FORT COLLINS, Colo. (AP) — A bison herd in northern Colorado is growing much faster than expected.

The herd at a natural area north of Fort Collins began three years ago as 10 genetically pure descendants of bison in Yellowstone National Park.

The Fort Collins Coloradoan reports the herd now numbers 76 animals, including a dozen calves born just this year. The natural area doesn't have enough room for more than 100 bison.

Herd managers expect to reach that number soon.

Colorado State University, the city of Fort Collins and Larimer County hope to raise enough genetically pure, disease-free bison to give or trade to American Indian tribes and conservation organizations.

Herd managers recently sent two bison bulls to a zoo in Oakland, California.