to harem males
(Voigt et al., 2001). This
finding was explained by potentially high additional energetic costs for
peripherals (e.g., for interactions with territorial males or females in the
roost, or for long-distance flights in search of mates at night). However, the
metabolic scope of S. bilineata is low, so that males might have to
"allocate energy economically to activities associated with female
recruitment and territorial defense"
(Voigt et al., 2001: 35).
Nevertheless, the flight and courting activity of S. bilineata males
increases significantly with the number of females in their territories, as
does the energy expenditure (Voigt and von
Helversen, 1999; Voigt et al.,
2001). This suggests that activities to maintain territories and
court females are not irrelevant for a males' physical condition.
An individual's body condition is often the most important determinant for
alternative mating tactics (e.g.,
Danforth, 1991;
Emlen, 1994;
Forsyth and Alcock, 1990;
Gross, 1982;
Martin and Taborsky, 1997;
Reynolds et al., 1993). Larger
and heavier males are typically dominant in male—male contests and
reproduce more often (see Gross,
1996, for overview). For S. bilineata males, no
differences in body size or weight were found between individuals using
alternative behavioral tactics. This is surprising because a large body could
be advantageous for S. bilineata males in fights for territory
ownership. Stockley et al.
(1994) reported that
differences in body weight between male common shrews (Sorex araneus)
adopting alternative tactics may vary with age. Males that differed in
mate-searching behavior had different body weights at an early stage of
maturation, but there was no difference in the adult body size of the two
types of males. S. araneus is short-lived, and territories of males
are newly established in spring each year. Early maturation seems to help
males gain advantages in establishing large territories. In S.
bilineata, however, territories are not available regularly and males
reach adult size and weight long before taking over a territory
(Heckel et al., 1999;
Tannenbaum, 1975). Therefore,
adult morphology may be important, but early maturation is unlikely to improve
chances for territory acquisition.
For S. bilineata, the courtship behavior of males is more likely
to select for an equal body size among male types. The courtship display of
males involves repeated hovering in front of females and the frequency and
duration of hovering displays may influence a male's reproductive success
(Voigt and von Helversen,
1999). It has been shown in other bats that hovering flight is
energetically very costly, and energy expenditure increases with body weight
(Voigt and Winter, 1999).
Larger and heavier S. bilineata males might have advantages in direct
male—male contests, but they would also have to face much higher
energetic costs for courtship. Under these premises, balancing selection for
high fighting ability and low energy expenditure in hovering flights would
result in an intermediate body size for all males.
Territorial males had on average higher reproductive success than
peripherals, and the social histories of males
(Figure 1) suggest that
peripherals attempt to acquire a territory sooner or later, but queuing for
territories (see Kokko and Johnstone,
1999) would probably not enable all males to acquire a territory.
Takeovers of territories are not frequent
(Heckel et al., 1999;
Tannenbaum, 1975), and the
number of peripherals clearly exceeded the number of takeovers during the
study. Territories may become available for peripherals unexpectedly if
territorial males disappear (e.g., due to death or predation). Such instances
give peripherals the chance to obtain a territory without challenging a
territorial male, but they would still have to compete for it with other
peripherals. This means that those peripherals, which are inferior in direct
male—male contests, may never be able to acquire a territory. They will
have to make the best of a bad job and try to reproduce without direct access
to females.
Further investigations are needed to understand reproduction of
peripherals. The relatively high reproductive success of some peripheral males
may indicate that peripheral behavior can be an acceptable alternative to
territoriality. These peripherals had no costs for territorial defense but
sired numbers of juveniles that were similar to that of some territorials.
Reproduction of peripherals is possible because territorial males are not able
to control the movements of the females in their harems
(Heckel et al., 1999; Heckel
and von Helversen, in preparation). Females are able to choose their mating
partners freely because harem males provide no paternal care. The social
dominance of harem males exhibited by the persistent maintenance of a
territory might indicate male quality and could therefore explain why most
females reproduced with territorials. Investigations that follow the behavior
and reproductive success of individual males over their lifetime could clarify
whether some peripherals could potentially compensate lower reproductive
success per year with longer persistence in the colony.
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ACKNOWLEDGEMENTS |
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We thank Frieder Mayer for technical advice and the use of lab facilities during the study. Eric Petit and Melanie Decker provided help in the laboratory, and Marco Tschapka, Christian Voigt, and Birgit Reuter contributed to fieldwork. Thanks go to Sonja Meister, Christian Voigt, Andrew Bourke, and two anonymous reviewers for critical discussion and helpful comments on the manuscript. We thank the Costa Rican authorities, especially the Ministerio del Ambiente y Energía and the Area de Conservación Cordillera Volcanica Central, for research permits and the Organization for Tropical Studies for the permission to work at the biological station "La Selva." This study was supported by a grant from the Deutsche Forschungsgemeinschaft (DFG).
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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