Why male orangutans do not kill infants

No immediate attentional bias towards or choice bias for male secondary sexual characteristics in Bornean orang-utans (Pongo pygmaeus)

Primate faces provide information about a range of variant and invariant traits, including some that are relevant for mate choice. For example, faces of males may convey information about their health or genetic quality through symmetry or facial masculinity. Because perceiving and processing such information may have bearing on the reproductive success of an individual, cognitive systems are expected to be sensitive to facial cues of mate quality. However, few studies have investigated this topic in non-human primate species. Orang-utans are an interesting species to test mate-relevant cognitive biases, because they are characterised by male bimaturism: some adult males are fully developed and bear conspicuous flanges on the side of their face, while other males look relatively similar to females. Here, we describe two non-invasive computerised experiments with Bornean orang-utans (Pongo pygmaeus), testing (i) immediate attention towards large flanges and symmetrical faces using a dot-probe task (N = 3 individuals; 2F) and (ii) choice bias for pictures of flanged males over unflanged males using a preference test (N = 6 individuals; 4F). In contrast with our expectations, we found no immediate attentional bias towards either large flanges or symmetrical faces. In addition, individuals did not show a choice bias for stimuli of flanged males. We did find exploratory evidence for a colour bias and energy efficiency trade-offs in the preference task. We discuss our null results and exploratory results in the context of the evolutionary history of Bornean orang-utans, and provide suggestions for a more biocentric approach to the study of orang-utan cognition.

Reproductive success of Bornean orangutan males: scattered in time but clustered in space

Abstract

The social and mating systems of orangutans, one of our closest relatives, remain poorly understood. Orangutans (Pongo spp.) are highly sexually dimorphic and females are philopatric and maintain individual, but overlapping home ranges, whereas males disperse, are non-territorial and wide-ranging, and show bimaturism, with many years between reaching sexual maturity and attaining full secondary sexual characteristics (including cheek pads (flanges) and emitting long calls). We report on 21 assigned paternities, among 35 flanged and 15 unflanged, genotyped male Bornean orangutans (Pongo pygmaeus wurmbii), studied from 2003 to 2018 in Tuanan (Central Kalimantan, Indonesia). All 10 infants born since mid-2003 with an already identified sire were sired by flanged males. All adult males ranged well beyond the study area (c. 1000 ha), and their dominance relations fluctuated even within short periods. However, 5 of the 10 identified sires had multiple offspring within the monitored area. Several sired over a period of c. 10 years, which overlapped with siring periods of other males. The long-calling behavior of sires indicated they were not consistently dominant over other males in the area around the time of known conceptions. Instead, when they were seen in the area, the known sires spent most of their time within the home ranges of the females whose offspring they sired. Overall, successful sires were older and more often resident than others.

Significance statement

It is difficult to assess reproductive success for individuals of long-lived species, especially for dispersing males, who cannot be monitored throughout their lives. Due to extremely long interbirth intervals, orangutans have highly male-skewed operational sex ratios and thus intensive male-male competition for every conception. Paternity analyses matched 21 immature Bornean orangutans with their most likely sire (only 10 of 50 genotyped males) in a natural population. Half of these identified sires had multiple offspring in the study area spread over periods of at least 10 years, despite frequently ranging outside this area. Dominance was a poor predictor of success, but, consistent with female mating tactics to reduce the risk of infanticide, known "sires" tended to have relatively high local presence, which seems to contribute to the males' siring success. The results highlight the importance of large protected areas to enable a natural pattern of dispersal and ranging.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00265-023-03407-6.

Beyond Dimorphism: Body Size Variation Among Adult Orangutans Is Not Dichotomous by Sex

Among extant great apes, orangutans are considered the most sexually dimorphic in body size. However, the expression of sexual dimorphism in orangutans is more complex than simply males being larger than females. At sexual maturity, some male orangutans develop cheek pads (flanges), while other males remain unflanged even after becoming reproductively capable. Sometimes flange development is delayed in otherwise sexually mature males for a few years. In other cases, flange development is delayed for many years or decades, with some males even spending their entire lifespan as unflanged adults. Thus, unflanged males of various chronological ages can be mistakenly identified as "subadults." Unflanged adult males are typically described as "female-sized," but this may simply reflect the fact that unflanged male body size has only ever been measured in peri-pubescent individuals. In this study, we measured the skeletons of 111 wild adult orangutans (Pongo spp.), including 20 unflanged males, 45 flanged males, and 46 females, resulting in the largest skeletal sample of unflanged males yet studied. We assessed long bone lengths (as a proxy for stature) for all 111 individuals and recorded weights-at-death, femoral head diameters, bi-iliac breadths, and long bone cross-sectional areas (CSA) (as proxies for mass) for 27 of these individuals, including seven flanged males, three adult confirmed-unflanged males, and three young adult likely-unflanged males. ANOVA and Kruskal-Wallis tests with Tukey and Dunn post-hoc pairwise comparisons, respectively, showed that body sizes for young adult unflanged males are similar to those of the adult females in the sample (all P ≥ 0.09 except bi-iliac breadth), whereas body sizes for adult unflanged males ranged between those of adult flanged males and adult females for several measurements (all P < 0.001). Thus, sexually mature male orangutans exhibit body sizes that range from the female end of the spectrum to the flanged male end of the spectrum. These results exemplify that the term "sexual dimorphism" fails to capture the full range of variation in adult orangutan body size. By including adult unflanged males in analyses of body size and other aspects of morphology, not as aberrations but as an expected part of orangutan variation, we may begin to shift the way that we think about features typically considered dichotomous according to biological sex.

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.

Infant loss during and after male replacement in gibbons

According to the sexual selection hypothesis, infanticide during resident male replacement is an adaptive strategy that has evolved because the killing of unweaned offspring sired by previous males shortens the inter-birth intervals of the mothers whose infants are targeted and thereby increases the reproductive fitness of the perpetrator. To test this hypothesis, we describe previously unreported cases of primary male replacement for two gibbon species (Hylobates lar and Nomascus nasutus), and review all other reported cases of primary male replacement in gibbons. Overall, infants were present in nearly half of all cases (16/33, 48%) and of the 18 infants present during replacement, 50% (N = 9) disappeared within 2 months of the event. In four of the five cases where there was sufficient demographic information to identify the likely sire of the subsequent offspring of females that lost infants, the new male was believed to be the sire. Infants were also less likely to die or disappear if the new male and original resident male were possible kin. However, there was no significant difference in the age of infants between those that died or disappeared following replacement and those that survived to weaning (p = .630). Our review of takeover-related infant loss in gibbons confirms that periods of male instability are risky for unweaned infants and that replacing males benefit from infant loss. Nevertheless, variability in the context of infant loss and difficulties related to data collection in the field make it difficult to test competing hypotheses concerning the mechanisms and functions of infanticide in the small apes.

Possible Male Infanticide in Wild Orangutans and a Re-evaluation of Infanticide Risk

Infanticide as a male reproductive tactic is widespread across mammals, and is particularly prevalent in catarrhine primates. While it has never been observed in wild orangutans, infanticide by non-sire males has been predicted to occur due to their extremely long inter-birth intervals, semi-solitary social structure, and the presence of female counter-tactics to infanticide. Here, we report on the disappearance of a healthy four-month-old infant, along with a serious foot injury suffered by the primiparous mother. No other cases of infant mortality have been observed at this site in 30 years of study. Using photographic measurements of the injury, and information on the behavior and bite size of potential predators, we evaluate the possible causes of this injury. The context, including the behavior of the female and the presence of a new male at the time of the injury, lead us to conclude that the most likely cause of the infant loss and maternal injury was male infanticide. We suggest that in orangutans, and other species where nulliparous females are not preferred mates, these females may be less successful at using paternity confusion as an infanticide avoidance tactic, thus increasing the likelihood of infanticide of their first-born infants.

Aggression by male bonobos against immature individuals does not fit with predictions of infanticide

The selective advantage of male infanticide is enhancement of reproductive success of the aggressor. This implies that aggression is directed at individuals sired by others, infant loss shortens the mother's inter-birth interval, and the aggressor has a greater likelihood of siring the next offspring of the victims' mother. As these conditions are not always met, the occurrence of male infanticide is expected to vary, and hominoid primates offer an interesting example of variation in male infanticide. Infanticide has been reported in gorillas and chimpanzees but appears to be absent in orangutans and bonobos. One argument for the absence of infanticide in bonobos is reduction of male aggression. However, given that male aggression against immature individuals occurs and that females engage in behavior that is considered to be counterstrategy against male infanticide, the risk of male infanticide may pose a potential threat. Here, we explored whether aggression by male bonobos fits predictions of male infanticide. Male aggression toward immature individuals was rare and did not have lethal consequences, but the majority of observed cases exposed targets to risks of injury. Males did not target their own offspring less frequently than unrelated immatures, and the risk of being the target of male aggression increased with the targets' age. Overall, these results do not match the predictions of the adaptive male infanticide hypothesis. Instead, aggression by males may promote the emigration of the targets and older males may reinforce their superior status toward individuals that will soon compete for the same resources.

The slow ape: High infant survival and long interbirth intervals in wild orangutans

Orangutans (Pongo spp.) are reported to have extremely slow life histories, including the longest average interbirth intervals of all mammals. Such slow life history can be viable only when unavoidable mortality is kept low. Thus, orangutans' survivorship under natural conditions is expected to be extremely high. Previous estimates of orangutan life history were based on captive individuals living under very different circumstances or on small samples from wild populations. Here, we combine birth data from seven field sites, each with demographic data collection for at least 10 years (range 12-43 years) on wild orangutans to better document their life history. Using strict criteria for data inclusion, we calculated infant survival, interbirth intervals and female age at first reproduction, across species, subspecies and islands. We found an average closed interbirth interval of 7.6 years, as well as consistently very high pre-weaning survival for males and females. Female survival of 94% until age at first birth (at around age 15 years) was higher than reported for any other mammal species under natural conditions. Similarly, annual survival among parous females is very high, but longevity remains to be estimated. Current data suggest no major life history differences between Sumatran and Bornean orangutans. The high offspring survival is remarkable, noting that modern human populations seem to have reached the same level of survival only in the 20th century. The orangutans' slow life history illustrates what can be achieved if a hominoid bauplan is exposed to low unavoidable mortality. Their high survival is likely due to their arboreal and non-gregarious lifestyle, and has allowed them to maintain viable populations, despite living in low-productivity habitats. However, their slow life history also implies that orangutans are highly vulnerable to a catastrophic population crash in the face of drastic habitat change.

Sexual conflict. The evolution of infanticide by males in mammalian societies

Male mammals often kill conspecific offspring. The benefits of such infanticide to males, and its costs to females, probably vary across mammalian social and mating systems. We used comparative analyses to show that infanticide primarily evolves in social mammals in which reproduction is monopolized by a minority of males. It has not promoted social counterstrategies such as female gregariousness, pair living, or changes in group size and sex ratio, but is successfully prevented by female sexual promiscuity, a paternity dilution strategy. These findings indicate that infanticide is a consequence, rather than a cause, of contrasts in mammalian social systems affecting the intensity of sexual conflict.

Female economic dependence and the morality of promiscuity

In environments in which female economic dependence on a male mate is higher, male parental investment is more essential. In such environments, therefore, both sexes should value paternity certainty more and thus object more to promiscuity (because promiscuity undermines paternity certainty). We tested this theory of anti-promiscuity morality in two studies (N = 656 and N = 4,626) using U.S. samples. In both, we examined whether opposition to promiscuity was higher among people who perceived greater female economic dependence in their social network. In Study 2, we also tested whether economic indicators of female economic dependence (e.g., female income, welfare availability) predicted anti-promiscuity morality at the state level. Results from both studies supported the proposed theory. At the individual level, perceived female economic dependence explained significant variance in anti-promiscuity morality, even after controlling for variance explained by age, sex, religiosity, political conservatism, and the anti-promiscuity views of geographical neighbors. At the state level, median female income was strongly negatively related to anti-promiscuity morality and this relationship was fully mediated by perceived female economic dependence. These results were consistent with the view that anti-promiscuity beliefs may function to promote paternity certainty in circumstances where male parental investment is particularly important.

Fatality of a wild Bornean orangutan (Pongo pygmaeus morio): behavior and death of a wounded juvenile in Danum Valley, North Borneo

Reports of wild great ape fatalities have been very limited, and only two have described wild orangutan deaths. We found a wounded juvenile female Bornean orangutan on 7 October 2006 in the Danum Valley, Sabah, Malaysia, and observed the individual's behavior for 7 days until her death on 13 October 2006. The 5-6-year-old orangutan, which we had observed since 2004, was wounded in the left brachium, back, and right hand. The individual's behavior changed after injury; the mean nest-nest active time became significantly shorter than before injury (from 12 h 3 min to 9 h 33 min), the mean waking time became significantly later (0552-0629 hours) and the mean bedtime became significantly earlier (from 1747 to 1603 hours). In the activity budget, resting increased significantly from 28.0 to 53.3%. Traveling and feeding decreased significantly from 23.5 to 12.7% and from 45.6 to 32.8%, respectively. The rate of brachiation during traveling and nest making decreased, whereas ground activity increased from 0 to 9%. We observed one vomiting incident and four occurrences of watery diarrhea during the 7 days before the individual died. The results of an autopsy performed by a local veterinarian suggested that the cause of death was septicemia because of Pseudomonas aeruginosa infection of the severely contaminated wounds. The morphology and distribution of the wounds suggested they had been incurred during an attack by a large animal with fangs and/or claws. This juvenile female became independent of its mother at ~4-5 years of age, slightly earlier than average. This individual might have been vulnerable to predatory attack because of her small body size (~5 kg at death) and lack of the mother's protection.

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.