The evolution of cooperation in an arid-zone bird: bet-hedging, plasticity and constraints
Capilla-Lasheras, P
Date: 23 March 2020
Publisher
University of Exeter
Degree Title
Doctor of Philosophy in Biological Sciences
Abstract
The evolution of cooperation represents a long-standing puzzle in evolutionary biology. From insects to mammals, this behaviour has evolved multiple times in separate lineages. Even though inclusive fitness theory provides a solid theoretical framework to understand the evolution of cooperation, there are still many research challenges ...
The evolution of cooperation represents a long-standing puzzle in evolutionary biology. From insects to mammals, this behaviour has evolved multiple times in separate lineages. Even though inclusive fitness theory provides a solid theoretical framework to understand the evolution of cooperation, there are still many research challenges in the evolutionary study of cooperation. First, most of the evidence for the effects of cooperation on the reproductive success of beneficiaries in vertebrate societies is based on correlational studies, which can be confounded by several environmental factors. Second, there are recent theoretical formulations to explain the evolution of cooperation that have not been empirically tested yet but could shed new light on the selective pressures that facilitate the evolution of cooperation. Third, we still have a poor understanding of the sources of among individual variation in cooperative behaviours. In particular, few studies have investigated whether the level of cooperation expressed by individuals is heritable and, therefore, could respond to natural selection. In this thesis, I combine nine years of life-history and behavioural information with field experiments and genomics to investigate (i) the routes for non-breeding individuals to acquire indirect fitness benefits and (ii) the sources of among-individual variation in cooperation in white-browed sparrow-weavers (Plocepasser mahali), an arid-zone cooperative breeder. After a general introduction to the subject of cooperative breeding, in Chapter 2 I test a novel hypothesis for the evolution of cooperation, the ‘altruistic bet-hedging’ hypothesis. There, I show that non-breeding helpers reduce variation in the reproductive success of breeders without affecting their arithmetic mean reproductive success. Furthermore, I show that this reproductive variance compression appears to arise because helpers specifically reduce unpredictable rainfall-induced variation in reproductive success, just as hypothesised by global comparative studies of the evolution of cooperative breeding in birds. Then, I investigate alternative routes through which helpers may gain indirect fitness benefits. Specifically, in Chapter 3 I investigate the effects of helpers on pre- and post-natal maternal investment in reproduction. The findings in Chapter 3 provide clear evidence for maternal plasticity in pre-natal investment in reproduction (egg volume) in response to the number of helpers. Moreover, the helper effect of increased pre-natal maternal investment is associated with a decrease in post-natal maternal investment. In Chapter 4, I test the philopatry hypothesis for the evolution of sex differences in cooperation within animal societies and find strong support for this hypothesis in white-browed sparrow-weavers. Furthermore, Chapter 4 highlights the need to consider both sex differences in direct fitness benefits and costs when trying to understand sex differences in cooperation. Finally, in Chapter 5 I investigate among-helper variation in cooperative generosity, finding consistent individual differences and providing evidence for heritable variation in this trait. To conclude, in Chapter 6 I discuss the implications of these results for our general understanding of the evolution of cooperation in animal societies and highlight methodological approaches for future empirical studies of cooperation in the wild.
Doctoral Theses
Doctoral College
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