WEBSTER, MD; COHRS, CA; EISENSCHENK, JJ; OLSON, KA; MEULENERS, AM; St. John’s University; College of St. Benedict; St. John’s University; St. John’s University; College of St. Benedict: Acclimation to heat in the Cactus Mouse, Peromyscus eremicus.

We compared metabolic rate and body composition in two groups of P. eremicus housed under for three weeks at room temperature (23°C; control mice, N=9) or in constant heat (35°C; HAC mice, N=11). We measured oxygen consumption and total evaporative water loss at 26, 32, and 38&degC. Body mass was higher in control mice (18.8 + 2.3 g; mean + SD) than in HAC mice (17.1 + 1.3 g). HAC mice had higher total body water (72.6 + 2.6%, range 70.3�75.4%, N=3) compared to control mice (66.3 + 2.9%, range 64.3-69.7%, N=3). We found no differences in total evaporative water loss or VO₂ between the two groups. Both HAC and control mice dissipated more than 50% of metabolic heat production by evaporation at 38&degC. Plasma osmolarity was similar between groups (pooled mean 329 + 23 mOsm/kg), but hematocrit was higher in the HAC mice (31.2 + 3.5% compared to 38.2 + 4.8%). The mass of heart, kidney, liver, stomach, intestine was significantly lower in HAC mice. The constant high temperature of our experimental treatment differs from the thermally and hygrically complex environment experienced by P. eremicus in nature. Nevertheless, we hypothesize that that these mice do not physiologically acclimate to chronic heat exposure and instead, respond to heat stress behaviorally or by selecting favorable microclimates.