Exploring group, age and sex differences in primate personality: the importance of standardised measurement scales and investigating group demographics

Abstract

CHAPTER 1 INTRODUCTION: Studying individual differences in primates has become increasingly relevant for understanding potential sampling biases in the field of comparative study, as well as understanding age related change in behaviour and well-being for the individual animals. I outline the first studies conducted on primate personality and move on to the current literature on primate personality. I discuss how primate personality is useful for the evolutionary comparison of behaviours, how personality is a measure of well-being in these animals, and how this is relevant for animals in captivity and involved in cognitive testing. I highlight how we still know relatively little regarding group differences in personality, particularly within species and between different living environments. Current literature also does not have a full picture of all age differences for some species (eg. Capuchins). This dissertation sets out to investigate personality differences in wild and captive living environments, age differences and sex differences in primate personality.

CHAPTER 2: In order to investigate personality differences at a group level in wild and captive Japanese macaques, we tested living environment, age and sex differences in this species. We performed a large secondary data analysis on 5 groups of macaques, living in the wild and in captivity. We tested group location, sex and age differences and found two significant interactions in Dominance and Openness. Males, older macaques and captive macaques were significantly higher in Dominance than females, younger macaques and wild macaques respectively. Older macaques were significantly lower in Openness than younger macaques, with a two-way interaction for captive females being lower than captive males, and wild females being higher than wild males. Openness seemed to decline at a much faster rate in captive populations than it did in wild populations.

CHAPTER 3: I set out to test the role of individual differences on a string-pulling paradigm in a wild sample of Japanese macaques. Data collection was limited but two females performed the majority of trials; they both had different learning curves and displayed different string-pulling strategies. One female seemed more methodical and careful while one female seemed more haphazard and impatient. The two females’ personality differences are discussed in relation to their different string-pulling strategies. These observations are presented as a case study.

CHAPTER 4: In order to investigate personality differences at a group level in wild and captive chimpanzees, we tested living environment and sex differences in this species. We performed a large secondary data analysis on wild and captive chimpanzee groups, and compared males and females. We found that there was little difference between captive and wild chimpanzees in Dominance or Conscientiousness. However, chimpanzees in captivity were significantly higher in Openness, Extraversion and Agreeableness than chimpanzees in the wild, whereas chimpanzees in captivity were overall lower in Neuroticism than their wild counterparts. There were two instances of an interaction: captive females were higher in Openness than wild females, and captive females were higher in Agreeableness than wild females.

CHAPTER 5: In order to investigate personality differences in capuchins, we tested age and sex differences in this species. We performed a secondary data analysis on wild and captive brown capuchins, focussing on age differences between juveniles (0-7 years), young adults (8-16 years), mature adults (17-24 years) and old individuals (over 24 years). There were significant age and sex differences in Dominance, with males scoring higher than females and mature and older individual scoring higher than younger adults and juveniles. There was also a significant age difference in Openness, with juveniles and young adults scoring significantly higher than mature and older individuals. There were no significant age or sex differences in Neuroticism or Friendliness.

CHAPTER 6 DISCUSSION: The usefulness of group, age and sex differences in personality are discussed in relation to cognitive testing and experimental set ups, as well as the well-being of primates in captivity and into old age. The limitations of the chapters are discussed; different measurement scales were used, often with a brief subset of items. Future directions for research in primate personality are discussed; we recommend consistency with measurement scales, longer measurement scales (rather than sub-setting some items) and more sampling.