Is the reproductive capacity impeded in European brown hares? The importance of and difficulties in discriminating between physiological, ecological and anthropogenic factors of influence

The continuous decline in populations such as the European brown hare is al ways a danger sign. A reduced reproductive capacity might be one of the rea sons for this decline. However, it is very difficult to prove, because fecu ndity is determined by many factors. Prerequisites for evidence of changes are exact basic data on reproductive anatomy and physiology, seasonal breed ing periods, and actual anthropogenic influences such as environmental toxi cants. Male brown hares show high testicular activity from January to July, a nadir in September and the reactivation in November/December. Ignoring t hese seasonal changes led to false conclusions. The gonads are organs sensi tive to many toxicants, because of the specific processes of meiosis and ra pid mitosis. However, unequivocal information about the causality of action s is only possible under defined experimental conditions in laboratory anim als or in in vitro fertilisation systems. Results from one species are not necessary valid for others nor for free-ranging populations. Recently, the lack of clear causal relationships led to controversial discussions about t he action of chemicals such as endocrine disrupters. Evaluation of actual r isks by toxicant pollution requires the study of the accumulation of these chemicals in free living animals. Most available data came from carcasses. In future, progress requires new, advanced approaches. One of them is the d iagnosis of the reproductive status in living, anesthesized animals from de fined habitats by minimal-invasive methods such as ultrasonographical, endo crinological and spermatological analyses.