Cooperative breeding occurs when helpers provide alloparental care to the offspring of a breeding pair. One hypothesis of why helping occurs is that helpers gain valuable experience (skills) that may increase their own future reproductive success. However, research typically focuses on the effect of helping on short-term measures of reproductive success. Fewer studies have considered how helping affects long-term fitness measures. Here, we analyse how helping experience affects key breeding and fitness-related parameters in the Seychelles warbler (Acrocephalus sechellensis). Importantly, we control for females that have co-bred, as they have experience with direct reproduction. Helping experience alone had no impact on any of the metrics considered, except that helpers had an older age at first dominance. Females that had co-bred had longer dominant tenures, produced more recruits as dominant breeders and had a higher lifetime reproductive success than females that had never co-bred. Our results suggest that helping experience alone does not increase direct fitness in Seychelles warblers and highlights the importance of separating the effects of helping from co-breeding. Our findings also emphasise the importance of analysing the effect of helping at various life-history stages, as higher short-term fitness may not translate to an overall increase in lifetime fitness.

We poorly understand the factors shaping variation in fitness among individuals, i.e. in their ability to make a contribution to the future gene pool. While short-term fitness proxies, e.g. lifetime reproductive success (LRS), are commonly used to measure fitness, how well do these proxies perform? Multigenerational human genealogical data allow the estimation of individual genetic contributions (IGC) - a fitness approximation closer to its theoretical definition - over many more years than is possible for other species. Here we use genealogical data from two local populations in Switzerland to estimate the IGC for 2,623 individuals on average 308 years after they were born. We find that the number of grandoffspring predicts IGC best explaining 28 percentage points more variation than LRS. Overall, LRS explains 29% of the variation in IGC, and 33% when accounting for offspring survival to adulthood. This suggests that offspring reproductive success is a key determinant of individual fitness. Nevertheless, we find that LRS only slightly underestimates the IGC of offspring as family sizes increase, and hence we find little evidence for an offspring quality-quantity trade-off. Together these findings suggest that, albeit relatively imprecise, LRS is a largely unbiased fitness proxy in this historic human population.

An individual's lifetime reproductive success (LRS) measures its realized genetic contributions to the next generation, but how well does it predict this over longer periods? Here we use human genealogical data to estimate expected individual genetic contributions (IGC) and quantify the degree to which LRS, relative to other fitness proxies, predicts IGC over longer periods. This allows an identification of the life-history stages that are most important in shaping variation in IGC. We use historical genealogical data from two non-isolated local populations in Switzerland to estimate the stabilized IGC for 2230 individuals approximately 10 generations after they were born. We find that LRS explains 30% less variation in IGC than the best predictor of IGC, the number of grandoffspring. However, albeit less precise than the number of grandoffspring, we show that LRS does provide an unbiased prediction of IGC. Furthermore, it predicts IGC better than lifespan, and accounting for offspring survival to adulthood does not improve the explanatory power. Overall, our findings demonstrate the value of human genealogical data to evolutionary biology and suggest that reproduction—more than lifespan or offspring survival—impacts the long-term genetic contributions of historic humans, even in a population with appreciable migration.