Thermal biology of reproduction in viviparous skinks, Eulamprus tympanum: why do gravid females bask more?

In some reptiles, gravid females bask more, and/or maintain higher body temperatures than do males or non-gravid females. This phenomenon is usually explained in terms of the female or her offspring benefitting from accelerated embryogenesis and early birth, but the effect of increased basking on gestation period has not been studied. In a laboratory experiment, gestation periods of gravid female skinks (Eulamprus tympanum) decreased with the duration of access to radiant heat. Embryonic development was more rapid in the laboratory than in the field, and there were no apparent adverse effects of this accelerated gestation on females or offspring. Number and mass of offspring, survival rates of embryos, relative clutch mass and female mass before and after parturition were not influenced by the decrease in gestation period caused by increased basking. Females selected similar temperatures in the laboratory and field (32° C), despite the availability of higher temperatures in the laboratory. Thus, gestation in the laboratory was accelerated by spending longer periods at usual basking temperatures, rather than by selecting higher temperatures. In the field, mean and modal body temperatures of active animals were similar in gravid females, males and non-gravid females, but gravid females appear to bask more of the time, even in cloudy weather when other members of the population do not bask. Hence, an apparent similarity in thermal regimes of gravid and non-gravid animals may mask significant underlying differences in thermoregulatory strategies.