Body temperatures of female Sceloporus grammicus: Thermal stress or impaired mobility?

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; these observations were in accord with the thermal stress hypothesis. In June/July, when thermoregulation was difficult as a result of low ambient temperatures, lizards had low T(b)s overall, and reproductive females had considerably lower T(b)s than nonreproductive females. Thus, when thermal opportunities were limited, reproductive females had more difficulty thermoregulating than did nonreproductive females. These observations were in accord with the encumbrance hypothesis. The encumbrance hypothesis was also supported by the observation that females grew more slowly when reproductive than when nonreproductive. Thus, we suggest that the thermal stress and the encumbrance hypotheses are not alternatives but are complementary explanations for the thermal behavior of female Sceloporus.