Are Male Orangutans a Threat to Infants? Evidence of Mother–Offspring Counterstrategies to Infanticide in Bornean Orangutans (Pongo pygmaeus wurmbii)

Sumatran orangutan mothers differ in the extent and trajectory of their expression of maternal behaviour

Mothers play a crucial role in the early development and survival of mammalian offspring, and differences in maternal care may affect offspring's development. Whereas previous research has primarily focused on biological and socioecological factors to understand population-level variation in maternal behaviour, the individual as a source of variation remains understudied. We investigated between-individual variation in the average expression of, and plasticity in, six maternal behaviours in Sumatran orangutans (Pongo abelii), using 15 years of behavioural data. We found that mothers differed substantially in the average expression of four maternal behaviours, even after controlling for socioecological conditions, biological state characteristics and the offspring's influence on these behaviours. Furthermore, not controlling for these confounding effects exaggerated or masked between-individual variation. Mothers also substantially differed in how they adjusted three of the maternal behaviours during offspring development, meaning that mothers differed in behavioural plasticity. Our results suggest that Sumatran orangutan mothers are constrained in the average expression of maternal behaviours and their plastic responses, potentially resulting in consistent differences among mothers, otherwise called maternal personality. Our findings highlight that there is biologically meaningful variation around the population mean in maternal behaviour and present novel opportunities to study evolutionary processes that shape maternal behaviour.

All together now: Assessing variation in maternal and nonmaternal handling of wild Colobus vellerosus infants

Primatologists have a long-standing interest in the study of maternal care and nonmaternal handling (NH) of infants stemming from recognition that early social relationships can have enduring consequences. Though maternal care and NH often include expression of similar behaviors, they are regularly studied in isolation from each other with nonoverlapping terminology, thereby overlooking possible interplay between them and obfuscating potential developmental ramifications that ensue from trade-offs made between maternal (MH) and NH during infancy. To that end, identifying how MH and NH patterns interact and contribute to the total handling (TH) infants receive is a critical first step. We present durational handling data collected from 25 wild Colobus vellerosus infants from 2016 to 2017 and assess the relationships between TH, MH, and NH. Patterns of social affiliation are shaped in part by surrounding context, and therefore, we also assess whether NH and TH differ in their responsivity to various infant and social group characteristics. Ninety-four percent of observed handling was MH, while just 5.5% was NH. Young infants who received more MH (excluding nursing) also received more NH; there was no relationship between the two in older infants. Infants in larger groups participated in more handling of all types. Additionally, NH time was associated with infant sex and group stability. Non-nursing TH time was associated with group stability and infant cohort size. Though NH variation likely confers social-networking advantage, in this population NH is not a major contributor to TH and would not effectively replace reduced MH. The positive association between MH and NH during early infancy suggests that colobus mothers may play a mediating role in shaping infant socialization. This is a first step in elucidating how different forms of handling relate to one another in wild primates and in identifying the impact of handling on infant socialization.

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.

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.

Alternative reproductive tactics of unflanged and flanged male orangutans revisited

In many slowly developing mammal species, males reach sexual maturity well before they develop secondary sexual characteristics. Sexually mature male orangutans have exceptionally long periods of developmental arrest. The two male morphs have been associated with behavioral alternative reproductive tactics, but this interpretation is based on cross-sectional analyses predominantly of Northwest Sumatran populations. Here we present the first longitudinal analyses of behavioral changes of 10 adult males that have been observed in both unflanged and flanged morph. We also analyzed long-term behavioral data on an additional 143 individually identified males from two study sites, Suaq (Sumatra, Pongo abelii) and Tuanan (Borneo, Pongo pygmaeus wurmbii), to assess male mating tactics cross-sectionally in relation to population, male morph (unflanged and flanged), and other socio-ecological factors. Both our longitudinal and cross-sectional results confirm and refine previous cross-sectional accounts of the differences in mating tactics between the unflanged and the flanged male morphs. In the unflanged morph, males exhibit higher sociability, particularly with females, and higher rates of both copulation and sexual coercion than in the flanged morph. Based on our results and those of previous studies showing that females prefer flanged males, and that flanged males have higher reproductive success, we conclude that unflanged males face a trade-off between avoiding male-male contest competition and gaining mating access to females, and thus follow a "best-of-a-bad-job" mating strategy.

Costs of male infanticide for female capuchins: When does an adaptive male reproductive strategy become costly for females and detrimental to population viability?

OBJECTIVES

Infanticide in white-faced capuchins (Cebus capucinus imitator) typically occurs in association with alpha male replacements (AMRs). Although infanticide is likely adaptive for males, it imposes costs on females that are difficult to quantify without long-term demographic data. Here we investigate effects of AMRs and infanticide on female reproductive success and how these costs affect capuchin groups. We investigate (1) effects of AMR frequency on the production of surviving infants; (2) energetic and (3) temporal "opportunity costs" of infant loss; and (4) how AMR frequency impacts capuchin group sizes.

MATERIALS AND METHODS

We censused six groups (7-33 years/group, 74 adult females). We modeled surviving infant production in relation to AMR. We estimated a female's energy requirements for lost infants and the temporal cost relative to the median reproductive window. We simulated how varying AMR rates would affect future capuchin group sizes.

RESULTS

Females exposed to more frequent AMR tended to produce fewer surviving offspring. We estimate the average lost infant requires approximately 33% additional energy intake for its mother and represents 10% of the average reproductive opportunity window available to females. Simulated populations remain viable at the observed rate of AMR occurrence but decrease in size at even slightly higher rates.

DISCUSSION

While infanticide is adaptive for males, for females it affects lifetime reproductive success and imposes energetic and opportunity costs. Although capuchin populations have evolved with AMRs and infanticide, small increases in AMR frequency may lead to population decline/extinction. Infanticide likely plays a large role in population maintenance for capuchins.

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.

Broadening Perspectives on the Evolution of Human Paternal Care and Fathers’ Effects on Children

Unlike most mammals, human fathers cooperate with mothers to care for young to an extraordinary degree. Human paternal care likely evolved alongside our unique life history strategy of raising slow-developing, energetically costly children, often in rapid succession. Adaptive frameworks generally assume that paternal provisioning played a critical role in this pattern's emergence. We draw on nonhuman primate data to propose that nonprovisioning forms of low-cost hominin male care were potentially foundational and ratcheted up through evolutionary time, helping facilitate social contexts for later subsistence specialization and sharing. We then argue for expanding the breadth of anthropological research on paternal effects in families, particularly in three domains: direct care and teaching;social capital cultivation; and reduction of family conflict. Anthropologists can greatly contribute to conversations about the determinants of children's development across contexts, but we need to ask more expansive questions about the pathways through which caregivers (including fathers) affect child outcomes.