Lion manes represent a compromise between social benefits and ecological costs. Mane development of Panthera leo varies geographically with prevailing climate, but varying genetics, behavior and social systems, and nutrition may all influence such broad-scale correlations. We studied captive lions housed comparably across 12 degrees of latitude in North America and correlated mane variation with climatic, life-history, and husbandry variables. As predicted from heat-load considerations, mane length and density were inversely correlated with temperature; color variation was unrelated. Mane development was correlated with January but not July conditions, suggesting a stronger response to cold than to heat. Climate-induced variation in manes of captives accounted for up to 50% of variation seen. Developmental effects of climate appear to be responsible for many of the taxonomic names applied to African lions and may signify important biological differences between modern and Pleistocene lions.