Effects of Body Size and Fur on Heat Loss of Collared Lemmings, Dicrostonyx groenlandicus

Abstract

Collared lemmings ( Dicrostonyx groenlandicus ) differ from many arvicoline rodents by increasing in size and developing white pelage in response to short photoperiod. I examined the relative contributions of body size and fur changes to heat loss in photoperiod-induced morphs. Heat loss as a function of ambient temperature was calculated from metabolic data and a heat-transfer model, and measured directly on pelts. Winter-morph lemmings were significantly longer and heavier than summer morphs; they were significantly rounder than both summer morphs and colony lemmings. The white winter pelage was significantly longer, thicker, and more dense than that of summer morphs. Calculated heat losses did not differ between morphs; however, heat loss through winter pelts was significantly lower than for summer pelts. Simulations predicted that heat loss of lemmings was more sensitive to changes in fur thickness and density than body mass; however, optimal body size increased by 10 g for lemmings at low temperatures. These results suggest that changes in size and shape may be of significance; nevertheless, fur insulation has the greatest influence on the energy budget of collared lemmings. The energetics of environmentally induced changes in body size of small mammals should be re-evaluated in the context of pelage response. I conclude with an evaluation of the thermal-conductance index as a measure of fur insulation, with recommendations for its replacement by more biologically relevant measures.