Do small voles require less food than large voles?

Abstract

Many small mammals lose body mass in the fall and winter. Laboratory studies suggest that the cause of this varies from individual to individual, at least in meadow voles, Microtus pennsylvanicus. The short day lengths of fall induce this adaptation in some individuals, presumably to promote winter survival. Other individuals, side-by-side neighbors, are insensitive to this cue; they enter winter with a larger body mass that they may not be able to maintain if conditions become too harsh. A long-held assumption is that the photoregulated loss of mass allows energy to be conserved by decreasing the amount of food required for survival and, thus, decreasing foraging time. The present experiment tested the hypothesis that smaller, photoresponsive meadow voles do indeed consume less food and spend less time feeding than larger individuals, which are not photoresponsive. This hypothesis was tested in conditions meant to simulate some of the energy challenges faced by voles at the onset of winter. Representatives of each phenotype were housed in cages in which they had to leave their nests in order to feed, and in which food intake and time spent feeding could be monitored. At 25°C the two groups did not differ in either food intake or time spent feeding; the smaller animals required more food per gram of body mass. Food quality was reduced and, later, ambient temperature was decreased to 3°C. Food intake and feeding time were again almost identical in the two groups. It is suggested that the hypothesis failed to find support in this experiment because it does not take into account the higher thermoregulatory costs associated with a decrease in size.