Spur-thighed tortoises, Testudo graeca, in south-western Spain lay 3-4 clutches in shallow nests from April to June. In the present study the incubation temperature of nests laid in field enclosures in April, May and June was monitored over four years. Mean daily temperature throughout incubation averaged 27.9°C, but displayed a wide daily range, with average maximum values around 41°C (also in nests where hatching success was >0), and an absolute maximum of almost 50°C. Early (April) nests displayed lower mean daily temperatures than intermediate (May) and late (June) nests, although all nests reached similar high temperatures during the hottest month (July). Incubation temperatures were affected by nest vegetation cover. Incubation length varied from 67-129 days. Because the length of incubation was negatively correlated with nest temperature, early nests had longer incubation periods than intermediate and late nests. Hatching success averaged 61% and was mainly affected by variables related to maximum temperatures. Thus unsuccessful nests (i.e. no eggs hatching) were associated with higher temperatures or longer exposure to higher temperatures. Differences in hatching or nest success were not related to the nesting month, but might have been influenced by the location of the nest. Lethal temperatures for embryo development were frequently reached during July, therefore vegetation cover of the nest is likely to play an important role in avoiding deleterious nest environments.

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