Sexual harassment and female gregariousness in the South American sea lion, Otaria flavescens

Strategies, costs and counter-strategies to sexual coercion

Sexual conflict, the conflict between the evolutionary interests of females and males over mating, occasionally results in the evolution of traits favourable for one sex and adverse for the other. In this context, males can use sexual coercion to increase their mating success, at the expense of their female targets' mate choice. An increasing number of studies highlight a great diversity of male and female behaviours that serve as strategies and counter-strategies, respectively, to sexual coercion. Previous studies have reviewed the literature on infanticide but not the literature on forced copulation, sexual harassment, intimidation or punishment. This qualitative review synthesises the empirical evidence and draws a unified framework of the ecology of sexual coercion across animals, presenting male sexually coercive strategies and co-evolved female counter-strategies that can reduce coercion and its fitness costs. Using examples from insects to humans, it shows that different strategies of sexual coercion can lead to the evolution of similar counter-strategies. These counter-strategies include female promiscuity, deception of males (e.g. concealed ovulation or pseudo-oestrus), avoidance of certain males and association with others for protection, female aggregation to dilute coercion and ultimately physical resistance by single or allied females. Extending previous work, this review provides compelling evidence of sexually antagonistic coevolution amid sexual coercion. It also calls for future work to clarify, first, which individual traits are linked to greater coercion rates in males and a higher likelihood of receiving coercion in females and, second, any causal relationships between different strategies of sexual coercion and the evolution of different social and mating systems.

Socially bonded females face more sexual coercion in a female-philopatric primate

Sexual coercion is a manifestation of sexual conflict increasing male mating success while inflicting costs to females. Although previous work has examined inter-individual variation in male sexually coercive tactics, little is known about female counter-strategies. We investigated whether social bonding mitigates the extent of sexual coercion faced by female mandrills (Mandrillus sphinx), as a putative mechanism linking sociality to fitness. Surprisingly, females faced the most coercion from those males with whom they formed the strongest bonds, while the strength of a female-male bond was also positively correlated with coercion from all other males. Finally, greater social integration in the female network was positively correlated with coercion, through a direct 'public exposure' mechanism and not mediated by female reproductive success or retaliation potential. Altogether, this study shows that neither between- nor within-sex bonds are protective against sexual coercion and identifies, instead, a hidden cost of social bonding.

Sexual Interference Behaviors in Male Adult and Subadult Tibetan Macaques (Macaca thibetana)

Simple Summary

Sexual interference behaviors (interruption/harassment) by male nonhuman primates can lead copulating individuals to separate and is hypothetically a form of male–male competition for access to sexually receptive females. Tibetan macaques (Macaca thibetana) provide an example of male sexual interference that can be used to discuss the sexual competition hypothesis. We found male sexual interference in this species showed significant seasonal variation. Age did not affect the proportion or type of interference behaviors that a male performed, but his social status did. Dominant males more often interrupted copulations. Subordinate males more often directed harassment behaviors toward dominant males, which reduced copulation duration, especially the post-ejaculatory phase of copulation. Our results suggest that sexual interference (interruption or harassment) may be a tactic to reduce the mating success of other males by either preventing ejaculation or reducing the duration of the post-ejaculatory phase, which is critical for sperm transport and, thus, reproductive success.

Abstract

Male nonhuman primate sexual interference, which includes copulation interruption and copulation harassment, has been related to reproductive success, but its significance has been challenging to test. Copulation interruption results in the termination of a copulation before ejaculation, whereas copulation harassment does not. We conducted this study using the all-occurrence behavior sampling method on sexual interference behaviors of seven adult and four subadult male Tibetan macaques (Macaca thibetana) in mating and non-mating seasons at Mt. Huangshan, China, from August 2016 to May 2017. Our results showed that males’ individual proportion of copulation interruption and harassment was higher during the mating season than during the non-mating season. In addition, dominant males more often performed interruption, whereas subordinate males more often performed harassment. We found no difference in the individual proportion of copulation interruption or harassment between adult and subadult males. Adult and subadult males both directed copulation interruption and harassment more often toward the mating male than toward the mating female. Lastly, the post-ejaculation phase of copulation was shorter when copulation harassment occurred than when it did not. Our results suggest that sexual interference may be an important mating tactic that adult and subadult males use in male–male sexual competition.

Spontaneous Yawning and its Potential Functions in South American Sea Lions (Otaria flavescens)

Spontaneous yawning is a widespread behaviour in vertebrates. However, data on marine mammals are scarce. In this study, we tested some hypotheses on the functions of yawning in a captive group of South American sea lions (Otaria flavescens). According to the Dimorphism Hypothesis, species showing low levels of sexual dimorphism in canine size do not show sex differences in yawning distribution; this was supported by our findings, since yawning did not differ between the sexes. Yawning was more frequently performed during resting/sleeping contexts, thus supporting the Drowsiness Hypothesis. Yawning and self-scratching are considered reliable indicators of short-term anxiety in sea lions, since they immediately increased after conflicts both in aggressors and victims (Social Distress Hypothesis supported). In the long-term, yawning was not correlated with individuals' dominance status, thus showing that anxiety is similarly experienced by dominants and subordinates. The last two findings can be explained by the social competition of this species, that involves individuals independently from their sex, age or ranking status. Therefore, the exposure to frequent stressful events can induce similar levels of anxiety in all the subjects (Resource Inequity Hypothesis supported). In conclusion, spontaneous yawning in sea lions seems to share similar functions with other social mammals, suggesting that this behaviour is a possible plesiomorphic trait.

Differential female sociality is linked with the fine-scale structure of sexual interactions in replicate groups of red junglefowl, Gallus gallus

Recent work indicates that social structure has extensive implications for patterns of sexual selection and sexual conflict. However, little is known about the individual variation in social behaviours linking social structure to sexual interactions. Here, we use network analysis of replicate polygynandrous groups of red junglefowl (Gallus gallus) to show that the association between social structure and sexual interactions is underpinned by differential female sociality. Sexual dynamics are largely explained by a core group of highly social, younger females, which are more fecund and more polyandrous, and thus associated with more intense postcopulatory competition for males. By contrast, less fecund females from older cohorts, which tend to be socially dominant, avoid male sexual attention by clustering together and perching on branches, and preferentially reproduce with dominant males by more exclusively associating and mating with them. Collectively, these results indicate that individual females occupy subtly different social niches and demonstrate that female sociality can be an important factor underpinning the landscape of intrasexual competition and the emergent structure of animal societies.

Male Violence and Sexual Intimidation in a Wild Primate Society

Sexual violence occurring in the context of long-term heterosexual relationships, such as sexual intimidation, is widespread across human populations [1-3]. However, its evolutionary origins remain speculative because few studies have investigated the existence of comparable forms of sexual coercion in animals [4, 5], in which repeated male aggression toward a female provides the aggressor with delayed mating benefits [6]. Here, we test whether male aggression toward females functions as sexual coercion in wild chacma baboons (Papio ursinus). We found support for all three main predictions of the sexual coercion hypothesis [7]: male aggression (1) is greatest against cycling females, (2) is costly and represents the main source of injuries for cycling females, and (3) increases male mating success with their victims in the future. Detailed analysis of chronological sequences between aggression and matings ruled out other coercive mechanisms, such as short-term harassment and punishment, by showing that aggression and matings are temporally decoupled. This decoupling may explain why some forms of sexual violence have been largely overlooked in well-studied animal populations despite their likely impact on the fitness of both sexes. Finally, we found no support for alternative hypotheses such as a female preference for aggressive males [8, 9]. This new, detailed study of the forms and intensity of sexual intimidation in a wild primate suggests that it may be widespread across mammalian societies, with important implications for understanding the evolution of mate choice and sexual conflict in mammals, as well as the origins of human sexual violence.

Female social response to male sexual harassment in poeciliid fish: a comparison of six species

Sexual harassment is common among poeciliid fish. In some fishes, males show a high frequency of sneak copulation; such sexual activity is costly to the females in terms of foraging efficiency. In mosquitofish (Gambusia holbrooki), when males are present, the distance between females tends to decrease, and this behavior has been interpreted as an adaptive strategy to dilute the costs of male sexual activity. In this study, the tendency to reduce distance in the presence of a male has been investigated in females of six poeciliid species (Girardinus metallicus, Girardinus falcatus, G. holbrooki, Poecilia reticulata, Xiphophorus hellerii, and Xiphophorus mayae) that exhibit different male mating strategies and different levels of sexual activity. Results revealed large interspecific differences in the pattern of female aggregation. Females of species with a high frequency of sneak copulations tended to reduce their social distance in the presence of a male. By contrast, species that rely mainly on courtship showed little or no variation in social distance. The proportion of sneak copulations predicts the degree of variation in female social response, but the amount of total sexual activity does not, suggesting that the change in females' social distance when a male is present may indeed serve to reduce the costs of male sexual harassment.

Sex, scarring, and stress: understanding seasonal costs in a cryptic marine mammal

We investigated variation in adrenal activity in the dugong, a vulnerable cryptic marine mammal species, in a population inhabiting the lower thermal limit of their range. We sampled 319 live wild dugongs and examined faecal glucocorticoid (fGC) concentrations in relationship to sex, reproductive state (immature, sexually mature, or pregnant), and season, as well as associations with body condition, male-induced aggressive conspecific interactions, and environmental temperature. Physiological validations with apparently healthy and unhealthy animals indicated that fGC levels reliably reflect adrenal activation in dugongs. Glucocorticoid levels and body condition varied seasonally, with individuals expressing lowest fGC levels and an improving body condition over summer and autumn. Best body condition was observed in winter, which may have enhanced the ability of the animals to cope with thermal and nutritional challenges and helped to sustain the subsequent costs of reproduction (including mating activity in spring). Immature dugongs, especially those recently estranged from their mothers, may be particularly vulnerable to stress during winter, and also at risk of injury from adult conspecifics. During the spring mating period, mature and sexually active males with large erupted tusks had high fGC and poorer body condition compared with all dugongs, except pregnant females, in all other seasons. This finding was consistent with high levels of body scarring on adults of both sexes, resulting from agonistic behaviour by mature males. Competitive reproductive behaviour, inherent in a promiscuous mating system, may have stressful consequences for male dugongs, especially given that strenuous mating activity follows unfavourable thermal and nutritional conditions. Overall, fGC levels appear to be a good proxy for stressors in dugongs associated with season and/or temperature, intra-specific aggression, and pregnancy, but not lactation and/or maternal care. Understanding baseline fGC levels will allow wildlife managers to identify additional and/or unusual stressors that may threaten wild dugong populations.

Polyandry Has No Detectable Mortality Cost in Female Mammals

In several taxonomic groups, females mate with several males during a single reproductive cycle. Although there is evidence that polyandry provides some benefits to females, it often involves mortality costs. However, empirical evidences of mortality costs of polyandry have so far been reported only in invertebrates. Whether polyandry has mortality costs in vertebrates is currently unknown. In the present study, we aimed to fill the gap by investigating the relationships between the level of polyandry (measured either by male relative testes mass or the percentage of multiple paternities) and female patterns of mortality across mammals. While we found that the two metrics of female mortality co-varied with pace of life, we did not find any evidence that polyandry leads to either decreased median lifespan or increased aging rate in mammals. We discuss such an absence of detectable mortality costs of polyandry in female mammals in light of recent advances in the study of mammalian reproductive biology and life-history tactics.

Effects of the distribution of female primates on the number of males

The spatiotemporal distribution of females is thought to drive variation in mating systems, and hence plays a central role in understanding animal behavior, ecology and evolution. Previous research has focused on investigating the links between female spatiotemporal distribution and the number of males in haplorhine primates. However, important questions remain concerning the importance of spatial cohesion, the generality of the pattern across haplorhine and strepsirrhine primates, and the consistency of previous findings given phylogenetic uncertainty. To address these issues, we examined how the spatiotemporal distribution of females influences the number of males in primate groups using an expanded comparative dataset and recent advances in bayesian phylogenetic and statistical methods. Specifically, we investigated the effect of female distributional factors (female number, spatial cohesion, estrous synchrony, breeding season duration and breeding seasonality) on the number of males in primate groups. Using bayesian approaches to control for uncertainty in phylogeny and the model of trait evolution, we found that the number of females exerted a strong influence on the number of males in primate groups. In a multiple regression model that controlled for female number, we found support for temporal effects, particularly involving female estrous synchrony: the number of males increases when females are more synchronously receptive. Similarly, the number of males increases in species with shorter birth seasons, suggesting that greater breeding seasonality makes defense of females more difficult for male primates. When comparing primate suborders, we found only weak evidence for differences in traits between haplorhines and strepsirrhines, and including suborder in the statistical models did not affect our conclusions or give compelling evidence for different effects in haplorhines and strepsirrhines. Collectively, these results demonstrate that male monopolization is driven primarily by the number of females in groups, and secondarily by synchrony of female reproduction within groups.

Consequences of local Allee effects in spatially structured populations

The ideal free distribution model incorporating the Allee effect was published by Fretwell and Lucas (1970), but went almost unnoticed within the ecological literature. The model is relevant to populations distributed among patchy habitats. It predicts a sporadic but substantial decline in populations at high densities, which in turn induces the rapid growth of new populations. In this paper, I show that the simple process explained by this model can be used to change our view of several phenomena within the field of population ecology, behavioural ecology and conservation. The ecological consequences of the model are well known. A key feature of Fretwell and Lucas's model is what I call the "Allee paradox:" there is a range of local population densities at which local individual fitness is less than the potential mean gain that could be obtained in the environment; however, individuals cannot disperse. This paradox can be used to explain why helping appears before suitable breeding areas are fully occupied, and why breeding females aggregate when male coercion is a reproductive cost. The model also predicts high clustering between related populations, and, in conservation biology, it can identify unfounded concerns about the dangers of extinction, delays in recolonisation processes after human-induced population decline, and latency periods in the initial phases of expansion of invasive species.