The fitness consequences of sociability in eastern grey kangaroos (Macropus giganteus)

Abstract

The level of individuals’ sociability, their propensity to associate and form close relationships with others, has fitness costs and benefits, but these have mostly been observed in females and in more socially complex species. My objective was to examine the social patterns of eastern grey kangaroos (Macropus giganteus) and investigate how these may relate to aspects of individuals’ fitness. The eastern grey kangaroo is a highly social macropod that exhibits high fission-fusion dynamics. I studied a population of wild kangaroos in Sundown National Park, Queensland, Australia, where research on females’ social patterns has been conducted since 2010. I collected data on both females’ and males’ grouping associations from January 2014 to February 2016. Adult kangaroos do not engage in allogrooming as a form of social exchange or allomothering, and there is no evidence of any other forms of cooperative behaviour. In Chapter 2, I studied how the social and space-use patterns of adult males differed among size classes to examine potential age and dominance related changes in males’ reproductive strategies. Large males were the most dominant, and they roamed further and had more adult female associates; however, their strength of association with their top ten female associates (i.e. “top 10 score”) was weaker than that of small males. Older, more dominant males likely adopt a strategy of maximising the number of females they are exposed to, as they have priority access to receptive females. Small males had a smaller space-use area, as they may have been investing more time into foraging for growth. Their higher top 10 score with adult females may have been due to their smaller range, but could also be indicative of a reproductive tactic that increases their chances of mating with a close associate when she first becomes receptive. I explored the link between males’ sociability and their reproductive success in Chapter 3. I examined the paternity of 136 young-at-foot from 96 known mothers for 58 adult males, and performed further analyses on 25 males. I found that more dominant males sired more young-at-foot, but their gregariousness and top 10 score were not related to their overall reproductive success in our site. The stronger the association between a male and female, the higher their probability of sharing an offspring. This may be because males that spend more time with a particular female are more likely to be present when she is in oestrous, but may also be due to females’ preference for more familiar males. In Chapter 4, I examined the testosterone (T) and glucocorticoid (GC) metabolite levels of 476 faecal samples collected from 31 adult males. I assessed whether the patterns of males’ hormone levels were consistent with the challenge hypothesis, social buffering theory, and an extended version of the steroid/peptide theory (a hypothesis that stipulates low T levels are related to greater nurturance behaviour). I found that males’ T levels were higher during months with more courtship and male-male competition, which is consistent with the challenge hypothesis, but it is not known whether heightened T was in response to competition or for increased spermatogenesis. There was no correlation between males’ GC levels and their sociability, thus I did not find any evidence for social buffering effects in male kangaroos. There was no evidence of testosterone being lower in more sociable males, but I found that the more frequently males were present in our site, the lower their T levels; if males that were infrequently present in our site were active roamers, this may have been indicative of T-mediated roaming behaviour. In Chapter 5, I explored whether females’ survival was related to their body condition, gregariousness, top 10 score, boldness, rainfall, and temperature. Of 138 females observed in 2010-2011, only 60 were present in the final year of study. I did not find evidence of survival benefits of having close social relationships, as top 10 score was not a significant predictor of survival. I found that survival between periods was higher when there was more rainfall in the previous month and females in better body condition had higher survival. Females with larger group sizes had a higher probability of survival showing that there is a direct fitness benefit of being gregarious. As predation is unlikely to be a cause of mortality in these females, I suggest that gregarious females are gaining foraging benefits from reduced vigilance when in larger groups. My thesis contributes to our understanding of the costs and benefits of sociability by examining a species with high fission-fusion dynamics, in which adults do not engage in complex cooperative behaviours or groom one another as a form of social exchange. Research comparing the social patterns of different species of mammals and the fitness consequences of their social relationships could help us understand what aspects of species’ social relationships are linked to particular fitness consequences.