Bird wings are shaped by thermoregulatory demand for heat dissipation

The tendency for animals in warmer climates to be longer-limbed (Allen’s Rule) is widely attributed to the demands of thermoregulation. However, it remains unclear whether this pattern is driven by selection for efficient heat retention at low temperatures or increased demand for heat dissipation at high temperatures. Using comparative phylogenetic models, we find that occurrence in warmer climates is associated with longer wing bones for 1,520 species of passerine birds. The highly vascularized musculature along these bones functions in heat exchange during flight when the overwhelming thermoregulatory challenge is dissipating heat, suggesting longer wing-bone length is driven by heat dissipation demands. Our findings reveal the pervasive impacts of thermoregulation on even the most critical features of vertebrate anatomy, and establish a clear mechanism linking spatial and temporal trends in appendage length. One-Sentence Summary Bird wing bones are longer in warmer climates to meet higher demand for heat loss. ### Competing Interest Statement The authors have declared no competing interest.