Abstract:
While feeding, mammalian browsers (primarily eat woody plants) encounter secondary
metabolites such as tannins. Browsers may bind these tannins using salivary proteins,
whereas mammalian grazers (primarily eat grasses that generally lack tannins)
likely would not. Ruminant browsers rechew their food (ruminate) to increase the
effectiveness of digestion, which may make them more effective at binding tannins
than nonruminants. Few studies have included a sufficient number of species to consider
possible scaling with body mass or phylogenetic effects on salivary proteins.
Controlling for phylogeny, we ran inhibition radial diffusion assays of the saliva of
28 species of African herbivores that varied in size, feeding strategy, and digestive
system. We could not detect the presence of salivary proline-rich proteins that bind
tannins in any of these species. However, using the inhibition radial diffusion assay,
we found considerable abilities to cope with tannins in all species, albeit to varying
degrees. We found no differences between browsers and grazers in the effectiveness
of their salivary proteins to bind to and precipitate tannins, nor between ruminants
and nonruminants, or scaling with body mass. Three species bound all tannins,
but their feeding niches included one browser (gray duiker), one mixed feeder (bush
pig), and one grazer (red hartebeest). Five closely related species of small ruminant
browsers were very effective in binding tannins. Megaherbivores, considered generalists
on account of their large body size, were capable of binding tannins. However the grazing white rhinoceros was almost as effective at binding tannins as the megaherbivore
browsers. We conclude, contrary to earlier predictions, that there were
no differences in the relative salivary tannin-binding capability that was related to
common ancestry (phylogeny) or to differences in body size.
