Group size predicts prenatal investment and offspring growth in a cooperatively breeding bird

Cooperative breeding, where seemingly altruistic helpers forgo reproduction to assist in raising the offspring of others, is a puzzle for biologists because it appears to contradict evolutionary principles of selfishness. One potential explanation is that helpers enhance group productivity, thus improving the direct fitness of breeders and providing indirect genetic and group-living benefits for helpers. In many species, however, helper presence does not actually enhance offspring production, raising the question of whether helpers instead either improve offspring quality (rather than quantity) or allow breeders to reduce prenatal investment (thereby increasing breeders' condition and future reproduction). In arrow-marked babblers (Turdoides jardineii), helpers do not increase the number of fledglings produced. To test whether helpers instead improve offspring quality or compensate for reduced prenatal investment, we investigated how egg volume, offspring growth, and nestling provisioning rates vary with group size. Breeding females in larger groups laid smaller eggs, but also produced heavier fledglings than those from smaller groups. This suggests that helpers in larger groups not only facilitate reduced prenatal maternal investment, but actually over-compensate for reduced egg sizes. Interestingly, larger groups did not feed nestlings more, suggesting that over-compensatory nestling growth may result from larger groups providing higher-quality food. Our results show that helping can be selected for via different mechanisms acting on both breeders and offspring, and that these can apply simultaneously within a single species. We highlight that variation in life history may explain inter-specific differences in mechanisms by which helpers improve group productivity.