The association of tooth wear with sociality of free-ranging male koalas (Phascolarctos cinereus Goldfuss)
Logan, M & Sanson, GD 2002, Australian Journal of Zoology, vol. 50, no. 1, pp. 621–626.
This study found that tooth wear is associated with the reproductive efforts of male koalas. Koalas with very low and high tooth wear were found to have decreased reproductive efforts, while koalas with intermediate tooth wear were found to exert higher reproductive effort.
Reproductive effort by the male koalas observed in this study was quantified as the number of trees used per koala and amount of time spent bellowing by each koala. These variables are associated with reproductive effort because koalas are thought to maintain their dominance by marking both feeding and non-feeding trees with their scents and bellowing territorially, behaviours which reflect a koala’s level of dominance and reproductive effort by extension. Individuals with low and medium tooth wear were observed spending more time bellowing and using more trees per day for non-feeding activities than those males with very low or high tooth wear. By contrast, the koalas with high tooth wear were not observed using trees other than those accessed for feeding. If age and level of tooth wear in koalas are correlated, the relationship between age and sociality in male koalas thus appears to be U-shaped rather than linear.
In animal reproduction, reproductive effort can be explained to some extent by a cost-benefit ratio that is associated with age. For younger koalas, reproductive risks are initially high but later decrease as their ability to compete increases. In this study, koalas with very low degrees of tooth wear were most likely younger; thus, the potential costs of reproduction likely outweigh the benefits. As a result, reproductive effort for these young koalas was low. Similarly, individuals with high tooth wear may compensate their reduced energy intake as a result of the increased energy requirement for feeding with reduced reproductive effort. Koalas with low-to-medium tooth wear, on the other hand, were likely to be more skilled competitors than young males, and hence the benefits of reproduction begin to outweigh the costs, resulting in higher reproductive effort.
While the authors of this study recognise that its results came from only a select few individuals studied, they suggest that its findings serve as an indicator of the potential limitations tooth wear may impose on sociality and reproductive output in other wild koala populations. The results of this study contribute to our understandings of the biological processes that affect koalas at the individual and population levels.
Summarised by Millie Thng
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