Life with fur and without: experimental field energetics and survival of naked meadow voles

Hair is considered to be a basic mammalian feature that provides protection and insulation, promoting energy conservation and survival. To quantify th e functional significance of mammalian pelage, we tested the shortterm expe rimental effects of fur removal in a natural population of the California v ole, Microtus californicus, in winter. The daily energy expenditure (DEE) o f seven voles was directly measured in the field using stable isotopes, fir st with the animals in their natural condition and then again after experim ental removal of pelage by shaving. The initial meant-SD DEE of 96.0 +/- 23 .1 kJ/day increased by only about 10%, to 106.3 +/- 21.4 kJ/day, following shaving. The voles showed a body mass loss of 5%, about half of which was d ue to fur removal. Comparing slightly larger samples of all animals whose l ocal survival could be documented, we found 100% survival over the 5 initia l days of the experimental manipulation in 16 control animals and nine shav ed animals; over the following 3 weeks the survival of shaved mice did not differ significantly from that of controls. We were surprised that the aver age increase in energy expenditure of voles without fur was so modest, thou gh the range of individual values was great, and likewise we were surprised that shaved voles survived as well as they did. M, californicus survives n aturally in winter under conditions of social aggregation that include hudd ling together of individuals in nests; this situation probably provided our experimentally shaved voles an opportunity to minimize the energetic disad vantages of pelage loss. They may also have employed a variety of compensat ory physiological and behavioral responses, including reduction in activity time and food intake, and perhaps a related small decline in body mass. Ou r limited sample sizes made it difficult to detect subtle differences that may have been biologically significant in the system we studied. Nonetheles s, we can reaffirm that fur has an insulative value that promotes energy ec onomy and survival. However, we also conclude that mammalian physiology and behavior are sufficiently complex and flexible that a variety of responses can be deployed to promote survival under unusual circumstances such as th ose of our experimental test.