REPRODUCTIVE DEMANDS AND MASS GAINS - A PARADOX IN FEMALE RED SQUIRRELS (TAMIASCIURUS-HUDSONICUS)

1. We studied the response of lactating red squirrels to reproductive demands that were experimentally increased by litter-size manipulation s. 2. Females with experimentally increased reproductive demands ('aug ment' females) gained significantly more body mass during the first ha lf of lactation than did unmanipulated ('control') females. However, a ugment females lost more mass following the mid-point of lactation tha n control females, so that the net mass difference between early lacta tion and late summer was negligible for both treatments. 3. Measuremen ts of total body water at the mid-point of lactation confirmed that ma ss gains during early lactation reflected changes in body fat levels, indicating that females adjusted their energy stores according to thei r reproductive demands. 4. We also analysed the relationship between e arly lactation mass gain and natural litter size among a larger group of control females, studied at the same site between 1990 and 1994. Th ere was a significant, positive relationship between natural litter si ze and female mass gain. Furthermore, females characterized by the lar gest gains in body mass had the highest levels of juvenile survival to emergence, indicating that early lactation mass gain is an important component of parental investment. 5. These responses suggest that (i) energy storage during early lactation is used to reduce daily energy r equirements during late lactation; (ii) breeders use demands during ea rly lactation to 'forecast' requirements during late lactation; and (i ii) that despite the elevated energetic demands of lactation, individu als can quickly adjust their energy budgets from slightly positive to highly positive levels.