Extrinsic modification of vertebrate sex ratios by climatic variation

Evidence for the influences of climate on early development, maternal condi tion, and offspring viability in terrestrial vertebrates suggests that clim atic change has the potential to induce variation in offspring sex ratios i n 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 influ enced the mass of their sons, but not daughters, and increasingly warmer an d snowy winters led to increasingly male-biased offspring sex ratios, indep endently of changes in the mean age of hinds. Moreover, hinds that were the mselves born following warm, snowy winters were smaller as adults, produced significantly lighter sons, and tended to produce more sons than hinds bor n 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.