Developmental Plasticity in Response to Temperature Aligns with Allen's but not Bergmann's Rule

Increases in average global temperatures have caused researchers to revisit how species and populations are expected to respond to changing temperatures. Allen's rule and Bergmann's rule describe two ecogeographic patterns where species in high (cold) latitudes have shorter limbs and larger bodies, respectively, than species at low (warm) latitudes. This pattern is purported to be due to the improved heat retention of short-limbed, large bodies, and improved heat dissipation of long-limbed, small bodies. While Allen's and Bergmann's rules are generally assumed to reflect adaptive outcomes, we sought to investigate the role of developmental plasticity in response to rearing temperature in producing these expected morphologies. To do so, we conducted a systematic review and meta-analysis to test the hypotheses that body shape and size will change plastically in response to experimentally manipulated thermal rearing environments and conform to the patterns predicted by Allen's and Bergmann's rules. Via a phylogenetic meta-analysis, we examined the relationship between temperature experienced by subjects during early life and subsequent body shape and size metrics. Our analysis of 40 studies included 243 estimates from 4 mammal species and 21 bird species, with some species contributing multiple estimates. Developmental plasticity generally produced morphologies in line with Allen's rule, though not Bergmann's rule. Changes in morphology were more prominent under artificially cold conditions than warm conditions. Mammals were generally more responsive than birds, though birds were better-represented in our final dataset. These results suggest that developmental plasticity may contribute to the ability of populations to respond to climate change and that broader biogeographical patterns may emerge from plastic responses.