A Comparison of Female Mating Strategies in Pan troglodytes and Pongo spp

Flanged males have higher reproductive success in a completely wild orangutan population

Male orangutans (Pongo spp.) exhibit bimaturism, an alternative reproductive tactic, with flanged and unflanged males displaying two distinct morphological and behavioral phenotypes. Flanged males are larger than unflanged males and display secondary sexual characteristics which unflanged males lack. The evolutionary explanation for alternative reproductive tactics in orangutans remains unclear because orangutan paternity studies to date have been from sites with ex-captive orangutans, provisioning via feeding stations and veterinary care, or that lack data on the identity of mothers. Here we demonstrate, using the first long-term paternity data from a site free of these limitations, that alternative reproductive tactics in orangutans are condition-dependent, not frequency-dependent. We found higher reproductive success by flanged males than by unflanged males, a pattern consistent with other Bornean orangutan (Pongo pygmaeus) paternity studies. Previous paternity studies disagree on the degree of male reproductive skew, but we found low reproductive skew among flanged males. We compare our findings and previous paternity studies from both Bornean and Sumatran orangutans (Pongo abelii) to understand why these differences exist, examining the possible roles of species differences, ecology, and human intervention. Additionally, we use long-term behavioral data to demonstrate that while flanged males can displace unflanged males in association with females, flanged males are unable to keep other males from associating with a female, and thus they are unable to completely mate guard females. Our results demonstrate that alternative reproductive tactics in Bornean orangutans are condition-dependent, supporting the understanding that the flanged male morph is indicative of good condition. Despite intense male-male competition and direct sexual coercion by males, female mate choice is effective in determining reproductive outcomes in this population of wild orangutans.

A composite menstrual cycle of captive orangutans, with associated hormonal and behavioral variability

Knowledge of species-typical reproductive endocrinology profiles is crucial for testing hypotheses pertaining to the evolutionary history, reproductive parameters, and life history of a species, and for managing the well-being of individual animals in human care. Large-scale empirical measurements of ovarian hormones, however, are rare for most primate species, including orangutans. In this study, we used enzyme immunoassays (EIA) to quantify estrogen (estrone conjugates; E₁ C) and progesterone (pregnanediol-3-glucuronide; PdG) levels for 98 cycles in 7 cycling zoo-housed female orangutans (10-43 years old). We use a subset of these cycles (N = 44) to create the first composite menstrual cycle for orangutans, which serves as a valuable baseline for future comparative analyses and veterinary considerations. Similar to previous studies, we determined the mean ovarian cycle length of orangutans to be 29.7 days (N = 98 cycles), although we illustrate evidence of both intra- and interindividual variation in ovarian steroid production. Given that this study took place in captivity, we consider how energetic and psychosocial aspects of the zoo environment, such as greater food availability and potential stress, may affect the reproductive physiology and sexual behavior of these females. Furthermore, we discuss the role that age and genetic background may play in producing variability. Finally, we test whether ovarian hormone levels correlate with the reproductive behaviors of these female orangutans using associated behavioral data. Our results suggest that matings are more common during the periovulatory period than outside of it, but do not support a consistent link between hormonal indices of fecundability and mating behaviors in these individuals.

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.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00265-020-02948-4.

Sociosexual behavioral patterns involving nulliparous female orangutans (Pongo sp.) reflect unique challenges during the adolescent period

The primate adolescent period is characterized by a series of changes in physiology, behavior, and social relationships. Orangutans have the slowest life history and the longest period of dependency of all primates. As members of a semisolitary species with high levels of sexual coercion, adolescent female orangutans face a unique combination of challenges when achieving independence from their mother. This study examined the mating behavior of adolescent female orangutans and compared it with that of adult females to assess whether mating behavior reflects distinct strategies at these different points in the life cycle. Data were collected in Gunung Palung National Park on the island of Borneo over 20 years. Mating events from adolescent (n = 19) and adult females (n = 26) were scored and compared. Adolescent female mating events had significantly higher mating scores (indicating more proceptivity) than those of adult females (β = 1.948, p = .001). Adolescent females also engaged in elaborate sociosexual interactions with different flanged males, behaviors that were never observed during mating events of adult females. These interactions involved characteristic behavior on the part of both the adolescent females and the flanged males. Given these findings and the documentation of similar accounts of adolescent female-flanged male mating from the island of Sumatra, we propose that adolescent female orangutans display distinctive behavioral repertoires throughout the genus Pongo which serves to overcome male ambivalence toward nulliparous females, establish familiarity, and evaluate coercive tendencies in flanged males. We suggest that these behavioral patterns are an integral part of female social development in a female philopatric, but highly dispersed species where consistent social support is absent after ranging independence is achieved.

Roving females and patient males: a new perspective on the mating strategies of chimpanzees

Mating strategies are sets of decisions aimed at maximizing reproductive success. For male animals, the fundamental problem that these strategies address is attaining mating access to females in a manner that maximizes their chances of achieving paternity. For chimpanzees (Pan troglodytes), despite substantial interest in mating strategies, very little attention has been paid to the most fundamental problem that mating strategies need to solve: finding mates. Only a single model, Dunbar's general model of male mating strategies, exists to explain mate-searching behaviour in chimpanzees. Under this model, males in most populations are regarded as pursuing a 'roving' strategy: searching for and sequestering fertile females who are essentially passive with respect to mate searching. The roving mating strategy is an assumption deeply embedded in the way chimpanzee behaviour is considered; it is implicit in the conventional model for chimpanzee social structure, which posits that male ranging functions both to monitor female reproductive state and to ward these females from other groups of males through collective territoriality: essentially, ranging as mating effort. This perspective is, however, increasingly at odds with observations of chimpanzee behaviour. Herein, I review the logic and evidence for the roving-male mating strategy and propose a novel alternative, a theoretical framework in which roving is a strategy pursued by female chimpanzees in order to engage successfully in promiscuous mating. Males, unable to thwart this female strategy, instead maximise the number of reproductive opportunities encountered by focusing their behaviour on countering threats to health, fertility and reproductive career. Their prolonged grooming bouts are seen, in consequence, as functioning to mitigate the negative impacts of socially induced physiological stress. In this new framework, the roving-male strategy becomes, at best, a 'best of a bad job' alternative for low-ranking males when faced with high levels of competition for mating access. Male chimpanzees do not search for mates, but for one another, for food, and, at times, for rivals in other communities. To the extent that female promiscuity functions to counter infanticide risk, mate searching by female chimpanzees-and any associated costs-can be seen as an unavoidable consequence of male sexual coercion. This novel framework is a better fit to the available data than is the conventional account. This review highlights the desperate need for additional work in an area of chimpanzee biology that has been somewhat neglected, perhaps in part because assumptions of roving males have remained unquestioned for too long. It also highlights the need, across taxa, to revisit and revise theory, and to test old assumptions, when faced with contrary data.

Evolution of life history and behavior in Hominidae: towards phylogenetic reconstruction of the chimpanzee-human last common ancestor

The origin of the fundamental behavioral differences between humans and our closest living relatives is one of the central issues of evolutionary anthropology. The prominent, chimpanzee-based referential model of early hominin behavior has recently been challenged on the basis of broad multispecies comparisons and newly discovered fossil evidence. Here, we argue that while behavioral data on extant great apes are extremely relevant for reconstruction of ancestral behaviors, these behaviors should be reconstructed trait by trait using formal phylogenetic methods. Using the widely accepted hominoid phylogenetic tree, we perform a series of character optimization analyses using 65 selected life-history and behavioral characters for all extant hominid species. This analysis allows us to reconstruct the character states of the last common ancestors of Hominoidea, Hominidae, and the chimpanzee-human last common ancestor. Our analyses demonstrate that many fundamental behavioral and life-history attributes of hominids (including humans) are evidently ancient and likely inherited from the common ancestor of all hominids. However, numerous behaviors present in extant great apes represent their own terminal autapomorphies (both uniquely derived and homoplastic). Any evolutionary model that uses a single extant species to explain behavioral evolution of early hominins is therefore of limited use. In contrast, phylogenetic reconstruction of ancestral states is able to provide a detailed suite of behavioral, ecological and life-history characters for each hypothetical ancestor. The living great apes therefore play an important role for the confident identification of the traits found in the chimpanzee-human last common ancestor, some of which are likely to represent behaviors of the fossil hominins.

Sexual conflict in primates

Sexual conflict is increasingly recognized as a major force for evolutionary change and holds great potential for delineating variation in primate behavior and morphology. The goals of this review are to highlight the rapidly rising field of sexual conflict and the ongoing shift in our understanding of interactions between the sexes. We discuss the evidence for sexual conflict within the Order Primates, and assess how studies of primates have illuminated and can continue to increase our understanding of sexual conflict and sexual selection. Finally, we introduce a framework for understanding the behavioral, anatomical, and genetic expression of sexual conflict across primate mating systems and suggest directions for future research.

Female reproductive strategies in orangutans, evidence for female choice and counterstrategies to infanticide in a species with frequent sexual coercion

Intersexual conflicts over mating can engender antagonistic coevolution of strategies, such as coercion by males and selective resistance by females. Orangutans are exceptional among mammals for their high levels of forced copulation. This has typically been viewed as an alternative mating tactic used by the competitively disadvantaged unflanged male morph, with little understanding of how female strategies may have shaped and responded to this behaviour. Here, we show that male morph is not by itself a good predictor of mating dynamics in wild Bornean orangutans but that female conception risk mediated the occurrence and quality of male-female interactions. Near ovulation, females mated cooperatively only with prime flanged males who they encountered at higher rates. When conception risk was low, willingness to associate and mate with non-prime males increased. Our results support the hypothesis that, together with concealed ovulation, facultative association is a mechanism of female choice in a species in which females can rarely avoid coercive mating attempts. Female resistance, which reduced copulation time, may provide an additional mechanism for mate selection. However, coercive factors were also important as prime males were frequently aggressive to females and females used mating strategies consistent with infanticide avoidance.

Why male orangutans do not kill infants

Infanticide is widespread among mammals, is particularly common in primates, and has been shown to be an adaptive male strategy under certain conditions. Although no infanticides in wild orangutans have been reported to date, several authors have suggested that infanticide has been an important selection pressure influencing orangutan behavior and the evolution of orangutan social systems. In this paper, we critically assess this suggestion. We begin by investigating whether wild orangutans have been studied for a sufficiently long period that we might reasonably expect to have detected infanticide if it occurs. We consider whether orangutan females exhibit counterstrategies typically employed by other mammalian females. We also assess the hypothesis that orangutan females form special bonds with particular "protector males" to guard against infanticide. Lastly, we discuss socioecological reasons why orangutan males may not benefit from infanticide. We conclude that there is limited evidence for female counterstrategies and little support for the protector male hypothesis. Aspects of orangutan paternity certainty, lactational amenorrhea, and ranging behavior may explain why infanticide is not a strategy regularly employed by orangutan males on Sumatra or Borneo.