Kinship and sociability in eastern grey kangaroos

Abstract

Animal grouping patterns are generally shaped by ecological constraints such as resource distribution or predation risk. Population density often increases competition, so that life-history consequences of association patterns should be particularly evident at increased densities. This thesis seeks to quantify how kinship affected association patterns in a species with fission-fusion grouping patterns at high density. I also aimed to measure individual variation in sociability and evaluate its fitness consequences. Eastern grey kangaroos (Macropus giganteus) are gregarious herbivores, with a fission-fusion social system. I thus investigated how kinship and sex-age class affected behavioural and spatial associations in a high-density population of kangaroos at Wilsons Promontory National Park, Victoria. In Chapter 2, I examined relationships between individuals of different sex-age classes using half-weight indices (HWIs, which measure pairwise association strengths) and lagged association rates, and calculated six individual-based network measures. I found little social structure in the population. Mean HWIs were similar within and between different sex-age combinations (mean HWIs = 0.01 to 0.02). Individuals of all sex-age classes had preferred and avoided associates overall. Adult females showed a weak relationship (r = 0.05) between relatedness and HWIs, but this effect disappeared when controlling for geographic distance or spatial overlap between dyads. Lagged association rates decreased exponentially for adult males with adult females accompanied by young-at-foot in the breeding season. Most other lagged association rates decreased slightly in a linear fashion and few were constant. Although females without young-at-foot appeared to be more social than females with young-at foot, all associations among individual non-juvenile kangaroos were weak (mean HWI = 0.01). Sample size had strong positive effects on mean HWI and all six network measures (r = 0.40 to 0.66), which is more likely to occur in large populations where association strengths are generally low and individuals have many weak associations. I then combined observational data with genetic information to investigate sex-biased dispersal in Chapter 3. I found weak fine-scale genetic structure among adult females but none among adult males. Mean relatedness among adults was low (r = –0.01 to –0.03). Immature males moved away from their mothers at a younger age than did immature females. Although median distances of detected dispersals were 2–2.5 km for both sexes, only 6% of sub-adult females dispersed compared to 34% of sub-adult males. Adult females had overlapping ranges far wider than the maximum extent of spatial genetic structure found. My results suggest that negative density dependence shapes spatial genetic structure. In Chapter 4, I examined development of the mother-offspring relationship after permanent emergence from the pouch. Mothers associated more closely with their juvenile daughters than with their sons, but neither sex associated closely with their mothers as adults. There were few differences in the behaviour of juvenile sons and daughters towards mothers, although daughters foraged slightly closer than sons to their mothers, and sons were sometimes weaned earlier than daughters if the mother had a subsequent pouch young. Mothers never intervened to defend their young-at-foot from aggressive individuals and it was the offspring that maintained spatial proximity to their mothers. Because kangaroo mothers have few interactions with their juvenile offspring other than nursing, it is unlikely that a close bond develops between mothers and young. Chapter 5 reports on eight adoptions that were detected through observations of marked mothers and young, and three more using microsatellite DNA. Four adoptions involved reciprocal switches and three were by mothers whose own pouch young disappeared. Adoptive mothers were not closely related to each other or to adoptees. Adoptive mothers and young associated as closely as did biological pairs. Adoptions tended to be more likely in high-density years and were probably caused by misdirected care, suggesting poorly developed mother-offspring recognition. Finally, in Chapter 6 I aimed to determine whether the strength of early social relationships correlated with offspring survival, growth and reproduction. Association indices with the mother and time spent alone with the mother were not correlated with juvenile survival, body condition as 2-year-olds or reproduction as 3-year-olds. Sons and daughters that spent a large proportion of time with their mothers at 18–21 months, however, were subsequently 6% larger and 18% heavier as 2-year-olds than those that did not associate with their mothers. Mother-offspring associations before weaning thus had a beneficial effect on growth, which likely reflects maternal care. I have shown in this thesis that weak but non-random associations occurred among individuals in this high-density population. Weak associations among adult females resulted from philopatry but were not based on kinship because density decreased mean relatedness among neighbouring females. Strong associations occurred between adult females and their young-at-foot and the strength of mother-offspring associations had a positive effect on juvenile growth. The benefits of these associations, however, were likely a direct result of greater maternal care rather than reduced predation or increased access to resources.