Captive roe deer (Capreolus capreolus) select for low amounts of tannic acid but not quebracho: fluctuation of preferences and potential benefits

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Abstract

Browsing ruminants have been shown to tolerate a certain amount of tannins in their natural diet, and preference trials with captive roe deer (Capreolus capreolus) have suggested an active selection for a low dose of hydrolysable tannins. In this study, we investigated the preference patterns for tannic acid, a source of hydrolysable tannins, and quebracho, a source of condensed tannins, in a series of preference trials with captive roe deer over time, using a pelleted feed that differed only in the respective tannin content. Additionally, two groups of four hand-raised roe deer fawns were fed either a control or a 3% tannic-acid containing diet and physiological parameters were compared after 7.5 months. There were large differences in preference patterns between the individual roe deer groups; quebracho was mostly avoided, whereas tannic acid was actively included in the diet in differing, low proportions. However, one group consistently preferred the quebracho diet over both the control or the tannic acid diet. For the tannic acid, the preference pattern often revealed an initial period of high preference, followed by a stable period of a moderate preference. The fawns on the tannic acid diet had a lower pellet intake and a higher relative mass gain than the fawns on the control diet; differences in salivary tannin-binding capacity and in blood antioxidant status were below significance. These results are the first indications of potential benefits of a low-dose tannin diet, which need further confirmation. The results of the preference trials demonstrate that the time pattern of tannin intake is not constant, and pose the question about the validity of short-term preference trials in general.

Introduction

Ecological (Cooper and Owen-Smith, 1985, Cooper et al., 1988) and experimental work on tannins (Distel and Provenza, 1991, McArthur et al., 1993) showed that ruminants generally seem to select against tannins in their natural forages. However, Cooper and Owen-Smith (1985) found a ‘threshold’ of 5% condensed tannin content below which they could not detect any influence of tannin concentration on feeding preference. For duiker (Cephalophus) species, an avoidance of tannins could not be demonstrated in trials with natural feedstuffs (Faurie and Perrin, 1993, Müller et al., 1998). Additionally, Tixier et al. (1997) found that free-ranging roe deer (Capreolus capreolus) selected a diet of higher tannin content than the average tannin content of all available forage in the habitat. After a detailed analysis they concluded that this could not simply be a side effect of a selection for plants with high soluble carbohydrate contents, which also had high tannin levels. Experimental work by McArthur et al. (1993) supported the idea that browsing ruminants (adapted to dietary tannins) will tolerate a certain tannin load if that is the cost of ingesting otherwise higher quality forage. Finally, Verheyden-Tixier et al., 1999, Verheyden-Tixier and Duncan, 2000 performed preference trials with captive roe deer groups with identical pelleted diets that differed only in their tannin content, which indicated that the animals actively included a certain amount of tannins in their daily ration.

Historically, the negative effects of tannins on ruminants have been emphasized (Kumar and Singh, 1984). However, positive effects of tannins have been investigated more recently and have been reviewed for wild animals (Clauss, 2003) and comprise anthelmintic, antibacterial and antioxidative effects as well as a protection of valuable nutrients from forestomach fermentation; these postive effects could account for a potential selection for low doses of tannins. So far, potentially positive effects have not been demonstrated in wild ruminant species.

We adopted the trial design of Verheyden-Tixier et al. (1999), using two different tannin sources and increasing the time period of the individual preference trials (which had, in their study, lasted for 5–8 days with one exception of 15 days), as we wanted to test whether preference or avoidance patterns varied with time. Additionally, we wanted to compare the effect of mid-term tannin feeding on measurable physiological parameters of roe deer fawns by feeding them either a control or tannin—containing pelleted diet for 7.5 months.

Section snippets

Basal and experimental diets

A basic pelleted diet was formulated that met the nutritional requirements of captive roe deer and could be used as a sole feed. This diet consisted of lucerne meal (39.8% of original ingredient mass), crushed oats (39.1%), crushed wheat (7.6%), bran (9.9%), soy oil (0.6%), linseed oil (0.6%), a calcium-phosphorus-premix (1.2%) and a vitamin/mineral-premix (1.2%). Prior to the feeding experiments, this diet was fed for several weeks to the experimental animals. The tannin-containing diets were

Preference trials

The time period for which a preference trial was conducted had distinctive effects on the outcome of the trial. For example, regarding the choices of ‘Adult2’ with respect to tannic acid (Fig. 1), after five days this group ingested a pelleted diet with approximately 3.5% tannic acid. During the next six days, they chose higher tannin concentrations of 4–4.5%. In the following 10 days, however, there was a decrease in tannic acid preference and a stable choice pattern at a level of 3.0% tannic

Discussion

According to Hagerman et al. (1992), results derived from experimental feeding of tannic acid or quebracho are not necessarily the same as those that would be achieved, if natural tannin sources were used (which are, however, generally not available). Even if these two commercially available tannin sources are included in feeds in identical amounts, as in our trial, differences in the amount of biochemically active tannin substances will occur between the two preparations: whereas the tannic

Acknowledgements

We thank P. Baumann for his support in captive roe deer husbandry and management, K. Männer for his support in producing the pelleted roe deer diet, and the Zoological Garden of Rostock for the support of this study.

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