A long-held assertion has been that nocturnality is an escape mechanism for many nocturnal desert rodents because of limited tolerances to heat. To test this claim, we used a treadmill to examine the tolerances to high ambient temperatures (T a 's) of one subspecies of desert rodent, Merriam's kangaroo rat, Dipodomys merriami merriami, from contrasting environments. We simultaneously measured body temperature (T b ), evaporative water loss, and metabolic rates at an ecologically relevant speed

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... 0.6 km h Ϫ1 ) at different ambient temperatures (T p a Њ-42.5ЊC). We hypothesized that kangaroo rats from a more 25 xeric site would have greater abilities to remain active and maintain stable T b than those from a more mesic site, but mesicand xeric-site animals had comparable tolerances and were active until ЊC. At ЊC, however, T b of mesic-T p 42 T p 42.5 b a site animals increased more quickly than in xeric-site animals. Although most animals could not run more than 18 min at ЊC, most could run at ЊC for at least 30 min. T p 42.5 T p 40 a a Benefits of nocturnality for this species may reside more in purposes of water conservation and avoidance of predation and less on the direct regulation of T b , as T b is more labile than commonly thought.