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1. Mother-Offspring Proximity Maintenance in Wild Bornean Orangutans (Pongo pygmaeus wurmbii)

   Scott, Amy M; Susanto, Tri Wahyu; Mitra Setia, Tatang; Knott, Cheryl D (October 2019, International Conference on Biodiversity for Life: Sustainable Development of Indonesia Biodiversity)

   With a semi-solitary social organization, interactions between orangutans are infrequent, but still complex. Mother-offspring pairs are the basic social unit in orangutan social organization. The distance between a mother and offspring decreases in the presence of males compared to when the pair is alone or with a non-related female. In order to investigate who is responsible for this change in distance, we collected behavioral data on wild orangutans in Gunung Palung National Park, West Kalimantan, from May 2018-April 2019. We employed the Hinde Index as a measure of proximity maintenance. The Hinde Index is a ratio calculated from the number of approaches and leaves between two individuals. We found that for 97% of days (N=30) the offspring was responsible for proximity maintenance. There was no difference in the Hinde Index when mother-offspring pairs were alone compared to in the presence of a male (Nalone=29, Nmale=10, t = 0.917, df = 14.878, p-value = 0.374). However, controlling for individual variation by analyzing the data on a per follow basis, we found that proximity became more mother-maintained when males were present for 6 out of 7 follows, but this difference was not significant (N=7, t = 0.029, df = 6, p-value = 0.489). 

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2. The Context of Food Sharing and Tolerance in Wild Bornean Orangutans (Pongo pygmaeus wurmbii)

   Kane, Erin E; Susanto, Tri Wahyu; Knott, Cheryl D (August 2020, 42nd Annual Meeting of the American Society of Primatologists)

   When female chimpanzees, orangutans, and callitrichids share challenging‐to‐process resources with their offspring, they improve offspring access to foods and calories which would otherwise be unavailable. Adult chimpanzees share foods rarely, but when they do, sharing valuable resources solidifies inter‐individual bonds (e.g., when building coalitions or eliciting copulations). While maternal‐offspring food sharing has been studied in wild orangutans, the context in which adult orangutans share food and feed in proximity is poorly known. We use 27 years of research on orangutans in West Kalimantan, Indonesia, to examine this behavior. Food sharing and tolerance were observed during 2,131 follows between 1994‐2019. Mother‐infant food sharing occurred in 78%, of these follows, female‐female sharing in 22%, male‐female sharing in 32%, and male‐male in just 1%. Adult females shared foods at different rates with adult males than with offspring (Chi‐square = 49.27,p< .01,N= 589 events). Eighty‐one percent of mother‐offspring food sharing/tolerance was fruit, compared to only 71% of male‐female food sharing/tolerance. Durio, Lithocarpus, and Willughbeia (hard‐to‐process fruits) were most frequently shared by mothers. Twenty‐three percent of male‐female food sharing/tolerance occurred while eating termites; only 3% of mother‐infant sharing did. Only two of 350 mouth‐to‐mouth or hand‐to‐mouth transfers involved adult males and females. Mothers increase their offspring’s access to challenging resources, while food sharing/tolerance among adult males and females is not limited to valuable resources, but may indicate strong social tolerance or affiliation in generally solitary adults 

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3. Female choice and sexual coercion in wild orangutans (Pongo pygmaeus wurmbii)

   Scott, AM; Susanto, TW; Knott, CD (January 2021, 90th Annual Meeting of the American Association of Physical Anthropologists)

   null (Ed.)

   Due to extreme incongruence in parental investment, the potential for sexual conflict in orangutan reproductive strategies is high. Female orangutans prefer flanged males, but with intense male-male competition and sexual coercion, it is unclear how females are able to exert choice. We hypothesized that female orangutans use initiation and maintenance of associations with preferred males as a mechanism of female choice. We used encounter rates and behavioral measures of proximity maintenance to distinguish between the role of female choice and male coercion in male-female associations in Gunung Palung National Park, West Kalimantan, Indonesia. From May 2018-April 2019, we collected data on initiation, termination, and proximity maintenance during associations between males and cycling females (females without dependent offspring or with offspring over 6 years old). Encounters between cycling females and flanged males were more likely to be female-initiated (83.3%), while encounters with unflanged males were more likely to be male-initiated (80%) (N=16, p=0.035, Fisher’s exact test). Additionally, the Brown Index indicates significantly more female proximity maintenance when cycling females associated with flanged males than unflanged males (N=33, W = 198.5, p= 0.012). However, using long-term data on male-female associations, we found that dyads of flanged males and females were less likely to encounter other males compared to dyads of unflanged males and females (N=542, χ2= 3.3051, df=1, p=0.035). This indicates that flanged males may also use associations to mate guard females. These data indicate that there are behavioral manifestations of both female choice and male coercion in orangutan associations. Funders: NSF (DGE-1247312, BCS-1638823, BCS-0936199), Boston University, Leakey Foundation, Disney Conservation Fund, and US Fish and Wildlife Service (F18AP00898) 

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4. Female choice and sexual coercion in wild orangutans (Pongo pygmaeus wurmbii)

   Scott, AM; Susanto, TW; Knott, CD (January 2020, 6th Annual Meeting of the Northeastern Evolutionary Primatologists)

   null (Ed.)

   Due to extreme incongruence in parental investment, the potential for sexual conflict in orangutan reproductive strategies is high. Female orangutans prefer flanged males, but with intense male-male competition and sexual coercion, it is unclear how females are able to exert choice. We hypothesized that female orangutans use initiation and maintenance of associations with preferred males as a mechanism of female choice. We used encounter rates and behavioral measures of proximity maintenance to distinguish between the role of female choice and male coercion in male-female associations in Gunung Palung National Park, West Kalimantan, Indonesia. From May 2018-April 2019, we collected data on initiation, termination, and proximity maintenance during associations between males and cycling females (females without dependent offspring or with offspring over 6 years old). Encounters between cycling females and flanged males were more likely to be female-initiated (83.3%), while encounters with unflanged males were more likely to be male-initiated (80%) (N=16, p=0.035, Fisher’s exact test). Additionally, the Brown Index indicates significantly more female proximity maintenance when cycling females associated with flanged males than unflanged males (N=33, W = 198.5, p= 0.012). However, using long-term data on male-female associations, we found that dyads of flanged males and females were less likely to encounter other males compared to dyads of unflanged males and females (N=542, χ2= 3.3051, df=1, p=0.035). This indicates that flanged males may also use associations to mate guard females. These data indicate that there are behavioral manifestations of both female choice and male coercion in orangutan associations. Funders: NSF (DGE-1247312, BCS-1638823, BCS-0936199), Boston University, Leakey Foundation, Disney Conservation Fund, and US Fish and Wildlife Service (F18AP00898) 

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5. Female reproductive ageing persists despite high infanticide risk in chacma baboons and geladas

   https://doi.org/10.1098/rsos.241210  

   Feder, Jacob A; Schneider-Crease, India A; Beehner, Jacinta C; Bergman, Thore J; Seyfarth, Robert M; Silk, Joan B; Snyder-Mackler, Noah; Lu, Amy (January 2025, Royal Society Open Science)

   Across mammals, fertility and offspring survival are often lowest at the beginning and end of females’ reproductive careers. However, extrinsic drivers of reproductive success—including infanticide by males—could stochastically obscure these expected age-related trends. Here, we modelled reproductive ageing trajectories in two cercopithecine primates that experience high rates of male infanticide: the chacma baboon (Papio ursinus) and the gelada (Theropithecus gelada). We found that middle-aged mothers generally achieved the shortest interbirth intervals in chacma baboons. By contrast, old gelada females often showed shorter interbirth intervals than their younger group-mates with one exception: the oldest females typically failed to produce additional offspring before their deaths. Infant survival peaked in middle-aged mothers in chacma baboons but in young mothers in geladas. While infant mortality linked with maternal death increased as mothers aged in both species, infanticide risk did not predictably shift with maternal age. Thus, infanticide patterns cannot explain the surprising young mother advantage observed in geladas. Instead, we argue that this could be a product of their graminivorous diets, which might remove some energetic constraints on early reproduction. In sum, our data suggest that reproductive ageing is widespread but may be differentially shaped by ecological pressures. 

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