Fossil teeth of African animals show that during the past 10 million years, different plant-eating critters began grazing on grass at different times as many switched from a salad-bar diet of tree leaves and shrubs, says a University of Utah study. The first animals to hit the hay — technically warm-season grasses known as C4 plants — were zebras’ ancestors, starting 9.9 million years ago. Next, some but not all rhinos made the switch, beginning 9.6 million years ago. Grass-grazing spread 7.4 million years ago to the ancestors of elephants. Hippos began grazing on grass more slowly. And giraffes, with heads in the trees, never left the salad bar.
The study — by a Utah-led international team of researchers — is being published online in the journal Proceedings of the National Academy of Sciences. It constructed a 7-million-year record of dietary change — from 10 million to 3 million years ago — by analyzing carbon isotope ratios in 452 fossilized teeth from nine animal families living at three sites in Kenya also occupied by ape-like human ancestors.
“This record is the first to illustrate the dietary response among herbivore families to the appearance of warm-season grasses in East Africa” at least 10 million years ago, says the study’s first author, Kevin Uno, a doctoral student in geology at the University of Utah. “Grass is now the main food for many herbivores there.”
He adds: “The results paint a picture of differential dietary response to changes in climate and landscape from 10 million to 3 million years ago, a period that includes the appearance of hominids that eventually gave rise to humans.” The findings “demonstrate that different animals respond differently to ecological change,” says geochemist Thure Cerling, the study’s senior author and a distinguished professor of geology and geophysics, and biology at the University of