Investigations of reproductive fitness in living female European brown hares from different habitats

The reproductive fitness of 114 live wild female brown hares from 15 differ ent areas in Northrhine-Westphalia was assessed under general anesthesia. T he methods applied included the ultrasonographic imaging of all genital org ans and pregnancies as well as the measurement of the levels of sexual horm ones. The study period included the main breeding season (April 1998 and 19 99) and the period of reduced sexual activity (July 1999). The results sugg est a seasonal influence, but there were no indications of any pathological changes of the gonads or other reproductive organs. Neither the pregnancy rate of 69.8 % nor the health status and the number of fetuses (n = 2.02 +/ - 0.1) indicated any impairment of reproductive fitness. The proportion of lactating hares (19,8 %) in the group of non-pregnant animals indicated the participation in reproduction; the group of non-pregnant/non-lactating fem ales showed normal sexual cycles. Due to the variability of fecundity among the individuals no significant differences of reproduction rate between ha bitats with high population density or low population density could be show n. This particular issue should be verified in further studies including mo re animals. In conclusion, the results suggest that the decline in the Euro pean brown hare population is caused by a reduced survival rate rather than by a disturbed fertility.