Tannins in the nutrition of wild animals: a review

摘要

Many free-ranging wild animals consume significant amounts of tannins and other polyphenolics. Historically, attention has focused on their negative effects: Tannins reduce apparent digestibility, impair the use of absorbed nutrients, can be toxic and reduce the palatability of many forages. Thus, tannins act as feeding deterrants. However, recently the antioxidant and cardioprotective potential of tannins/polyphenolics has been emphasized in human nutrition.\udWild animals in captivity are fed a diet relatively low in tannins. It has been hypothesized that this might lead to a “lacking” in species that have evolutionarily adapted to and potentially become dependent on tannins. Potentially positive effects of tannins are a reduction in ruminal protein degradation with an increased flow of valuable amino acids into the lower gut, the prevention of bloat, a reduction of gastrointestinal parasites and of pathogenic bacteria, and the chelation of iron. With respect to iron chelation, it could be hypothesized that some species have evolutionarily adapted to a natural diet with low iron availability due to its tannin content, and therefore cannot restrict iron absorption on captive diets high in available iron.\udSeveral observations from zoos can be compared to these effects. Especially, but not exclusively during the transition from the wild into captivity, many folivorous animals suffer from gastrointestinal disorders such as bloat, enteritis and helmithic infestations. Long-lived species whose natural diet contains tannins can suffer from cardiovascular disease in captivity. Additionally, some folivorous, hindgut-fermenting species develop excessive iron storage on captive diets. It is tempting to suggest the lack of tannins as an underlying factor.\udThe potentially beneficial effects of a more natural dietary tannin supply should be investigated, especially with respect to the antioxidant properties of polyphenolics in the face of iron storage disease. However, studies on the possibility to prevent the mentioned conditions by other dietary means, most notably a reduction in overall concentrate intake, should also be performed.