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The European roe deer (Cupreolus cupreolus) is the smallest wild ruminant species in the northern hemisphere. Capreolini are represented in the fossil record of Eurasia by an array of forms extending from the Miocene to the Recent. Interestingly, a map of fossil occurrences shows that the historical range is fairly congruent with the recent distribution of Capreolirs; roe deer have always been temperate-zone animals of Eurasia (Lister et al. 1998). In contrast to many other large herbivores the roe deer population in Europe increased both in terms of distribution and density during the last 100 years. This emphasises the flexible response of the species to rapidly changing environmental conditions and their potential to adapt to new resources in modern agriculture and forestry.This paper reviews aspects of evolution, dietary selection, digestion and adaptation of the European roe deer to the changing environment.In terms of feeding type, roe deer have been classified as a typical concentrate selector (Hofmann 1989), browsing on highly digestible plant parts. However, this feeding strategy can not be sustained during winter when forage is less available and its digestibility is reduced. Body reserves generally play no major role in roe deer (Holand 1992).Roe deer have developed several mechanisms to adapt to seasonal variations in their environment. It is the only ungulate that is strictly monestric with a genetically fixed diapause. Fawns are born in May when the quality of browse is at an optimum, and twins or even triplets are common. It is concluded that the perfect synchrony between seasonal changes of the resources and the demand due to reproductive investment may be the key function for the success of roe deer in temperate and northern climates.

Energy expenditure depends on resource availability (e.g. food, body reserves). Reproduction is a major energetic challenge for mammals living under highly variable seasonal conditions. Lactation represents the most energy demanding time in a female's annual cycle. To understand how important adaptations facilitating lactation are, we studied its energetic costs.Two ruminant species of similar size (about 25 kg body mass) were used in an investigation to learn more about their adaptations to seasonality: The European roe deer (Capreolus cupreolus Linnaeus, 1758) and the European mouflon (Ovis orientalis musimon Schreber, 1782). Energy investment into lactation was estimated using stable isotope techniques with two different approaches. The D20 dilution technique for estimating milk intake described for sheep lambs by Coward et al. (1982) and the doubly labelled water method for measuring energy expenditure (Speakman 1997).Milk intake was estimated by the D20 dilution technique in 2-week-old roe deer fawns and mouflon lambs. Estimated milk intake rates were then transferred into the respective energetic investment into lactation. It was calculated as well from energy expenditure data in growing fawns during their first two weeks post partum using results of the doubly labelled water technique.Results for roe deer showed an energy investment into lactation of 5.6MJ/d metabolizable energy (ME) per fawn. In mouflon this value was approximately 5.7 MJ/d per lamb. Twins are extremely rare in mouflon, they routinely get only one offspring per year. Keeping in mind that roe deer often give birth to twins, or sometimes even triplets we measured more than twice the energy investment into lactation than predicted from theoretical calculations (Lechner-Doll et al. 2000).

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