Individual variation in the lifetime reproductive success of Seychelles warblers

Cooperative breeding is a system wherein more than two individuals raise offspring in a single breeding attempt. Helping is energetically costly, and helpers often sacrifice their personal reproduction, at least temporarily, to provide alloparental care. Why subordinates choose to help despite these apparent costs has been explored. However, studies have tended to focus on short-term fitness measures, as lifetime data are hard to obtain in most species due to system ecology. Here, I use the long-term Seychelles warbler (Acrocephalus sechellensis) dataset from a closed natural population with accurate fitness estimates to consider the short and life-long fitness consequences of helping behaviour to helpers (chapter 2) and helped offspring (chapter 3) and identified no positive fitness benefit in any of the metrics considered. However, I did find that the presence of a natal helper was associated with a reduced likelihood of males acquiring a dominant breeding position and reduced male lifetime reproductive success.

Whilst many studies use single-generation fitness proxies to estimate fitness, how well these proxies predict long-term genetic contributions is poorly understood. In chapter 4, I considered the relationship between single-generation life-history traits and fitness proxies and the genetic contributions of individuals to a population 15 years in the future. I found that life-history traits explained 8-33% of variation in genetic contributions, and lifetime reproductive success explained ≤56% of variation. The unexplained variation in genetic contributions is likely due to fitness having low additive genetic variance, environmental variation, and genetic drift. As such, the reproductive success of an ancestor does not necessitate the reproductive success of their descendants. My thesis improves our understanding of the evolution of cooperative breeding by highlight the importance of quantifying fitness at different life-history stages, as short-term benefits may not translate into improved longer-term fitness, and accounting for offspring sex-specific effects.