Agonistic Behaviour and Sexual Conflict in Atypical Reproductive Groups: The Case of Bearded Vulture Gypaetus barbatus Polyandrous Trios

Social behaviour in species forced to form atypical breeding coalitions is poorly documented. The saturation of optimum territories in the Bearded vulture Gypaetus barbatus population in the Pyrenees has led floating males to settle in already occupied territories, thereby forming polyandrous trios. We examined the patterns of intrasexual aggression in five trios (nine reproductive events in total) during courtship. Alpha males initiated 82% of agonistic encounters that were mainly aimed at preventing or disrupting copulations. During the fertile period in recently formed groups, intrasexual aggression had a negative influence on the frequency of heterosexual copulations, which may be a contributing factor to the lower productivity of polyandrous trios. Females rejected a higher proportion of beta male- than alpha male-initiated copulations, and rejected copulations with both alpha and beta males more often when the other male was present close by. These results indicate that alpha males cannot effectively prevent all copulation attempts by beta males and that females avoid harassment by minimizing sexual activity when both males are present. Aggression between males decreased with time, occurring less often in established than in recent trios, despite the fact that the frequency of heterosexual copulations – the cause of conflicts – was similar. The frequency of homosexual interactions tended to increase in established trios, suggesting that this behaviour may help to regulate aggression within these groups, although no significant relationship between homosexual interactions and aggression was found. In summary, reproductive conflicts in trios seem to be unavoidable, although they tend to decrease if the group is maintained. This suggests that, for birds in these groups, the maintenance of a quality territory is more important than solving sexual conflicts.     

Male‐Biased Mate Competition in King Penguin Trio Parades

Darwin devised sexual selection theory to explain sexual dimorphisms. Further developments of the theory identified the operational sex‐ratio (OSR) as one of its cornerstones, and it was commonly admitted that an OSR biased toward one sex would lead to stronger selection pressures toward that sex. Recent theoretical developments have challenged this view and showed that the OSR alone does not determine the direction of sexual selection, more particularly in mutually ornamented species exhibiting high and similar parental investment by both sexes. These developments, however, focused on mutual intersexual selection, and little is known about intrasexual selection of both males and females in species exhibiting such characteristics. The first aim of our study was to test the relative involvement of males and females in same‐sex contest over mates in the king penguin, a species exhibiting mutual ornamentation of the sexes, high parental investment by both sexes, and a male‐biased OSR. We investigated the sex composition of trio parades, which are groups of three individuals that compete for mates during pair formation. We found that these trios consist of a female trailed by two fighting males in 19 of 20 cases; the 20th trio was all male. The second aim of our study was to investigate the existence of within‐sex differences in colour ornaments between individuals involved in such trios and individuals already paired. While limited sample sizes precluded detection of statistically significant differences between trios vs. pairs, reflectance measurements suggested that the beak spot of males in trios were more strongly ultraviolet than the beak spot of males in pairs. We concluded that intrasexual selection in our colony follows the typical pattern of mate competition observed in species in which sexual dimorphisms and OSR are male biased, and discussed the ultraviolet difference within the framework of the king penguins' colour perception.     

Sex and Weapons: Contrasting Sexual Dimorphisms in Weaponry and Aggression in Snapping Shrimp

Sexual dimorphisms in weaponry and aggression are common in species in which one sex (usually males) competes for access to mates or resources necessary for reproduction – sexually dimorphic weaponry and aggression, in other words, are frequently the result of intrasexual selection. In snapping shrimp, the major chela (snapping claw) can be a deadly weapon, and males of many species have larger chelae than females, a pattern readily interpreted as resulting from intrasexual selection. Thus, males might be expected to show more sex‐specific aggression than females, and be more aggressive overall. We tested these predictions in two species of snapping shrimp in a territorial defense context. Neither of these predictions was supported: in both species, females, but not males, engaged in sex‐specific aggression and females were more aggressive than males overall. These contrasting sexual dimorphisms – larger weaponry in males but higher aggression in females – highlight the importance of considering the function of weaponry and aggression in contexts other than direct competitions over mates. In addition, species differences in the degree of sexual dimorphism in chela size were due to differences in female, not male, chela size, and the species with greater sexual dimorphism in weaponry was significantly less aggressive overall; also, while paired and solitary males did not differ in residual chela size, for the species with greater sexual dimorphism, females carrying embryos had smaller residual chela sizes. These results suggest that understanding the sexual dimorphisms in weaponry and aggression in snapping shrimp requires understanding the relative costs and benefits of both in females as well as males.     

Pre‐ and postcopulatory sexual selection and the evolution of sexually dimorphic traits in earwigs (Dermaptera)

Dermaptera (earwigs) is a relatively small polyneopteran order with approximately 2200 described species. They are characterized by a pair of forceps, which are hardened, unsegmented cerci at the caudal end of the abdomen. In most species, males have more exaggerated forceps than females, indicating an effect of sexual selection on them. Earwigs also exhibit astonishing diversity in the number, laterality and size of both male and female genital components. This characteristic has promoted the study of postcopulatory sexual selection in several representative species. Here, previous studies of earwigs that examined pre‐ and postcopulatory sexual selection are reviewed in detail. Related topics included here are sexually antagonistic coevolution, evolution of laterally asymmetrical morphologies, and developmental aspects of intra‐sexually dimorphic traits. A new terminology system for male genitalia is also proposed.     

Fitness Costs of Sexual Harassment–The Price of Persuasion

Females are often subjected to unwanted mating advances from males. Such advances can be costly to both parties. The short‐term costs of harassment to females have been widely explored in the literature; however, few studies have measured the direct fitness costs. Moreover, male costs are seldom considered. Conventional wisdom would lead us to hypothesise that sexual harassment is costly; thus, when males and females are housed together, harassment should reduce foraging, growth and reproductive output and may disrupt social interactions. This study quantified harassment costs in both sexes by observing behavioural responses and long‐term effects of unsolicited mating in a controlled setting. Sexually mature guppies were subjected to two housing treatments: equal sex ratios or single‐sex groups. The effects of male harassment on males and females were assessed by measuring behaviour, growth rate and the number of offspring produced over a period of 6 mo. Contrary to our expectations, our results indicated no significant differences in foraging and growth rates between mixed‐ and single‐sex shoals for either sex. Moreover, there was no significant difference in fry production between mixed‐ and all‐female shoals. Large males showed higher mortality when housed with females. Both sexes showed a reduction in shoaling when in mixed‐sex groups. Thus, there appear to be few direct costs of harassment for females in natural, mixed‐sex shoals, but males appear to bear significant harassment costs. The study provides insights into reproductive behaviour and life‐history traits.     

Pre‐ and postcopulatory sexual selection favor aggressive,young males in polyandrous groups of red junglefowl

A challenge in evolutionary biology is to understand the operation of sexual selection on males in polyandrous groups, where sexual selection occurs before and after mating. Here, we combine fine‐grained behavioral information (>41,000 interactions) with molecular parentage data to study sexual selection in replicated, age‐structured groups of polyandrous red junglefowl, Gallus gallus. Male reproductive success was determined by the number of females mated (precopulatory sexual selection) and his paternity share, which was driven by the polyandry of his female partners (postcopulatory sexual selection). Pre‐ and postcopulatory components of male reproductive success covaried positively; males with high mating success also had high paternity share. Two male phenotypes affected male pre‐ and postcopulatory performance: average aggressiveness toward rival males and age. Aggressive males mated with more females and more often with individual females, resulting in higher sexual exclusivity. Similarly, younger males mated with more females and more often with individual females, suffering less intense sperm competition than older males. Older males had a lower paternity share even allowing for their limited sexual exclusivity, indicating they may produce less competitive ejaculates. These results show that—in these populations—postcopulatory sexual selection reinforces precopulatory sexual selection, consistently promoting younger and more aggressive males.     

THE ROLE OF MULTILEVEL SELECTION IN THE EVOLUTION OF SEXUAL CONFLICT IN THE WATER STRIDER AQUARIUS REMIGIS

In evolution, exploitative strategies often create a paradox in which the most successful individual strategy “within” the group is also the most detrimental strategy “for” the group, potentially resulting in extinction. With regard to sexual conflict, the overexploitation of females by harmful males can yield similar consequences. Despite these evolutionary implications, little research has addressed why sexual conflict does not ultimately drive populations to extinction. One possibility is that groups experiencing less sexual conflict are more productive than groups with greater conflict. However, most studies of sexual conflict are conducted in a single isolated group, disregarding the potential for selection among groups. We observed Aquarius remigis water striders in a naturalistic multigroup pool in which individuals could freely disperse among groups. The free movement of individuals generated variation in aggression and sex‐ratio among groups, thereby increasing the importance of between‐group selection compared to within‐group selection. Females dispersed away from local aggression, creating more favorable mating environments for less‐aggressive males. Furthermore, the use of contextual analysis revealed that individual male aggression positively predicted fitness whereas aggression at the group level negatively predicted fitness, empirically demonstrating the conflict between levels of selection acting on mating aggression.     

SEX LINKAGE,SEX‐SPECIFIC SELECTION,AND THE ROLE OF RECOMBINATION IN THE EVOLUTION OF SEXUALLY DIMORPHIC GENE EXPRESSION

Sex‐biased genes—genes that are differentially expressed within males and females—are nonrandomly distributed across animal genomes, with sex chromosomes and autosomes often carrying markedly different concentrations of male‐ and female‐biased genes. These linkage patterns are often gene‐ and lineage‐dependent, differing between functional genetic categories and between species. Although sex‐specific selection is often hypothesized to shape the evolution of sex‐linked and autosomal gene content, population genetics theory has yet to account for many of the gene‐ and lineage‐specific idiosyncrasies emerging from the empirical literature. With the goal of improving the connection between evolutionary theory and a rapidly growing body of genome‐wide empirical studies, we extend previous population genetics theory of sex‐specific selection by developing and analyzing a biologically informed model that incorporates sex linkage, pleiotropy, recombination, and epistasis, factors that are likely to vary between genes and between species. Our results demonstrate that sex‐specific selection and sex‐specific recombination rates can generate, and are compatible with, the gene‐ and species‐specific linkage patterns reported in the genomics literature. The theory suggests that sexual selection may strongly influence the architectures of animal genomes, as well as the chromosomal distribution of fixed substitutions underlying sexually dimorphic traits.     

The evolution of sexually dimorphic earwig forceps: social interactions among adults of the toothed earwig, Vostox apicedentatus

Earwigs (Insecta, Dermaptera) are characterized by uniquelyelaborated cerci, commonly called forceps, the function of whichremains unclear. We studied intrasexual and intersexual interactionsin the laboratory to examine the context and pattern of forcepsuse in the toothed earwig. Vostox apictdenlatus (Caudell). Interactionsbetween pairs of earwigs were recorded in four social situations:(1) two males, (2) two males plus a virgin female, (3) two females,and (4) one male and one female. Forceps were used as both weaponsand display structures by males and females in all of thesesocial contexts. During pairwise male-male interactions, onemale clearly dominated the other male. Dominant males were moreactive and more likely to use their forceps in intrasexual interactionsthan were subordinate males. In interactions where there weretwo males and one female present, the male that dominated male-maleinteractions was able to maintain exclusive access to the female.There was no indication of active female choice during or aftercourtship. During intersexual interactions, only males usedtheir forceps during courtship. The behavioral repertoire involvingforceps was greater for males than for females, especially inintrasexual contests. There was no clear outcome of intrasexualinteractions among females. These results suggest that forcepsfunction mainly as weapons in male-male interactions and mayhave evolved, at least in part, as a result of sexual selection.Further research is required to test for female mate choiceand to separate the various mechanisms of sexual selection ifmate choice exists. Comparative studies are needed to determineif sexual selection was the original evolutionary mechanismleading to the development of these unusual structures or ifsexual selection is relegated to a secondary effect, leadingto the elaboration and sexualdimorphism of these structuresin selected groups of earwigs.     

Intra- and Inter-seasonal Variation in the Socio-Spatial Behavior of Adult Male Collared Lizards, Crotaphytus collaris (Reptilia, Crotaphytidae)

When individuals maintain strong inter‐seasonal philopatry to the same territories, males may be able to re‐establish territory occupancy without intense intra‐sexual aggression, and instead spend more time courting females early in the reproductive season. Furthermore, when some males have prior experience defending the same territories, it may be necessary for young males to exhibit higher levels of aggression because they are establishing a territory for the first time. We tested these hypotheses by examining within‐season (1992 and 1997) temporal variation in the social behavior of adult male collared lizards of known age and prior territorial experience in a population where inter‐season philopatry to territories is high. Contrary to expectations, the frequency of aggression exhibited by males with and without prior territorial experience did not differ. The frequency of intra‐sexual aggression was higher in 1992 than in 1997, perhaps because male competitors were more abundant in 1992. Although there was an interactive effect of year, male display and patrol were low at the beginning of the reproductive season in Apr. and May, reached peaks during midseason in June, and then decreased as reproduction ended in July. The size of territories showed a similar pattern, with males defending larger areas in June. Our data support the philopatry hypothesis in that the establishment of territories occurred without high levels of aggression early in the season, perhaps because territory boundaries have been well defined by high rates of patrol and advertisement during the middle of the previous season. Inter‐sexual interactions were most frequent in June rather than at the beginning of the reproductive season. Adult females are producing their second clutches and yearling females are producing their first clutches in June. The high rate of inter‐sexual encounters in June supports the hypothesis that males allocate more time to courtship when females are receptive because there are more reproductively active females at this time. The temporal pattern of activities in adult Crotaphytus collaris appears to function as a compromise between competing intra‐ and inter‐sexual social demands on males, allowing males to maximize mating opportunities as well as maintain future access to productive territories.     

Testing Functional Hypotheses for the Behavior of Resident Pine Voles, Microtus pinetorum, Toward Non-Residents

Members of a social group should attempt to maximize their fitness by maintaining an optimal group composition. Allowing an immigrant into the group may be beneficial or costly depending on the characteristics of the immigrant as well as the composition of the group. Therefore, we examined behavioral interactions between pine voles to test three functional hypotheses proposed to explain behavior of residents toward non‐residents: the resource defense, mate defense, and benefit of extra‐pair copulation hypotheses. To test these, we examined the effects of age, sexual experience and sex of non‐residents as well as the effects of sex of residents on the behavior of residents. Neither male nor female residents showed affiliative behavior toward non‐residents. Residents were more aggressive toward non‐residents than vice versa. The frequency of same‐sex aggression was greater than opposite‐sex aggression for male residents and this aggression was directed at adult male non‐residents to a greater degree than at subadult males. Resident males were least aggressive toward adult females. We found no differences in the behavior of females toward subadults, sexually naive adult non‐residents or sexually experienced adult non‐residents. Females also displayed similar amounts of aggression toward male and female non‐residents. Therefore, for males, aggression may function in defense of a mate while for females, aggression functions in resource defense. For both sexes, aggression is likely to play a role in the regulation of group composition.     

Male territoriality and ‘sex confusion’ in recently adapted lizards at White Sands

The evolution of intersexual interactions, like mate choice, during ecological speciation has received widespread attention. However, changes in intrasexual interactions, like male territoriality, during ecological divergence are largely unexamined. We conducted field experiments with adaptively diverged populations of the eastern fence lizard (Sceloporus undulatus) to determine whether territorial males behaved differently towards ecologically similar vs. dissimilar intruders. We performed trials with light‐coloured males from White Sands, New Mexico and dark‐coloured males from the surrounding desert. We found that intruders from White Sands elicited more aggression than intruders from dark‐soil habitat. We also documented a case of ‘sex confusion’ where white‐sand males courted dark‐soil intruders. We found population differences in signalling patch size that can explain both aggression bias and sex misidentification. We argue that direct selection (for population recognition or optimal signal transmission) and indirect selection (by‐products of ecological adaptation) should influence both intersexual and intrasexual interactions during ecological speciation.     

Effects of breeding site density on competition and sexual selection in the European lobster

The availability of breeding sites has been predicted to affectthe intensity of sexual selection, including mate competition,mate choice and ultimately, variation in mating success. Wetested the hypothesis that reduced density of shelters wouldcause an increase in the intensity of sexual selection in Europeanlobsters, Homarus gammarus. However, we found little supportfor our predictions. For example, within-sex competition bymales and by females was not more intense when shelters werescarce. Indeed, females attempted to evict one another fromshelters significantly more often when shelters were common.When shelters were abundant, shelter-holding males had greatermating success than males without shelters, yet females didnot show more interest towards these males during courtshipencounters. Mate attraction was more strongly related to largemale body size when shelters were scarce. Overall, the resultssuggest that reduced shelter density does not lead to more overtwithin-sex aggression in this species. Instead, we suggest thatimpacts of breeding resource availability on sexual selectionmay depend on the range over which resources are measured, withextreme scarcity of shelters rendering overt competition uneconomical.Furthermore, females may become more selective of male traitssuch as large size, which enhance male control of breeding sitesand hence protection of females.     

Agonistic behaviour and control of access to hiding places in two intertidal blennies, Lipophrys pholis and Coryphoblennius galerita (Pisces: Blenniidae)

In this study we investigated the relationships between dominance rank and access to shelters in captive groups of Lipophrys pholis and Coryphoblennius galerita, as well as the effects of group size and shelter availability. Dominance rank was strongly correlated with size in juvenile L. pholis and with sex and size in adult C. galerita, males being dominant over females of similar size. Access to shelters was significantly correlated with dominance rank. For both species, most interactions occurred out of shelters. Direct disputes over shelters were always initiated by the dominant fish and the initiator was always the winner. The rate of aggression per fish per unit time decreased with an increase in the number of fish in L. pholis but not in C. galerita. No significant differences were found in groups differing in the number of shelters. C. galerita showed a higher rate of agonistic interactions and a higher proportion of overt aggression than L. pholis. It is suggested that one of the functions of agonistic interactions in these fishes is the control of a set of shelters, in the network of pathways used by each individual within its home range, minimising the time required to hide in case of danger. Received in revised form: 29 January 2001 Electronic Publication     

It takes two: Seasonal variation in sexually dimorphic weaponry results from divergent changes in males and females

Sexually dimorphic weaponry often results from intrasexual selection, and weapon size can vary seasonally when costs of bearing the weapon exceed the benefits outside of the reproductive season. Weapons can also be favored in competition over nonreproductive resources such as food or shelter, and if such nonreproductive competition occurs year‐round, weapons may be less likely to vary seasonally. In snapping shrimp (Alpheus angulosus), both sexes have an enlarged snapping claw (a potentially deadly weapon), and males of many species have larger claws than females, although females are more aggressive. This contrasting sexual dimorphism (larger weaponry in males, higher aggression in females) raises the question of whether weaponry and aggression are favored by the same mechanisms in males and females. We used field data to determine whether either sex shows seasonal variation in claw size such as described above. We found sexual dimorphism increased during the reproductive season due to opposing changes in both male and female claw size. Males had larger claws during the reproductive season than during the nonreproductive season, a pattern consistent with sexual selection. Females, however, had larger claws during the nonreproductive season than during the reproductive season—a previously unknown pattern of variation in weapon size. The observed changes in female weapon size suggest a trade‐off between claw growth and reproduction in the reproductive season, with investment in claw growth primarily in the nonreproductive season. Sexually dimorphic weaponry in snapping shrimp, then, varies seasonally due to sex differences in seasonal patterns of investment in claw growth, suggesting claws may be advantageous for both sexes but in different contexts. Thus, understanding sexual dimorphisms through the lens of one sex yields an incomplete understanding of the factors favoring their evolution.     

Tail Length and Mutual Mate Choice in Bearded Tits (Panurus biarmicus)

Direct sexual selection via mutual mate choice can result in both sexes showing conspicuous traits. We experimentally tested whether this hypothesis can explain tail length in the bearded tit (Panurus biarmicus). In this species, both sexes have a long, graduated tail. Males have, however, a longer tail than females, suggesting perhaps that females are choosier than males in selecting mates. We used two choice set‐ups for each sex: shortened vs. control tail individuals and elongated vs. control tail individuals. We found that direct sexual selection seems to operate differently in the two sexes. In both set‐ups, females spent more time with the male with the longest tail, and they also showed sexual display behaviour only towards these males. Males spent more time with control than with short‐tailed females, but they did not discriminate between control and long‐tailed females. Moreover, males displayed preference towards both short‐ and long‐tailed females. Thus, females preferred long‐tailed males, whereas males did not always prefer long‐tailed females. Our study suggests that mutual mate choice has played a role in the evolution of long tails in bearded tits. It also suggests that the sexual dimorphism in tail length has evolved because mate choice exerts a stronger sexual selection pressure on males than on females.     

The contrasting role of male relatedness in different mechanisms of sexual selection in red junglefowl

In structured populations, competition for reproductive opportunities should be relaxed among related males. The few tests of this prediction often neglect the fact that sexual selection acts through multiple mechanisms, both before and after mating. We performed experiments to study the role of within‐group male relatedness across pre‐ and postcopulatory mechanisms of sexual selection in social groups of red junglefowl, Gallus gallus, in which two related males and one unrelated male competed over females unrelated to all the males. We confirm theoretical expectations that, after controlling for male social status, competition over mating was reduced among related males. However, this effect was contrasted by other sexual selection mechanisms. First, females biased male mating in favor of the unrelated male, and might also favor his inseminations after mating. Second, males invested more—rather than fewer—sperm in postcopulatory competition with relatives. A number of factors may contribute to explain this counterintuitive pattern of sperm allocation, including trade‐offs between male investment in pre‐ versus postcopulatory competition, differences in the relative relatedness of pre‐ versus postcopulatory competitors, and female bias in sperm utilization in response to male relatedness. Collectively, these results reveal that within‐group male relatedness may have contrasting effects in different mechanisms of sexual selection.     

Male-male competition and large size mating advantage in European earwigs, Forficula auricularia

European earwigs are sexually dimorphic in forceps shape and length. Male forceps are thought to be weapons in male contests for access to females, but recent findings suggest that females choose males on the basis of their forceps length. I investigated sexual selection on forceps length and body size and the occurrence of male-male competition. When I controlled for forceps length experimentally and statistically, relatively heavy males had greater copulation success than relatively light males. When I controlled for body size, males with relatively longer forceps had no tendency for greater copulation success than males with shorter forceps. Relatively heavy males more often took over copulations from smaller males than vice versa. Male contests were important for the outcome of mate competition, as males commonly interrupted and took over copulations. My results therefore suggest that intrasexual selection is significant in competition for copulations in male earwigs, and acts on body size. This contrasts with previous findings, which have shown intersexual selection on forceps length to be important. However, both modes of sexual selection may be acting through a two-stage process, where male-male competition first determines which males have access to females, and then through female choice among available males. Morphological measurements supported the conclusion that forceps length and body size are male secondary sexual characters, as these characters had large variance and skewed distributions in males, but were normally distributed in females. Copyright 2000 The Association for the Study of Animal Behaviour.     

Sex differences in the genetic architecture of aggressiveness in a sexually dimorphic spider

Sex differences in the genetic architecture of behavioral traits can offer critical insight into the processes of sex‐specific selection and sexual conflict dynamics. Here, we assess genetic variances and cross‐sex genetic correlations of two personality traits, aggression and activity, in a sexually size‐dimorphic spider, Nuctenea umbratica. Using a quantitative genetic approach, we show that both traits are heritable. Males have higher heritability estimates for aggressiveness compared to females, whereas the coefficient of additive genetic variation and evolvability did not differ between the sexes. Furthermore, we found sex differences in the coefficient of residual variance in aggressiveness with females exhibiting higher estimates. In contrast, the quantitative genetic estimates for activity suggest no significant differentiation between males and females. We interpret these results with caution as the estimates of additive genetic variances may be inflated by nonadditive genetic effects. The mean cross‐sex genetic correlations for aggression and activity were 0.5 and 0.6, respectively. Nonetheless, credible intervals of both estimates were broad, implying high uncertainty for these estimates. Future work using larger sample sizes would be needed to draw firmer conclusions on how sexual selection shapes sex differences in the genetic architecture of behavioral traits.