Photoperiodic effects on body size and energetics of the collared lemming, Dicrostonyx groenlandicus

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Abstract
Resting metabolic rates of adult collared lemmings, Dicrostonyx groenlandicus, acclimated from weaning to either long, "summer" (22L:2D) or short, "winter" (2L:22D) photoperiod at 15.degree. C were examined as a function of ambient temperature. Winter morphs were significantly heavier than summer morphs (73.8 .+-. 7.7 (SD) and 54.5 .+-. 7.2 (SD) g, respectively). However, there were no differences in mass-specific metabolic rates between treatments at 15 and 20.degree. C. At low ambient temperatures (0 and -10.degree. C), metabolic rates of summer morphs were significantly higher than those of winter morphs, indicating a shift in the thermoneutral zone with photopoeriod acclimation. There was no significant differences in core temperature between morphs at any ambient temperature. Thermal conductance of winter morphs was significantly lower (0.05 mL O2 (g .cntdot. h .cntdot. .degree.C)-1) than that of summer morphs (0.09 mL O2 (g .cntdot. h .cntdot. .degree. C)-1). Comparisons with other myomorph rodents do not support the contention that lemmings have unusually high metabolic rates. However, minimal thermal conductances of lemmings were much lower than expected on the basis of body size. These data suggest that although lemmings may differ in seasonal patterns of energetics from other microtines, there is little evidence to support the assertion that high rates of metabolism are characteristic of all microtines, or that observed basal rates represent a phylogenetic adaptation to cold.