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1. The performance of ectotherms integrated over time depends in part on the position and shape of the distribution of body temperatures (T-b) experienced during activity. For several complementary reasons, physiological ecologists have long expected that T-b distributions during activity should have a long left tail (left-skewed), but only infrequently have they quantified the magnitude and direction of T-b skewness in nature.

2. To evaluate whether left-skewed T-b distributions are general for diurnal desert lizards, we compiled and analysed T-b (Sigma = 9,023 temperatures) from our own prior studies of active desert lizards in three continents (25 species in Western Australia, 10 in the Kalahari Desert of Africa and 10 species in western North America). We gathered these data over several decades, using standardized techniques.

3. Many species showed significantly left-skewed T-b distributions, even when records were restricted to summer months. However, magnitudes of skewness were always small, such that mean T-b were never more than 1 degrees C lower than median T-b. The significance of T-b skewness was sensitive to sample size, and power tests reinforced this sensitivity.

4. The magnitude of skewness was not obviously related to phylogeny, desert, body size or median body temperature. Moreover, a formal phylogenetic analysis is inappropriate because geography and phylogeny are confounded (i.e. are highly collinear).

5. Skewness might be limited if lizards pre-warm inside retreats before emerging in the morning, emerge only when operative temperatures are high enough to speed warming to activity T-b, or if cold lizards are especially wary and difficult to spot or catch. Telemetry studies may help evaluate these possibilities.