DO SMALL VOLES REQUIRE LESS FOOD THAN LARGE VOLES

Many small mammals lose body mass in the fall and winter. Laboratory s tudies suggest that the cause of this varies from individual to indivi dual, at least in meadow voles, Microtus pennsylvanicus. The short day lengths of fall induce this adaptation in some individuals, presumabl y to promote winter survival. Other individuals, side-by-side neighbor s, are insensitive to this cue; they enter winter with a larger body m ass that they may not be able to maintain if conditions become too har sh. A long-held assumption is that the photoregulated loss of mass all ows energy to be conserved by decreasing the amount of food required f or survival and, thus, decreasing foraging time. The present experimen t tested the hypothesis that smaller, photoresponsive meadow voles do indeed consume less food and spend less time feeding than larger indiv iduals, which are not photoresponsive. This hypothesis was tested in c onditions meant to simulate some of the energy challenges faced by vol es at the onset of winter. Representatives of each phenotype were hous ed in cages in which they had to leave their nests in order to feed, a nd in which food intake and time spent feeding could be monitored. At 25 degrees C the two groups did not differ in either food intake or ti me spent feeding; the smaller animals required more food per gram of b ody mass. Food quality was reduced and, later, ambient temperature was decreased to 3 degrees C. Food intake and feeding time were again alm ost identical in the two groups. It is suggested that the hypothesis f ailed to find support in this experiment because it does not take into account the higher thermoregulatory costs associated with a decrease in size.