Sex allocation: comparative analyses of brood sex ratio in birds and mammals

Abstract

Sex allocation is one of the core concepts in evolutionary biology nevertheless, the patterns of sex allocation and evolutionary predictors of sex ratio variation had remained controversial. Here, using data from 78,025 and 628,503 sexed individual offspring of birds and mammals respectively, that represent 90 species and 315 species of birds sex allocation patterns are investigated. A phylogenetic control analysis showed that in mammals 13 species have significantly male-biased brood sex ratios whereas 9 species have significant female-biased brood sex ratios. However, across all species brood sex ratio does not deviate significantly from 50:50 in birds nor in mammals consistently with the Fisherian frequency-dependent selection. We also hypothesize that brood sex ratios are predicted by traits related to sexual size dimorphism, mating systems, and brood size, and found that species with larger brood sizes had significantly more female offspring in birds. To conclude, captive populations provide an excellent yet rarely tapped source of information to investigate sex allocation. We call for future studies to explore the conservation, and welfare management implications of sex allocations in captive animals.