Ecogeographic body size clines in turtles: analysis and synthesis of pattern and process

Abstract The mechanisms underpinning the evolution of ectotherm body size across temperature and latitude remain an enduring mystery. Oxygen limitation in warmer aquatic environments is often invoked to explain steeper temperature–size relationships of aquatic species across latitude, but the role of aquatic environments on body size evolution of aquatic species that respire primarily in air is unclear. We tested the generality of intraspecific temperature–size responses in Testudines, assessing whether turtles generally follow Bergmann’s Rule, and whether aquatic versus terrestrial turtles exhibit different temperature–size responses. Using a global dataset of turtle body sizes, we found that turtles generally show decreasing body size with increasing temperature. However, we also found that aquatic, semi-aquatic, and terrestrial turtles respond similarly to temperature. Our study affirms the possibility that temperature–size relationships are driven partly by oxygen limitation in other ectotherms, but casts doubt on hypotheses related to oxygen limitation for groups that respire primarily in air. We formalize and explore a hypothesis that temperature–size responses in turtles are a result of heat balance, arguing small size in warm environments reduces overheating risk, and large size in cool environments minimizes heat loss. Ultimately, we provide new insight and direction in the study of ecogeographic clines.