| Summary: | Females of some lizard species exhibit lower body temperatures (T(b)s) when reproductive (gravid or pregnant) than when not reproductive. Two hypotheses have been invoked to explain this phenomenon. One, the thermal stress hypothesis, is that the thermal optimum for embryos is lower than that of the female, and females thus actively select relatively low T(b)s. The other, the encumbrance hypothesis, is that females are encumbered by their clutch/litter and thus passively accept relatively low T(b)s. We collected field data on the thermal biology of Sceloporus grammicus at a high elevation site in Mexico during two seasons. In March, when thermoregulation was facilitated by high ambient temperature, Lizards had high T(b)s overall, and reproductive females had significantly lower T(b)s than males
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