Sexual aggression in mammals

The evolution of same-sex sexual behaviour in mammals

Same-sex sexual behaviour has attracted the attention of many scientists working in disparate areas, from sociology and psychology to behavioural and evolutionary biology. Since it does not contribute directly to reproduction, same-sex sexual behaviour is considered an evolutionary conundrum. Here, using phylogenetic analyses, we explore the evolution of same-sex sexual behaviour in mammals. According to currently available data, this behaviour is not randomly distributed across mammal lineages, but tends to be particularly prevalent in some clades, especially primates. Ancestral reconstruction suggests that same-sex sexual behaviour may have evolved multiple times, with its appearance being a recent phenomenon in most mammalian lineages. Our phylogenetically informed analyses testing for associations between same-sex sexual behaviour and other species characteristics suggest that it may play an adaptive role in maintaining social relationships and mitigating conflict.

Drop dead! Female mate avoidance in an explosively breeding frog

Males' and females' reproductive strategies may differ, potentially leading to sexual conflict. Increased efforts by males (harassment, forced copulation, intimidation) to gain access to females could even negatively affect female survival and thus lead to reproductive failure for both individuals. In anurans, a higher mortality risk of mating females has been reported in explosive breeding species. During these mating events, several males cling to a female, which are mostly unable to get rid of the unwanted males. This can lead to the female's death. From the literature, it seems that females of explosive breeding frogs have no means to reject unwanted males. Here we describe female mate avoidance behaviours in the European common frog. We observed three female avoidance behaviours, namely 'rotation', 'release call(s)' and tonic immobility (death feigning). These behaviours were significantly associated with smaller female body size, and smaller females were more successful in escaping amplexus. Tonic immobility as a tactic to avoid mating or male harassment has only been observed in a handful of species and only in one other amphibian. Our observations show that females in explosive breeding frogs may not be as passive and helpless as previously thought.

Mating in the cold. Prolonged sperm storage provides opportunities for forced copulation by male bats during winter

In a wide range of heterothermic mammals, hibernation interrupts the reproductive cycle by forcing reproductive delays. In hibernating bats with delayed fertilization, an opportunity for sperm competition is enhanced by extending a time-window between copulations and fertilization. In order to achieve greater fertilization success, males are expected to show adaptations for sperm competition by increasing their opportunities for mating over an extended period. We aimed to clarify the physiological and behavioral characteristics of male bats experiencing increased risks of sperm competition. We investigated the characteristics of the reproductive cycle of the little horseshoe bat (Rhinolophus cornutus), and examined whether males retain reproductive physiology related to sexual behavior, and attempt to copulate with females even during the hibernation period. Field observations and histological examinations of the reproductive cycle confirmed that females, having mated in the autumn, store spermatozoa in the uterus during hibernation and give birth in the early summer to just one offspring per year, thus males face a low certainty of successful fertilization. Although their testes regressed rapidly and their testosterone levels were lower during winter than in autumn, males stored motile spermatozoa in their cauda epididymides from autumn throughout the winter. During hibernation, we found that males occasionally aroused from torpor and attempted to mate forcibly with torpid females. Forced copulations appear to increase a male's chances of obtaining a mate while avoiding pre-copulatory female choice. Epididymal sperm storage could be advantageous for males in allowing them to extend their potential mating period even though their testes have regressed. We also found that some hibernating nulliparous females were ready for fertilization in spring after hibernation, whereas few parous females appeared in the same roost. In contrast to males, forced copulations would be maladaptive for females because they cannot opt for higher-quality males while in torpor. Females that have experienced sexual coercion when young may subsequently avoid hibernacula where adult males are present.

Modelling sexual violence in male rats: the sexual aggression test (SxAT)

Sexual assault and rape are crimes that impact victims worldwide. Although the psychosocial and eco-evolutionary factors associated with this antisocial behavior have repeatedly been studied, the underlying neurobiological mechanisms are still largely unknown. Here, we established a novel paradigm to provoke and subsequently assess sexual aggression (SxA) in adult male Wistar rats: the sexual aggression test (SxAT). Briefly, male Wistar rats are sexually aroused by a receptive female, which is exchanged by a non-receptive female immediately after the first intromission. This protocol elicits forced mounting and aggressive behavior toward the non-receptive female to different degrees, which can be scored. In a series of experiments we have shown that SxA behavior is a relatively stable trait in rats and correlates positively with sexual motivation. Rats with innate abnormal anxiety and aggressive behavior also show abnormal SxA behavior. In addition, central infusion of oxytocin moderately inhibits aggressive behavior, but increases forced mounting. Finally, we identified the agranular insular cortex to be specifically activated by SxA, however, inhibition of this region did not significantly alter behavior in the SxAT. Altogether, the SxAT is a paradigm that can be readily implemented in behavioral laboratories as a valuable tool to find answers regarding the biological mechanisms underlying SxA in humans, as well as social decision-making in general.

Male chimpanzee sexual coercion and mating success at Ngogo

Chimpanzees (Pan troglodytes) have a complex mating system in which both sexes use multiple tactics. Most copulations occur in group contexts, and high-ranking males can gain high mating and reproductive success, but females typically mate with multiple males and the potential for sperm competition is high. Also, male-female dyads sometimes form temporary exclusive mating associations (consortships). Male aggression to receptive females is common. Several studies have supported the hypothesis that this is sexual coercion, but debate exists regarding the importance of coercion relative to that of female choice. The number of adult males in a community can influence the balance between these processes. In the large Ngogo community, male dominance ranks and rates of aggression to fully-swollen females were positively related to mating success as estimated by copulation rates and by proportions of copulations achieved. Aggression rates were higher than at other sites, overall and per male, especially during periovulatory periods, and increased with the number of males associating with a female. Aggression impaired female foraging efficiency. Males initiated most copulations and females rarely refused mating attempts. Male-to-female grooming was positively associated with male mating success and with the proportion of copulations that females initiated, but the amount of grooming was typically small and whether grooming-for-mating trading occurs is uncertain. These results are consistent with the hypothesis that sexual coercion is an important component of male chimpanzee mating strategies in many sociodemographic circumstances, but also show that male tactics vary both in response to and independently of those circumstances.