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.

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. 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.

https://doi.org/10.1016/j.anbehav.2021.09.012 0003-3472/© 2021 The Authors. Published by Elsevier ND license (http://creativecommons.org/licenses/by-n Sexual coercion is widespread in the animal kingdom. Its direct forms, including harassment and forced copulation, have largely been investigated as an expression of (alternative) male reproductive strategies, rather than the result of a sexual conflict between the sexes. Likewise, the frequent occurrence of forced copulations in orang-utans (Pongo spp.) has been attributed to male strategies and more recently also to concealed female fecundity. So far, however, the immediate contextual variables leading to forced copulations have rarely been examined. We compared two orang-utan populations, Suaq (Pongo abelii, Sumatra) and Tuanan (Pongo pygmaeus, Borneo), both characterized by an individual-based fission efusion lifestyle, whereas their socioecology differs. We assessed how the occurrence of femaleresisted and voluntary copulations was affected by female reproductive state, male morph (unflanged or flanged), measures of maleemale competition, maleefemale relationship and ecological factors. Besides female reproductive state and male morph, predictors of female resistance were related to male emale competition. First, female resistance was more likely towards subordinate males who were displaced from proximity to the female by another male during that association. Second, the presence of additional flanged males increased the probability of female resistance. Third, the latency to both the arrival of another male and to the end of the association after sexual interactions was shorter if there was female resistance. We conclude that sexual coercion in orang-utans is highly dependent on the vicinity of more dominant males and can only be understood in the light of sexual conflict: While males force copulations when at risk of losing access to a female and thus follow a ‘now-or-never’ strategy, female resistance follows a ‘not-you-now’ pattern, which is ultimately consistent with an infanticide avoidance strategy. © 2021 The Authors. Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Acoustic individual discrimination has been demonstrated for a wide range of animal taxa. However, there has been far less scientific effort to demonstrate the effectiveness of automatic individual identification, which could greatly facilitate research, especially when data are collected via an acoustic localization system (ALS). In this study, we examine the accuracy of acoustic caller recognition in long calls (LCs) emitted by Bornean male orangutans (Pongo pygmaeus wurmbii) derived from two data-sets: the first consists of highquality recordings taken during individual focal follows (N = 224 LCs by 14 males) and the second consists of LC recordings with variable microphone-caller distances stemming from ALS (N = 123 LCs by 10 males). The LC is a long-distance vocalization. We therefore expect that even the low-quality test-set should yield caller recognition results significantly better than by chance. Automatic individual identification was accomplished using software originally developed for human speaker recognition (i. e. the MSR identity toolbox). We obtained a 93.3% correct identification rate with high-quality recordings, and 72.23% with recordings stemming from the ALS with variable microphone-caller distances (20-420 m). These results show that automatic individual identification is possible even though the accuracy declines compared with the results of high-quality recordings due to severe signal degradations (e. g. sound attenuation, environmental noise contamination, and echo interference) with increasing distance. We therefore suggest that acoustic individual identification with speaker recognition software can be a valuable tool to apply to data obtained through an ALS, thereby facilitating field research on vocal communication.

In many solitary mammalian species, females live alone in relatively small home ranges whereas males roam widely and converge around, and compete for matings with fertile females. In primates, orangutans are the only diurnal semi solitary species showing this roving male promiscuity mating system. Here, we develop and test hypotheses about the nature of male-male competition in Bornean orangutans by examining its sexually selected long-distance calls to disentangle the mechanisms through which males compete over mating access. This study uses a novel approach to disentangle the mechanisms of male-male competition. We established an acoustic localization system (ALS) comprising 20 recorders installed in a grid that allowed accurate localization in an area of 450 ha. With this procedure, we triangulated 1615 long calls over 109 days spread over 10 months to examine the males' ranging and calling decisions. A male's choice of area was determined mainly by local fruit availability. Once there, however, his calling behavior depended primarily on the number of sexually attractive females and the number of other flanged males present. Both these variables also predicted the proportion of calls to which individual males responded, and the proportion of males present that responded to long calls of other males. We conclude that intra-sexual competition among Bornean flanged males comprises a combination of contest and scramble competition best summarized as confrontational assessment.Significance statementWhere males range widely in search of scattered fertile females and these females do not broadcast their reproductive state, males could pursue a variety of strategies. Flanged male Bornean orangutans emit long calls serving to attract females and repel male rivals. In order to disentangle whether the competitive tactics of flanged males correspond to scramble or contest competition in their long-distance communication system, we used an acoustic localization approach with a grid of recorders installed in the research are. We found that flanged males go to areas with much food and then engage in vocal, and subsequently, physical battles with rivals, probably because their relative strength fluctuates rapidly. We call this unusual form of contest competition confrontational assessment.

Orangutans (genus Pongo ) are the largest arboreal mammals, but Bornean orangutans ( P. pygmaeus spp .) also spend time on the ground. Here, we investigate ground use among orangutans using 32,000 hr of direct focal animal observations from a well‐habituated wild population of Bornean orangutans ( P. p. wurmbii ) living in a closed‐canopy swamp forest at Tuanan, Central Kalimantan, Indonesia. Ground use did not change with increasing observation time of well‐habituated individuals, suggesting it was not an artifact of observer presence. Flanged males spent the most time on the ground (ca. 5% of active time), weaned immatures the least (around 1%). Females and immatures descended mainly to feed, especially on termites, whereas flanged males traveled more while on the ground. Flanged males may travel more inconspicuously, and perhaps also faster, when moving on the ground. In addition, orangutans engaged in ground‐specific behavior, including drinking from and bathing in swamp pools. Supplementary records from 20 ground‐level camera traps, totaling 3986 trap days, confirmed the observed age‐sex biases in ground use at Tuanan. We conclude that ground use is a natural part of the Bornean orangutan behavioral repertoire, however it remains unclear to what extent food scarcity and canopy structure explain population differences in ground use. Am. J. Primatol. 77:1216–1229, 2015. © 2015 Wiley Periodicals, Inc.

The long call is an important vocal communication signal in the widely dispersed, semi‐solitary orangutan. Long calls affect individuals' ranging behavior and mediate social relationships and regulate encounters between dispersed individuals in a dense rainforest. The aim of this study was to test the utility of an Acoustic Location System (ALS) for recording and triangulating the loud calls of free‐living primates. We developed and validated a data extraction protocol for an ALS used to record wild orangutan males' long calls at the Tuanan field site (Central Kalimantan). We installed an ALS in a grid of 300 ha, containing 20 SM2+ recorders placed in a regular lattice at 500 m intervals, to monitor the distribution of calling males in the area. The validated system had the following main features: (i) a user‐trained software algorithm (Song Scope) that reliably recognized orangutan long calls from sound files at distances up to 700 m from the nearest recorder, resulting in a total area of approximately 900 ha that could be monitored continuously; (ii) acoustic location of calling males up to 200 m outside the microphone grid, which meant that within an area of approximately 450 ha, call locations could be calculated through triangulation. The mean accuracy was 58 m, an error that is modest relative to orangutan mobility and average inter‐individual distances. We conclude that an ALS is a highly effective method for detecting long‐distance calls of wild primates and triangulating their position. In combination with conventional individual focal follow data, an ALS can greatly improve our knowledge of orangutans' social organization, and is readily adaptable for studying other highly vocal animals. Am. J. Primatol. 77:767–776, 2015. © 2015 Wiley Periodicals, Inc.

Proactive, that is, unsolicited, prosociality is a key component of our hyper-cooperation, which in turn has enabled the emergence of various uniquely human traits, including complex cognition, morality and cumulative culture and technology. However, the evolutionary foundation of the human prosocial sentiment remains poorly understood, largely because primate data from numerous, often incommensurable testing paradigms do not provide an adequate basis for formal tests of the various functional hypotheses. We therefore present the results of standardized prosociality experiments in 24 groups of 15 primate species, including humans. Extensive allomaternal care is by far the best predictor of interspecific variation in proactive prosociality. Proactive prosocial motivations therefore systematically arise whenever selection favours the evolution of cooperative breeding. Because the human data fit this general primate pattern, the adoption of cooperative breeding by our hominin ancestors also provides the most parsimonious explanation for the origin of human hyper-cooperation.

Long calls by flanged male Bornean orang-utans (Pongo pygmaeus wurmbii) serve as a long-distance communication signal in this semi-solitary species and allow individuals to adjust their ranging behavior. Long calls can be heard up to circa 1 km in dense rainforest. Only flanged males emit them, in various contexts: spontaneously (where no disturbances from the environment are perceived by human observers), when highly aroused by another male's long call or a falling tree nearby, or right after having pushed over a dead tree themselves. In this study, acoustic analyses of orang-utan long calls at Tuanan in Central Kalimantan not only confirm the discrimination of individual males by their long calls but also demonstrate the discrimination of context based on the long calls' acoustic structure, which is further supported by the females' ranging responses according to long call contexts. Females with dependent offspring move away from spontaneous long call sources but appear to ignore long calls elicited by disturbance. Hence, Bornean orang-utan females perceive measurable differences in acoustic characteristics of long calls given in different contexts. These findings concur with vocal discrimination of contexts in other non-human primates.