Explaining the seasonal decline in litter size in European ground squirrels

In European ground squirrels Spermophilus citellus as in many ground squirr el species, late born litters are composed of fewer young than early born l itters. Two alternative though not mutually exclusive hypotheses may explai n this seasonal pattern of change in litter size. On the one hand, the prod uction of few large young late in the season may be an adaptation to time l imitations on the offspring, that have to complete growth and fattening pri or to hibernation. Then one would expect a trade-off between offspring numb er and size as the breeding season progresses. At its extreme, this hypothe sis would predict that total maternal effort should be equal independent of litter size. Alternatively, litter size may be determined by physiological limitations on the mother, in that highly constrained mothers breed later and produce smaller litters. Then one would expect reduced overall maternal effort in highly constrained mothers of smaller litters. In this case, a t rade-off between litter size and offspring size would not be expected. We f ound that total maternal effort in terms of gestation length and the durati on of lactation increased with increasing litter size, thus supporting the second hypothesis. Lactation was not terminated at natal emergence. It exte nded a relatively long period of time beyond the time of first litter emerg ence depending on litter size. During prolonged lactation, individual young of large litters made up body mass to young of small litters. As a consequ ence, juvenile weaning body mass was unaffected by litter size although off spring body mass at natal emergence was inversely related to litter size. T his additional weight gain in young of large litters compensated for initia l survival disadvantages and presumably affected fecundity at yearling age.