American Bison Adjust Diet For Changing Seasons

During the growing season, grasses become less nutritious for grazing animals. North American bison deal with that fact by adjusting their diet during the spring and fall, expanding their pallet to include woody shrubs and flowering plants, say researchers from the University of Colorado Boulder.

"We wanted to know if bison might change what they eat as the season goes on and how that affects the microbial composition of their gut," Gaddy Bergmann, a doctoral candidate in the Department of Ecology and Evolutionary Biology at CU-Boulder, explained in a news release. "This study shows that it may be beneficial for the species to have other plants, bushes, and trees available to browse from."

For their study, fecal samples of roughly 300 bison living in the Konza Prairie Biological Station, located in northern Kansas, were examined for plant types. This revealed that the large animals are able to consume higher quantities of woody shrubs in the early spring and fall when fresh grass, their meal of choice, isn't as readily available.

Using the same group of animals, researchers then examined the bison's lower digestive tract and traced how the abundance of the plant microbes changed over time. That's when it was confirmed that some bacteria – an indication of certain plant types – became more abundant during the growing season as the bison switched to more energy-rich foods in the summer and autumn.

Ultimately, feeding behaviors such as this make bison increasingly susceptible to seasonal changes, significantly impacting bison populations. While an estimated 20 to 30 million bison once roamed throughout North American landscapes, by the end of the 1800s there were only just a few thousand remaining. Other factors that contributed to such a decline include hunting and habitat encroachment.

The study, recently published in the journal PLOS ONE, has implications for future bison management and conservation throughout the American West.

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