Reproductive Demands and Mass Gains: A Paradox in Female Red Squirrels (Tamiasciurus hudsonicus)

We studied the response of lactating red squirrels to reproductive demands that were experimentally increased by litter-size manipulations. Females with experimentally increased reproductive demands ('augment' females) gained significantly more body mass during the first half of lactation than did unmanipulated ('control') females. However, augment females lost more mass following the mid-point of lactation than control females, so that the net mass difference between early lactation and late summer was negligible for both treatments. Measurements of total body water at the mid-point of lactation confirmed that mass gains during early lactation reflected changes in body fat levels, indicating that females adjusted their energy stores according to their reproductive demands. We also analysed the relationship between early lactation mass gain and natural litter size among a larger group of control females, studied at the same site between 1990 and 1994. There was a significant, positive relationship between natural litter size and female mass gain. Furthermore, females characterized by the largest 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. These responses suggest that (i) energy storage during early lactation is used to reduce daily energy requirements during late lactation; (ii) breeders use demands during early lactation to 'forecast' requirements during late lactation; and (iii) that despite the elevated energetic demands of lactation, individuals can quickly adjust their energy budgets from slightly positive to highly positive levels.