HERPETOLOGICAL JOURNAL, Vol. 9, pp. 125-132 (1999)
A LATITUDINAL CLINE OF DARK PLASTRAL PIGMENTATION IN THE
TORTOISE TESTUDO HERMANNI IN GREECE
RONALD E. WILLEMSEN1 AND ADRIAN HAILEY2
1MonteCassinostraat 35, 7002 ER Doetinchem, The Netherlands
252 Mascotts Close, London NW2 6NS, UK
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 (Tb) and substrate (Ts) temperatures in populations from northern, central and southern Greece. Tb was generally above Ts, showing that heat would generally be lost rather than gained through the plastron, and the mean difference Tb-Ts was greatest in the north: +6.6oC, compared to +2.4oC in the south. Mean Tb was lowest in the south (26.9oC, compared to 29.3oC in the north) and the slope of Tb on Ts 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 Tb 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.