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The existence of cooperative breeding seemingly contradicts classical selection processes, as helpers forgo their own reproduction to assist others’. Hamilton’s rule quantifies the value of helping with the equation rb>c where r is the relatedness between the helper and recipient, b is the benefit of the help, and c is the cost of not pursuing independent breeding. Despite the unusual nature of cooperative breeding, it is widespread, taxonomically and geographically. Comparative studies have found that cooperative breeding is correlated with monogamous breeding, presumably to maintain relatedness between helpers and recipients, ensuring sufficient incentives for helpers to persist. Further comparative research has found that cooperative breeding is more common in climatically-stochastic environments. Helpers may assist in buffering the unfavourable conditions in ‘poor’ seasons, or allow breeders to maximise reproductive outputs in times of plenty.
To investigate the interplay of these two correlates (mating system and environment), I investigated the breeding behaviour of Chestnut-crowned babblers breeding on the fringe of the Strzelecki Desert, Australia. I used several assignment methods to determine their mating system and found that the babblers are largely (~80%) monogamous and showed no intra-specific brood parasitism, or extra-group paternity. I then explored the meteorological influence on the timing of breeding and its role in predicting breeding events. I expanded on this by focusing on the climatic impacts on female investment in both clutch size and egg volume, and compared dominant to subordinate breeders. Lastly, I examined the effect that rainfall and temperature has on chick growth and output, again accounting for dominance status. Overall I detected specific climate windows for both rainfall and temperature, which had an effect on breeding frequency. Clutch sizes increased in response to higher rainfall in the preceding four months. Rainfall also increased the survival of subordinate chicks, but had no significant impact on dominant chick survival. Finally, temperature had a negative effect on body mass of chicks from dominant, but not subordinate broods.
Overall, this study found support for the relationship between cooperative breeding and monogamy, with high levels of monogamy, and groups being comprised of close kin. Support for the benefits of helpers was subtler, but it appears helpers allow breeders to breed under all but the most extreme climatic conditions. |
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