Allometry reveals trade-offs between Bergmann's and Allen's rules, and different avian adaptive strategies for thermoregulation

Animals tend to decrease in body size (Bergmann's rule) and elongate appendages (Allen's rule) in warm climates. However, it is unknown whether these patterns depend on each other or constitute independent responses to the thermal environment. Here, based on a global phylogenetic comparative analysis across 99.7% of the world's bird species, we show that the way in which the relative length of unfeathered appendages co-varies with temperature depends on body size and vice versa. First, the larger the body, the greater the increase in beak length with temperature. Second, the temperature-based increase in tarsus length is apparent only in larger birds, whereas in smaller birds, tarsus length decreases with temperature. Third, body size and the length of beak and tarsus interact with each other to predict the species' environmental temperature. These findings suggest that the animals' body size and shape are products of an evolutionary compromise that reflects distinct alternative thermoregulatory adaptations. Bergmann's and Allen's rules are speculated to describe alternative strategies of thermal adaptation. Here, Frohlich et al. explore global variation across avian species to show that the way in which relative length of beaks and tarsi co-vary with ambient temperature depends on body mass and vice versa.

Automated summary

Animals in warm climates tend to decrease in body size and elongate appendages. This study shows that the relative length of unfeathered appendages varies with temperature depending on body size. Body size, beak length, and tarsus length interact to predict the species' environmental temperature. These findings suggest that body size and shape are products of thermoregulatory adaptations.