A northern Colorado bison herd's growth rate is unexpectedly high.

In fact, the herd is outgrowing the natural area they reside in.

The area only provides enough space for 100 bison, which the heard is expecting to reach soon.

The Fort Collins Coloradoan reports that 3 years ago the herd only consisted of 10 animals, but now the number has raised to 76. This includes the dozen calves that were also born this year alone.

The managers of the herd just sent two of the bison bulls to an Oakland, California zoo.