The feeding habits of mammals haven't always been what they are today, particularly for omnivores, finds a new study. Some groups of mammals almost exclusively eat meat - take lions and tigers and other big cats, for example. Other mammals such as deer, cows and antelope are predominantly plant-eaters, living on a diet of leaves, shoots, fruits and bark.

But particularly for omnivores that live on plant foods in addition to meat, the situation wasn't always that way, finds a new study by researchers working at the National Evolutionary Synthesis Center in Durham, North Carolina.

Past studies have shown that animals with similar diets tend to share certain characteristics. But this study is the first of its kind to look across all mammal groups, including omnivores, to reconstruct how mammal diets have changed over evolutionary time.

To do that, the researchers compiled previously published diet data for more than 1500 species representing more than one third of mammals alive today, including primates, ungulates, bats, rabbits and rodents.

By mapping that data onto the mammal family tree, the researchers were able to trace backwards in time and infer what the ancestors of each species most likely ate.

They found that while some groups of mammals maintained steady diets, others changed their feeding strategies over time.

Today's omnivores in particular - a group that includes primates, bears, dogs and foxes - came from ancestors that primarily ate plants, or animals, but not both, said co-author Samantha Price of the University of California Davis.

While omnivorous mammals weren't always that way, plant-eaters and meat-eaters have diversified within a more well-worn path. Radical shifts were unlikely for these animals. Mammals that eat meat for a living, for example, never gave up their taste for flesh without transitioning through an omnivorous stage first.

"Direct transitions from carnivory to herbivory were essentially nonexistent," said co-author Louise Roth of Duke University. "It's an intuitive result because it takes very different kinds of equipment to have those kinds of diets," she added.

"Plant- and animal-based foods require different digestive chemistries and different processing mechanisms in the mouth and stomach," explained co-author Samantha Hopkins of the University of Oregon. The kinds of teeth adapted for tearing and slicing meat, for example, are remarkably different from the large, flat-topped molars adapted for grinding nuts and roots.

"[Given these differences] it makes sense that you couldn't easily transition from one to the other in one step," Price added.

The researchers found that diet is also linked to how fast mammals spawn new species. As new species arise and others go extinct, on balance the plant-eaters proliferate faster than their meat-eating counterparts, with omnivores lagging behind both groups.

"If there was an evolutionary race to evolve 100 species, it would take three times longer for omnivores compared to herbivores, and carnivores would be in the middle," Price said.

The study will appear in the online early edition of Proceedings of the National Academy of Sciences the week of April 16, 2012. Biologist Kathleen Smith of Duke University was also an author on this study. Price, S., S. Hopkins, et al. (2012). "Tempo of trophic evolution and its impact on mammalian diversification." Proceedings of the National Academy of Sciences. Data deposited in the Dryad Digital Repository can be found here.