MUELLER, P.J.: Desert rodents stockpile food in the form of body fat.

Seasonal, nutritional and age-related factors influencing body composition are frequently studied, but there is less information on species-level variation across habitat types. Because of the unpredictable distribution of food resources in xeric environments, desert-dwelling rodents might accumulate greater fat reserves than their mesic-zone relatives. To investigate this question, I compared, by total body lipid extraction, the body fat content of six species of wild-derived Peromyscus mice: three species of desert provenance and three of mesic habitats. All mice were born in captivity, maintained on identical, ad libitum diets, and housed (two mice per cage) in cages large enough to permit running, jumping and climbing exercise. Thus all mice had the opportunity to lay down significant adipose tissue or, alternatively, to remain lean through either low food intake or exercising. Although mice (all male) varied in age from 4 to 22 months, there was no discernable relationship in these data between age and body fat content. However, xeric zone mice had significantly greater body fat (17.0 � 1.3 % of body weight) than did mesic zone mice (9.5 � 1.2 %). The fattest species (P. californicus) also had livers significantly fattier than the genus average, while livers of the leanest species (P. aztecus) contained very little fat. There was also no relationship between food intake and lipid accretion, e.g., the species with the highest (mass-adjusted) food intake (P. leucopus) had a low body fat content, while the species with the lowest food intake (P. aztecus) also had a very low body fat content. Therefore, factors more complex than simple appetite or food availability regulate body composition in these species. Storage of body fat may allow desert dwelling animals to survive through periods of drought and food insufficiency.