Are Bison Adopting Sedentary Eating Habits?

Bison that once roamed the continent in search of nutritious grass may be adapting to a wider diet to suit a more sedentary lifestyle.

“If you’re going to be keeping bison either for conservation or commercial use it’s nice to know they can feed on other stuff besides grasses or forbs,” said Gaddy Bergmann, a PhD student at the University of Colorado-Boulder and lead author of a study recently published in PLOS ONE.

Bergmann and his coauthors collected bison scat samples from April to the end of October in 2011 at the Konza Prairie Biological Station, a field research reserve operated by Kansas State University and the Nature Conservancy in the Flint Hills of northeastern Kansas.

They found that the diet of the bison (Bison bison) there changed during the seasons. The herbivores grazed mostly on grasses and forbs in the summer, when these plants are most nutritious due to their fresh shoots. But come fall, these plants become overripe and bison browse more on shrubs and more tree-like vegetation.

“They were browsing more in fall and spring and grazing more in the summer,” Bergmann said. “We don’t have data on what their diet was like 100 years ago when they were still migrating and there were millions of them,” he continued, but adds that they have likely changed their diet since then.

The researchers also found there was a shift in the makeup of their gut microbes over the seasons, potentially as the bison responded to a change in diet. When compared to grain-fed bison, the gut microbe content changed significantly, staying the same over the year to match the consistent diet.

Bergmann said that the wide mouths of bison are better suited to grazing than those of species like giraffes (Giraffa camelopardalis), whose mouths are adapted to picking leaves off trees.

But he said this may be good news for bison ranchers as it means they may not always need to supplement their grassy diets in the fall and spring with hay.

“The bison numbered about 30 million up until the 1880s when they were almost exterminated,” he said. Now, bison ranges are fragmented by fences, roads and other, and the animals have to make some shifts in diet as a result.