Yawning isn't just for humans. Here's why fish, birds and other animals yawn too, according to an expert.

Humans aren't alone when it comes to yawning — all vertebrates do it too, including fish, amphibians, reptiles, birds and mammals. But why?

The "evolutionarily ancient" act of yawning likely serves a few purposes, according to Andrew Gallup, a professor of behavioral biology at Johns Hopkins University.

"The fact that it is conserved so widely across the animal kingdom suggests that it likely holds an evolutionary function, and research suggests that it functions in a variety of domains," he said on "CBS Mornings Plus" Friday.