Anthropology, Anthropological Institute and Museum, University of Zurich, Switzerland
Saturday All day, Plaza Level
Humans are characterized by huge brains and high reproductive rates in comparison to great apes, which we aim to explain using a comparative phylogenetic approach. Variation in brain size relative to body mass has been found to correlate negatively with reproductive rates in most mammals. Cooperatively breeding carnivores are the exception. This is interpreted as the effect of energy subsidies during breeding, which allow species with allomaternal help to alleviate the trade-off between energy used for reproduction and for brain growth and maintenance. Because the number of mammals with allomaternal care is relatively limited, we here report on a study of its correlates in over 600 bird species.
In addition to basic data on brain and body mass, life history variables and development types, we quantified the energy expenditure of each parent and additional helpers, and the combined input per offspring. We predicted that energy subsidies of nestlings affect brain size or fertility mainly in altricial birds, as brain growth occurs mainly between hatching and fledging in this group. However, we found a stronger effect of the total energetic input of caretakers in precocial birds, indicating the critical role of supporting adult-sized brains in relatively very small immature bodies. In sum, our results provide independent evidence for the cooperative breeding hypothesis, which postulates that a change towards a significant allomaternal component in child care stood at the beginning of the ever-increasing brain size of the human lineage.