A latitudinal cline of dark plastral pigmentation in the tortoise Testudo hermanni in Greece

The area of dark pigmentation on the plastron of the tortoise Testudo hermanni shows a latitudinal cline over about 400 km in Greece, with populations in the south being darker. The carapace did not show the clinal trend, and pigmentation was not significantly related to longitude or altitude. We examined several possible explanations for the cline, including an effect of incubation temperature, random genetic variation, and adaptation to several environmental variables. The most likely explanation is selection for thermoregulation, with decreased dark pigmentation in the north reducing heat loss to the substrate by infra red radiation during activity. This hypothesis was supported by data on body (T(b)) and substrate (T(s)) temperatures in populations from northern, central and southern Greece. T(b) was generally above T(s), showing that heat would generally be lost rather than gained through the plastron, and the mean difference T(b)-T(s) was greatest in the north: +6.6 °C, compared to +2.4 °C in the south. Mean T(b) was lowest in the south (26.9°C, compared to 29.3°C in the north) and the slope of T(b) on T(s) was about 1 (compared to 0.5 in the north). Thermoregulation in southern Greece is similar to that of tropical tortoises, with avoidance of overheating being the major problem, rather than elevation of T(b) for activity.