BRANTA — Claire Spottiswoode
Behavioural ecology and tropical life-histories in African birds
Institution: University of Cambridge, UK
Supervisors: NB Davies
Details: PhD 2005 (Completed)
Address: Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK (Jun 2007) Email
Subject Keywords: Life history, coloniality, parasitism, predation
Species Keywords: Sociable Weavers Philetairus socius
Evolutionary success results not only from the struggle for existence, but also the struggle to reproduce, and resolving the trade-offs in investment that arise is the domain of life-history theory. In tropical birds, investment in survival tends to be favoured over reproduction, contrasting with the short lives and high fecundity of better-studied north-temperate species. I explore two key questions generated by life-history variation, using African birds as a model system and a combination of comparative analyses and a detailed field study of a single species.
First, what factors permit such long life in the tropics? In a field study in Malawi and South Africa, I tested whether longer-lived species invest more in immunity, and found that species with higher annual survival rates are indeed able to raise a stronger immune response to a simulated infection. Species may also delay breeding and live in cooperative kin groups in order to extend their lifespans; however, kin groups are by definition composed of genetically related individuals susceptible to similar parasite strains, suggesting that they might carry a cost of increased parasite transmission, and should hence invest more in immune defence. Such a pattern was found in a comparison between cooperative breeders and related species breeding in pairs, suggesting that elevated disease transmission may be a previously overlooked cost of cooperation among family members. The foregoing comparative studies examine outcomes of long-term evolutionary processes in relatively stable environments; however, the world is rapidly changing. I go on to attempt to predict among-species variation in the advancement of spring arrival times of migratory birds in response to climate change. I put forward a hypothesis concerning interspecific variation in the trade-off between the survival costs of migration and sexually selected reproductive benefits of early arrival, and provide a comparative test showing that, as predicted, more strongly sexually selected species seem have advanced their arrival dates the most.
Second, can variation among individuals within a species living in different microenvironments help to illuminate the ecological causes of such large-scale evolutionary patterns? I carried out a detailed field study of a single species with an extreme but variable life-history, the colonially-breeding Sociable Weaver Philetairus socius of the Kalahari desert. Offspring in larger colonies succumbed more to predation, parasitism and starvation than those in smaller colonies. Life history theory predicts that adults in bigger colonies should hence favour their own survival and reduce their investment in reproduction, and indeed they live longer and lay smaller eggs producing poorer-quality young. Cross-fostering and food supplementation experiments implied that some of these differences are determined either genetically or via maternal effects. I investigated the latter through assays of yolk antioxidants and androgens, and found that indeed these vary with colony size, which may account for some of the phenotypic differences observed. Thus life-history variation among colonies of different sizes may provide a microcosm of patterns across species, and by reducing fitness differences across colonies may help to maintain a stable distribution of colony sizes.