The cost of associating with males for Bornean and Sumatran female orangutans: a hidden form of sexual conflict?

Abstract

Sexual coercion, in the form of forced copulations, is relatively frequently observed in orangutans and generally attributed to their semi-solitary lifestyle. High ecological costs of association for females may be responsible for this lifestyle and may have prevented the evolution of morphological fertility indicators (e.g., sexual swellings), which would attract (male) associates. Therefore, sexual conflict may arise not only about mating per se but also about associations, because males may benefit from associations with females to monitor their reproductive state and attempt to monopolize their sexual activities. Here, we evaluate association patterns and costs for females when associating with both males and females of two different orangutan species at two study sites: Suaq, Sumatra (Pongo abelii), and Tuanan, Borneo (Pongo pygmaeus wurmbii). Female association frequency with both males and females was higher in the Sumatran population, living in more productive habitat. Accordingly, we found that the cost of association, in terms of reduced feeding to moving ratio and increased time being active, is higher in the less sociable Bornean population. Males generally initiated and maintained such costly associations with females, and prolonged associations with males led to increased female fecal cortisol metabolite (FCM) levels at Tuanan, the Bornean population. We conclude that male-maintained associations are an expression of sexual conflict in orangutans, at least at Tuanan. For females, this cost of association may be responsible for the lack of sexual signaling, while needing to confuse paternity. Significance statement Socioecological theory predicts a trade-off between the benefits of sociality and the ecological costs of increased feeding competition. Orangutans’ semi-solitary lifestyle has been attributed to the combination of high association costs and low predation risk. Previous work revealed a positive correlation between association frequencies and habitat productivity, but did not measure the costs of association. In this comparative study, we show that females likely incur costs from involuntary, male-maintained associations, especially when they last for several days and particularly in the population characterized by lower association frequencies. Association maintenance therefore qualifies as another expression of sexual conflict in orangutans, and especially prolonged, male-maintained associations may qualify as an indirect form of sexual coercion.

After the smoke has cleared: Extended low fruit productivity following forest fires decreased gregariousness and social tolerance among wild female Bornean orangutans (Pongo pygmaeus wurmbii)

“…We hypothesized that, as a fission-fusion species, orangutans would respond to extreme and long-term fruit scarcity by not only adjusting their activity patterns and diets, but also by associating less with conspecifics. We focussed on associations among parous females, as adult females have full control over the initiation and maintenance of associations with other adult females (as opposed to associations with adult males) and can choose freely when, and when not, to associate with each other (Galdikas 1985b;Kunz et al 2021). Furthermore, theory predicts that female behaviour and social relationships will be the most directly affected by variation in fruit availability (Sterck et al 1997;Young & Isbell 2002), and the comprehensive longitudinal study of female orangutans is enabled by their philopatry (e.g., van Noorwijk et al 2012;Ashbury et al 2020).…”

Male chimpanzee sexual coercion and mating success at Ngogo

“…Evidence for ecological costs of coercion also exists for female bottlenose dolphins (Tursiops truncatus) in Shark Bay, who used preferred habitat less when male alliances aggressively sequestered them (Wallen et al, 2016), and noncoercive, but mating-related, malefemale association imposes ecological costs on female Bornean orangutans (Kunz et al, 2021). Elevated glucocorticoid secretion induced by male aggression like that shown for female chimpanzees at Kanyawara (Muller et al, 2009b) also imposes costs, as would any need to divert energy from reproduction to immune functioning when females are wounded (ibid.…”

Slow loris (Nycticebus borneanus) consumption by a wild Bornean orangutan (Pongo pygmaeus wurmbii)

“…Since male orangutans disperse over large areas (Arora et al 2012;Nietlisbach et al 2012), and we know that the male Molong did not grow up within the TORS area, he may have had earlier exposure to the effective handling of a loris in his natal range. However, the rare opportunity for a orangutan to witness a successful catch if slow loris density is similarly low across the study area, and the relatively low rate of association of an orangutan with conspecifics (Kunz et al 2021), make it unlikely that catching slow lorises will spread widely through social learning in this population (van Schaik et al 2003).…”