Salivary tannin-binding proteins: A foraging advantage for goats?

Abstract

Tannins are a key chemical defense that plants use against mammalian herbivores. To cope with tannins, many herbivores have evolved salivary tannin-binding proteins that precipitate tannins in forage items, and thus minimize their deleterious effects. In the past, one specific type of salivary tannin-binding protein was identified (proline-rich proteins), and has been found in a range of mammalian species. However, there are other proteins in the saliva of mammalian herbivores that also have a high tannin-binding affinity, which may allow for tolerance of low-quality food resources. Domestic goats (Capra hircus) are able to survive on low quality, tannin-rich food items. However, it has been reported that they do not have proline-rich proteins in their saliva. Thus, the question that arises is, do they have other tannin-binding proteins in their saliva, or do they rely on other physiological mechanisms (e.g. gut microflora) to negate the negative effects of tannins? We explored whether goats had tannin-binding proteins in the saliva by using a series of laboratory assays. As a starting point, we tested for the presence of proline-rich proteins in goat saliva using two different approaches: (1) sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) using Laemmli's (1970) destaining method, and (2) comparative SDS-PAGE gels using Beeley et al. (1991) method for staining and destaining to probe for proline-rich proteins. We found, as in previous studies, that goats did not have proline-rich proteins, but that they did have other proteins in their saliva that have a high tannin-binding affinity. To test for the tannin-binding affinity of goat saliva, we then performed an inhibition assay based on a modified radial diffusion assay. For the inhibition assay, we mixed goat saliva with three tannic acid solutions as well as extracts from seven common South African tree species. To determine whether goat saliva could significantly reduce the amount of tannins left to bind after interacting with the tannin-rich solutions, we used a Wilcoxon test. Our inhibition assays revealed that goat saliva is able to significantly bind to and precipitate tannic acid and tannins in common South African tree species. The presence of these other as yet unclassified proteins, coupled with other physiological mechanisms, helps explain how goats survive on tannin-rich foods, and why they are so successful in areas dominated by highly defended food items.