ENERGY-EXPENDITURE DURING LACTATION IN RELATION TO LITTER SIZE IN FREE-LIVING GOLDEN-MANTLED GROUND-SQUIRRELS

Spermophilus saturatus maternal daily energy expenditure (DEE) increased slightly, but significantly, with litter size, by only c10% over the typical range of 3-5 young produced. Maternal body mass, total daily time spent in surface activity, and time spent foraging were not correlated with litter size. The increase in maternal energy metabolism with increasing litter size was small compared to the energy actually contained in milk. Milk production must have been essentially proportional to litter size, since the mass of individual pups at emergence from the natal burrow, as well as age, were independent of litter size. That the DEE is so similar for females producing different numbers of young suggests that differences in maternal workload at the peak of lactation associated with differences in litter size are not likely to be the basis of any trade-off that might eventually be observed between immediate level of fecundity and future fecundity and survival. Because energy intake appears to limit reproductive effort, the number of young produced by an individual female may be determined by individual variation in behaviour and habitat quality. -from Authors