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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 infrared 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^oC, compared to +2.4^oC in the south. Mean T_b was lowest in the south (26.9^oC, compared to 29.3^oC 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.

Key words: cline, pigmentation, Testudo, thermoregulation, tortoise

Correction: Nussear, Simandle & Tracy have pointed out in a Forum article (Herpetol. J. 10, 119-122, 2000) that the proposed thermoregulatory advantage of dark plastral pigmentation will not work. We agree that although colour can affect absorption of the short-wave IR in sunlight, colour does not matter to emission of long-wave IR by a tortoise (or to absorption of long-wave IR emitted by the ground). Plastral colour can therefore have no effect on thermoregulation, and this explanation of the cline must join the other four already rejected. Of those considered, the only hypothesis remaining is random genetic differentiation; this would explain why pigmentation varied with latitude but not with altitude.