Geographic variation in the effects of heat exposure on maximum sprint speed and Hsp70 expression in the western fence lizard Sceloporus occidentalis

We examined whether western fence lizards Sceloporus occidentalis occurring in thermally divergent environments display differential responses to high temperature in locomotor performance and heat-shock protein (Hsp) expression. We measured maximum sprint speed in S. occidentalis from four populations at paired latitudes and elevations before and after exposure to an experimental heat treatment and then quantified hind-limb muscle Hsp70 expression. Lizards collected from northern or high-elevation collection sites suffered a greater reduction in sprint speed after heat exposure than lizards collected from southern or low-elevation sites. In addition, lizards from northern collection sites also exhibited an increase in Hsp70 expression after heat exposure, whereas there was no effect of heat exposure on Hsp70 expression in lizards from southern collection sites. Across all groups, there was a negative relationship between Hsp70 expression and sprint speed after thermal stress. This result is significant because (a) it suggests that an increase in Hsp70 alone cannot compensate for the immediate negative effects of high-temperature exposure on sprint speed and (b) it demonstrates a novel correlation between an emergent property at the intersection of several physiological systems (locomotion) and a cellular response (Hsp70 expression). Ultimately, geographic variation in the effects of heat on sprint speed may translate into differential fitness and population viability during future increases in global air temperatures.