Congenital malformations in Japanese macaques (Macaca fuscata) at Takasakiyama

Primates and disability: Behavioral flexibility and implications for resilience to environmental change

Congenital malformations, conditions, injuries, and illness can lead to long-term physical impairment and disability in nonhuman primates. How individual primates change their behaviors flexibly to compensate for their disabilities can inform our understanding of their resilience and ability to adjust to environmental change. Here, we synthesize the literature on nonhuman primates and disability, addressing the questions: how does disability influence behavior in primates? What insights can we take from the literature to better understand and predict the capacity of primates to modify their behaviors in the face of human-induced environmental change? We conducted a systematic review of the literature on spontaneous physical impairment and disability in captive, free-ranging, and wild primates. We surveyed 2807 articles on Web of Science and Scopus and identified 114 studies that fit our predetermined inclusion criteria. Behavioral plasticity, maternal and conspecific care, and the potential for innovation of novel behaviors allow many primates with disabilities to compensate when faced with challenges that are outside the scope of usual circumstances. We also found that 60% of the publications connected primate physical impairment and disability to human activities, suggesting an entangled relationship among humans, the environment, and primate disability. Disability and physical impairments provide an opportunity to examine how primates modify their behavior when presented with challenging conditions, and their potential resilience to a changing environment.

My primate studies

In this paper, I describe my 62 years in primatology focusing on some of the key findings from fieldwork conducted in Japan, India, and Africa. My first study on nonhuman primates described in detail the division of a troop of Japanese macaques at Takasakiyama. After that, I had an opportunity to work on Hanuman langurs at Dharwar, India. These langurs lived in one-male, multi-female groups. This type of group structure was maintained through takeovers by all-male parties. The adult male and all juvenile males were chased out of the group. By this process, the one-male, multi-female group system was maintained. The incoming adult male bit and killed all infants in the group. Mothers who lost their infants went into estrus and mated with the newly arrived male. For many years, scientists ignored these events or ruled them out as abnormal behavior. My work on Japanese macaques suggested that concentrated resources created by artificial feeding exaggerated dominance rank hierarchies among individuals, whereas it is comparatively relaxed in the natural environment. I also investigated the population dynamics of a troop and the life histories of individuals. From these studies, I documented the frequency of twin births, the carrying of dead infants by mothers, and the occurrence of physical malformations. These observations were made possible through artificial feeding, revealing the merits and demerits of this approach. I pointed out that authors and journal editors must be careful to acknowledge important elements of the environment where studies are conducted, and these should be described when reporting results in scientific articles. My studies of chimpanzees were conducted at Bossou, Guinea. I suggested that there are males who lived outside of bisexual groups. Chimpanzees in this population made and used many kinds of tools. Some of them were observed only at Bossou, and a few were only discovered 20 years after the establishment of Bossou as a research site. After decades of research on tool use in this species, I also suggested that there are cultural zones throughout the geographic distribution of chimpanzees.

Infant Survival Among Free-Living Bonnet Macaques (Macaca radiata) in South India

Female reproductive success depends to a large extent on infants’ ability to survive to maturity. While most studies of female reproductive success have focused on the effects of individuals’ sociodemographic factors (e.g., age/parity, dominance rank) on offspring survival among wild primates living in less disturbed habitats, little research has focused on offspring survival in urban or periurban animals. Here we investigated sociodemographic and anthropogenic determinants of infant survival (up to 1 yr of age) in free-ranging bonnet macaques (Macaca radiata) living in a periurban environment in Southern India. We conducted the study from November 2016 to May 2018, on two groups of bonnet macaques at the Thenmala tourist site in the state of Kerala. Fifty infants were born across two birth seasons. Of these infants, 29.2% died or disappeared in 2017 and 26.9% died or disappeared in 2018. We found that infant survival was strongly influenced by the mother’s parity: infants of experienced mothers had a better chance of survival than those of first-time mothers. We also found that male infants were more likely to die than female infants. However, we found no effects of mothers’ dominance rank, or of frequency of mothers’ interactions with humans and time spent foraging on anthropogenic food, on infant survival. Our results, consistent with findings from other wild primate species, show that even in challenging human-impacted environments, experienced bonnet macaque mothers have greater success than inexperienced ones.

Social grooming efficiency and techniques are influenced by manual impairment in free-ranging Japanese macaques (Macaca fuscata)

Animals born with physical impairments may particularly require behavioural flexibility and innovation to survive and carry out social activities, such as grooming. Studies on free-ranging Japanese macaques on Awaji Island, Japan, have shown that individuals with congenital limb malformations exhibited compensatory behaviours while grooming, such as increased mouth and elbow use for removing ectoparasites. The aim of this study is to explore disabled and nondisabled grooming techniques to determine whether and to what extent disabled monkeys develop novel grooming techniques, and if there is disability-associated variation in grooming efficiency. We hypothesized that modified grooming techniques used by disabled monkeys fulfilled the social and relaxing functions of grooming, however, that grooming by manually impaired individuals may still carry a hygienic cost to the recipients. Grooming behavioural data were collected by video in 2007 on 27 adult females (11 with CLMs). With a detailed grooming-related ethogram, we transcribed 216 2-minute continuous grooming video samples. We analyzed the data using generalized linear mixed effects models in R. We found that monkeys with manual impairment were less efficient groomers, as measured by removal and movement efficiency during grooming. However, there were no significant differences associated with the number of grooming movements per sample among the focal animals. Additionally, with a behavioural sequential analysis, we isolated 8 distinct grooming techniques and 3 novel disability-specific movements. Our results indicate that innovation and modification of movement types does not entirely compensate for manual disability, and that manual impairment carries a cost to the hygienic function of grooming. However, for the grooming recipient, the experience of being groomed by a disabled or nondisabled groomer is likely similar, and through movement compensation, disabled monkeys are able to engage in the social aspect of grooming without incurring any disability-associated costs.

Individuals in urban dwelling primate species face unequal benefits associated with living in an anthropogenic environment

In primates, living in an anthropogenic environment can significantly improve an individual's fitness, which is likely attributed to access to anthropogenic food resources. However, in non-professionally provisioned groups, few studies have examined whether individual attributes, such as dominance rank and sex, affect primates' ability to access anthropogenic food. Here, we investigated whether rank and sex explain individual differences in the proportion of anthropogenic food consumed by macaques. We observed 319 individuals living in nine urban groups across three macaque species. We used proportion of anthropogenic food in the diet as a proxy of access to those food resources. Males and high-ranking individuals in both sexes had significantly higher proportions of anthropogenic food in their diets than other individuals. We speculate that unequal access to anthropogenic food resources further increases within-group competition, and may limit fitness benefits in an anthropogenic environment to certain individuals.