Why do social species live longer? - Investigating interactions between helping and senescence in cooperatively breeding animals

Individuals in species with a social breeding system often live longer than individuals in less social species. In some cooperatively breeding species, including humans, care for offspring is shared between the parents (breeders) and their helpers. Helpers can enable parents to reduce their investment in parental tasks, thereby conserving the parents’ resources that can be redirected to increase their survival and future reproduction. These observations lead to the intriguing possibility that receiving help can alleviate senescence – the progressive decline of somatic function with age, causing reduced survival and fecundity – and lead to increased longevity in parents. Because high longevity in parents increases constraints on independent breeding opportunities for helpers – an underlying driver of cooperative breeding – reduced senescence caused by helping may reinforce cooperative breeding behaviour. I will test the novel hypothesis that this positive feedback between helping and longevity leads to reduced senescence in parents and increased helping. Using a combination of empirical research, experiments and theoretical modelling, I will investigate how helping and senescence interact, both on individual and evolutionary timescales. I will use measures of fitness (reproduction and survival), physiological condition and an indicator of biological ageing (age-related telomere length), from the exceptional 30-year longitudinal dataset on cooperatively breeding Seychelles warblers (Acrocephalus sechellensis), to quantify how and why helping explains individual variation in senescence in parents, and test the hypothesis that delayed senescence in breeders promotes helping. Using evolutionary theoretical models, I will study how the impact of helping on parental senescence affects the evolution of cooperative breeding. Through this multifaceted approach my study will provide new and important insights into the impact of senescence on the evolution of social behaviour. The results will also contribute to our understanding of the factors and mechanisms that cause individual variation in reproductive and survival senescence in social species.