Reproductive effort and costs of reproduction in female European ground squirrels

Reproductive effort, factors affecting reproductive output and costs of rep roduction were studied in primiparous yearling compared to multiparous olde r female European ground squirrels (Spermophilus citellus). Yearling female s weaned smaller litters than older ones. Litter size increased with posthi bernation body mass at the expense of slightly lighter young for yearling b ut not for older mothers. In older females, on the other hand, emergence bo dy mass influenced offspring mass, whereas litter size was affected by oest rus date. High reproductive effort entailed reproductive costs in terms of reduced subsequent fecundity but not subsequent survival for both yearling and older females. The production of large litters and long duration of lac tation delayed subsequent oestrus, which, iii turn, correlated negatively w ith litter size. During the second half of lactation, oestradiol levels wer e significantly elevated, indicating the initiation of follicular maturatio n processes. Oestradiol levels during that time correlated negatively with current, but positively with subsequent litter size. We therefore assume th at inhibitory effects of lactation on gonadal development may mediate the n egative relationship between reproductive effort and subsequent reproductiv e timing in adults. This effect is absent in yearlings because they are rep roducing for the first time. Reproductive output in yearlings was influence d by interactions between structural growth and puberty.