Extrinsic modification of vertebrate sex ratios by climatic variation

Evidence for the influences of climate on early development, maternal condition, and offspring viability in terrestrial vertebrates suggests that climatic change has the potential to induce variation in offspring sex ratios in such organisms. Using long-term data at individual and population levels, we investigated the influence of global climatic variation, as a result of the North Atlantic Oscillation (NAO), on offspring sex ratios of red deer in Norway. The state of the NAO during the fetal development of hinds influenced the mass of their sons, but not daughters, and increasingly warmer and snowy winters led to increasingly male-biased offspring sex ratios, independently of changes in the mean age of hinds. Moreover, hinds that were themselves born following warm, snowy winters were smaller as adults, produced significantly lighter sons, and tended to produce more sons than hinds born following cold, dry winters. In light of the fact that these observations defy explanation according to previous hypotheses of adaptive modification of offspring sex ratios, we present the extrinsic modification hypothesis, which suggests that sex ratios may evolve in variable environments through natural selection independently of maternal strategies of sex allocation.