BODY-FAT AS AN INDICATOR OF NUTRITIONAL CONDITION FOR THE BROWN LEMMING

Our laboratory studies show that brown lemmings (Lemmus sibiricus) have the capacity to store large amounts of fat (>40% of body mass) and that the amount of fat stored is directly related to food intake. Nevertheless, lemmings and other microtine rodents maintain relatively low fat deposits in the field (often less-than-or-equal-to 5% of body mass). Apparently, they have not evolved to allocate much energy to fat reserves under natural conditions. Fat content of microtine rodents generally differs little between sexes, but in the field it does differ between seasons. Fat deposits tend to increase during the nonbreeding season, which is generally a time with low availability of high-quality food. Thus, body fat is not clearly related to nutritional condition in the field, except that starved animals have low fat reserves at death (<3% of wet body mass). Our experimental results indicated that high body fat in the laboratory can be accounted for by higher digestibility of energy in laboratory diets and lower levels of activity in cages.