Toward a general explanation for latitudinal clines in body size among chelonians

Abstract

For many species, the population-mean body size increases with latitude. Although heat conservation and starvation resistance are frequently invoked to explain latitudinal clines, these explanations seem incompatible with the natural history of turtles, which also increase in size with latitude. We collate the population-mean body size for nearly 100 populations of North American freshwater turtles belonging to seven species. We test the hypothesis that temperature–size relationships in turtles are driven by strong seasonality in the north, which leads to maturation at a larger size and the production of relatively fewer, larger clutches per season. Our results, however, do not support the seasonality hypothesis. We present two new hypotheses that can explain the generality of temperature–size responses in chelonians. First, oxygen consumption is temperature sensitive, placing a premium on small size in warm aquatic environments, because smaller size reduces total oxygen demand during activities requiring submergence. Second, overwintering physiology of turtles might drive size clines, because a decrease in mass-specific metabolic rate with size might result in a disproportionate increase in the capacity of large individuals to buffer lactic acid in anoxic conditions. The pursuit of a general explanation for temperature–size relationships in chelonians would benefit from comparing temperature–size responses between aquatic and terrestrial species.