Effects of birch phenolics on a grazing and a browsing mammal: A comparison of hares

Mountain hares (Lepus timidus L.) commonly have high proportions of birch (Betula spp.) in their winter diets, whereas European hares (Lepus europaeus) do not. The effects of a birch extract added to laboratory diets offered to mountain hares and European hares on the digestibility and sodium balance were measured. The extract added contained total phenolics equivalent in amount to that occurring in diets containing 0, 40, 60 and 80% dry matter of birch twigs. Increasing dietary phenolic concentration led to reduced voluntary food intake and apparent protein digestibility in both hare species. No effects on apparent dry matter digestibility were observed. The highest concentration of birch extract caused severe sodium losses via the urine by European hares but not by mountain hares. The effects of the birch extract within the digestive system appear to be similar in the two hare species, whereas the mountain hare appears to be better adapted to the toxic effects that disrupted sodium balance in the European hare. These preliminary results suggest that detoxification rather than digestive abilityper se may contribute to different mammalian herbivores' feeding strategies.