Sociality evolves when the benefits of grouping outweigh the costs. Living with relatives can sometimes help to offset some of these costs, and fission-fusion dynamics can help to maximize benefits. Individual differences in sociability can exist and such differences have been linked to fitness consequences in several species. Differences in other personality traits, those that show within-individual consistency and significant differences among individuals, can also influence fitness. However, more research is needed on the relationships among personality traits, and their fitness correlates, in natural populations to better understand the adaptive nature of individual differences in personality traits.
The overall objective of my PhD was to understand some of the constraints, correlates, and fitness consequences of individual differences in sociability in an herbivorous marsupial, the eastern grey kangaroo (Macropus giganteus). Eastern grey kangaroos forage in open-membership groups that frequently change in size and composition. Females show natal philopatry but not complex cooperative interactions. I conducted field observations of a wild population of eastern grey kangaroos in Sundown National Park in south-east Queensland, Australia, over more than two years. I documented life-history traits, grouping patterns, and behavioural responses of over 240 identified females, and analysed these data in addition to two previous years’ worth of data collected by other researchers.
My first aim (Chapter 2) was to determine the extent to which females’ boldness and sociability measures were consistent over multiple years, and how stable correlations among these traits were across time. I also determined whether females’ association patterns could be explained to some degree by similarity in boldness, as measured by flight initiation distance. Individual differences in boldness and two measures of sociability (foraging group size and number of preferred associates) were significantly consistent across four years, and correlations between these traits tended to show consistency over time. Some social assortment between females of similar personalities was observed, but this was not consistent over time, and boldness was not related to preferential associations, suggesting that females were not actively choosing to associate with others based on their similarity in personality.
In Chapter 3, I explored the relationships between kinship and association strengths among female kangaroos over four years. I used both biparental genetic relatedness estimates and mother-daughter relationships as measures of kinship, and conducted one set of analyses that considered both adults and sub-adults, and one restricted to adult females. Both analyses showed weak, yet significant, positive correlations between association strengths and biparental relatedness for all four years. Mother-daughter status of adult females explained almost twice the variation in pairs’ association strengths as did biparental relatedness. Although space use overlap explained a high proportion of females’ association strengths, controlling for this revealed that a significant relationship between kinship and associations persisted among adult females.
Chapter 4 investigated short-term intrinsic and environmental variables that might constrain the social interactions of adult females. I examined the influences of body condition, reproductive state, food availability, and interactions among these measures, on both aggregation and association patterns of individuals. In months of higher lagged rainfall (a proxy for food availability) females’ mean group sizes and the number of different nearest neighbours females had were smaller, while distances between nearest neighbours were larger. Similar to findings from other kangaroo populations, females with larger dependent young foraged in smaller groups, at a greater distance from their neighbours, and grouped with fewer different individuals. Compared to females in poor condition, females in better condition foraged in smaller groups, fed closer to their nearest neighbours when food was plentiful, and had fewer different neighbours.
In Chapter 5 I explored relationships between measures of sociability and female reproductive success, using three approaches that incorporated data across different temporal scales. Parity had a considerable influence on both offspring production and survival at nearly all stages of offspring development - first time mothers had poor success. Contrary to our expectations, the survival of young to weaning was negatively related to several measures of sociability. Females that scored highly on a composite measure of their social network size, and females that had a large number of preferred associates, were less likely to wean a young in a year. At a shorter time scale, the young of females that foraged at closer distances to their nearest neighbours were less likely to survive the period from permanent pouch emergence to weaning.
This thesis contributes to understanding fine- and broad-scale social patterns in a widely distributed gregarious marsupial. Individual differences in sociability, although influenced to some extent by short-term variables, were consistent over time, correlated with females’ boldness levels and related to females’ reproductive success. The relationships of mothers and their daughters explained the majority of the correlations between kinship and association patterns, although most associations among adults likely occurred between unrelated individuals. Continued research into the individual consistency and fitness consequences of sociability and other personality traits in this population, as well as in other species, will further our understanding of the importance of these measures from an adaptive perspective.