Keeping your cool: thermoregulatory performance and plasticity in desert cricetid rodents

Small mammals in hot deserts often avoid heat via noctumality and fossoriality, and are thought to have a limited capacity to dissipate heat using evaporative cooling. Research to date has focused on thermoregulatory responses to air temperatures (T-a) below body temperature (T-b). Consequently, the thermoregulatory performance of small mammals exposed to high T-a is poorly understood, particularly responses across geographic and seasonal scales. We quantified the seasonal thermoregulatory performance of four cricetid rodents (Neotoma albigula, Neotoma lepida, Peromyscus eremicus, Peromyscus crinitus) exposed to high T-a, at four sites in the Mojave Desert. We measured metabolism, evaporative water loss and T-b using flow-through respirometry. When exposed to T-a >= T-b, rodents showed steep increases in T-b, copious salivation and limited evaporative heat dissipation. Most individuals were only capable of maintaining T-a -T-b gradients of similar to 1 degrees, resulting in heat tolerance limits (HTLs) in the range T-a =43-45 degrees C. All species exhibited a thermoneutral T-b of similar to 35-36 degrees C, and T-b increased to maximal levels of similar to 43 degrees C. Metabolic rates and rates of evaporative water loss increased steeply in all species as T-a approached T-b. We also observed significant increases in resting metabolism and evaporative water loss from summer to winter at T-a within and above the thermoneutral zone. In contrast, we found few differences in the thermoregulatory performance within species across sites. Our results suggest that cricetid rodents have a limited physiological capacity to cope with environmental temperatures that exceed T-b and that a rapidly warming environment may increasingly constrain their nocturnal activity.