Functional differentiation of African grazing ruminants: an example of specialized adaptations to very small changes in diet

We assess whether interspecific differences in craniodental morphology within a single ruminant feeding guild, the grazers, represent anatomical adaptations to subtle differences in diet. Differences in craniodental anatomy follow a distinct taxonomic pattern that is paralleled by dietary niche differentiation recorded in species' stable carbon (δ 13 C) and nitrogen isotope (δ 15 N) compositions, strongly supporting a hypothesis for functional divergence within the grazers. We propose that the evolutionary origin of grazers were multifold; at least two and up to four different types of grazing can be discerned within the 11 taxa studied here alone. However, correspondence between craniodental adaptations and isotopic differences across species are not found when only δ 13 C data are considered (i.e. morphological differences do not reflect varying proportions of C 3 browse to C 4 grass consumed). This implies that alternate anatomical adaptations to grazing are not related to differences between variable (part‐time browsing) and obligate grazers, as previously predicted. Rather, anatomical differences correlate strongly with changes in δ 15 N, which we infer to reflect functional responses to changes in diet quality associated with the degree of feeding selectivity and short‐, medium‐, or tall‐grass grazing. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 755–764.