CHAPPELL, Mark A; HAMMOND, Kimberly A: Maximal aerobic capacity in combined cold and exercise in deer mice

Small endotherms must expend energy on both thermoregulation and locomotion, and often the two demands occur simultaneously. Several studies have investigated whether energy costs of thermoregulation and locomotion are additive or if metabolic heat produced by locomotor activity can be substituted for thermogenic requirements. However, little is known about how maximal aerobic performance is affected by interactions between thermogenesis and exercise. These interactions may be important for small endotherms in cold environments. For example, previous work suggests that deer mice (Peromyscus maniculatus) living at high altitudes are often active at temperatures requiring near-maximal rates of metabolic heat production. Accordingly, we tested whether temperature affected this species’ maximal oxygen consumption in exercise (VO₂max) by running animals to VO₂max on a treadmill at 22, 0, and -15 °C. For comparison, we also measured maximal thermogenic VO₂ (VO₂sum) using heliox. Exercise-induced VO₂max was about 17% greater at 0 °C than at 22 °C (P= .00004). However, VO₂max at -15 °C was considerably less than at 0 °C (P= .028) and did not significantly differ from the 22 °C value. Also, endurance (duration of exercise prior to exhaustion) was 51% greater at 0 °C than at -15 °C (P = .006). Heliox-induced VO₂sum was greater than VO₂max at 22 °C (P= .00006) but did not differ from VO₂max at 0 or -15 °C. These results suggest that moderate heat production can slightly enhance VO₂max during exercise, but thermogenesis at low temperatures reduces exercise performance.