WEAVERBIRD_DEFENCE · Unravelling an extended phenotype: sexual selection and the evolution of nest architecture in weaverbird defence against brood parasitism

A major challenge for evolutionary biologists is to explain how selection maintains biodiversity. Coevolution between closely associated species contributes substantially to the diversity of life. Yet little is known about how coevolutionary pressures between different species interact with selection from intraspecific social interactions. I will address this fundamental gap in our knowledge by analysing how sexual selection interacts with the evolution of host defences against a brood parasite. Cuckoos and their hosts provide a textbook example of coevolution, by reciprocally selecting for better parasitic tricks or better host defences. I will determine whether sexual selection influences the trajectory and pace of coevolutionary change and investigate whether this is why similar coevolutionary arms races have such diverse outcomes. The weaverbirds and their brood parasite the Diederik cuckoo provide an unrivalled opportunity to test this hypothesis given that weaverbirds exhibit a sexually-selected extended phenotypic trait: an elaborate nest that the females choose before mating with the male and laying in his nest. This trait also potentially doubles as a deterrent to brood parasites, by preventing them from gaining access to the nest. Second, this trait can be precisely quantified by applying state-of-the-art computational and analytical techniques to this novel context. Third, populations of weaverbirds have been introduced to islands where they have existed in the absence of selection from brood parasitism for over one hundred generations, providing the opportunity to partition out effects of different selection pressures. With this interdisciplinary approach I will investigate whether nest architectural defences have evolved to combat brood parasitism, how the evolution of these defences interacts with sexual selection, and how the evolution of nest architecture influences the evolution of other forms of defence against brood parasites.