Saliva of the graminivorous Theropithecus gelada lacks proline-rich proteins and tannin-binding capacity

Gelada baboons are the sole survivors of the genus Theropithecus and the only known graminivorous primates. They developed special adaptations to their diet such as high-crowned teeth for processing hard and abrasive feed. The fine-tuning of salivary protein composition might be another key mechanism that is used by species for adapting to the environment and competing with rivals for exploiting new ecological niches. In order to test whether gelada (graminivorous) and hamadryas baboons (omnivorous) differ in their salivary protein composition, we compared whole saliva samples of captive Theropithecus gelada and Papio hamadryas using gel electrophoresis and tannin-binding assay. We hypothesized that the amount of proline-rich salivary proteins with tannin-binding capacity is higher in baboons consuming a feed with high dicot/monocot rations. Dicots produce tannins as a chemical defense system, discouraging animals from eating them. In contrast to dicots, monocots do not synthesize tannins. The presence of tannin-binding proteins in saliva should effectively inactivate the dicot tannin-based defense mechanism and increase the dietary breadth and/or the capability to switch between monocots and dicot leaves. The lack of such tannin-binding proteins in saliva would indicate a narrow dietary spectrum more restricted to monocots. We found T. gelada to completely lack proline-rich proteins (PRPs) and tannin-binding capacity similar to a great variety of other grazing mammals. In contrast, P. hamadryas does possess PRPs with tannin-binding activity. The findings support a growing body of evidence suggesting a high-level specialization of T. gelada to grass diets. However, it remains unclear, whether loss of salivary tannin-binding capacity drove the gelada into its narrow feeding niche, or whether this loss is the result of a long process of increased specialization. Thus, from an ecological point of view, T. gelada appears to be more vulnerable to environmental changes than other baboon species owing to its narrow dietary traits.