Energy Expenditure During Lactation in Relation to Litter Size in Free-Living Golden-Mantled Ground Squirrels

Publisher Website
Google Scholar
Abstract
(1) In a population of golden-mantled ground squirrels Spermophilus saturatus Rhoads, we directly measured daily energy expenditure (DEE) and behaviour at the peak of lactation (days 31-35 after parturition) in females that were producing one to five young. DEE was measured by a labelled isotopic technique and included all physiological work but excluded the energy content of milk. (2) Maternal DEE increased slightly, but significantly, with litter size, but only about 10% over the typical range of three to five 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 proportion to litter size, since the mass of individual pups at emergence from the natal burrow, as well as age, were independent of litter size. (3) To understand these observations we reconstructed the daily energy balance by taking into account the additional energy transferred from the mother to the young in the form of milk. The estimated daily metabolizable energy intake at the peak of lactation reached a level that may represent the limit for an animal of this size. (4) Our observation that the DEE is so similar for females producing different numbers of young is surprising. It 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 for this species, we suggest that the number of young produced by an individual female may be determined by individual variation in behaviour and habitat quality.