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.

Those findings suggest yawning may have a role in promoting changes in our state of alertness or activity patterns.

"We often yawn frequently before we go to sleep or after we wake up," Gallup said. "Yawns have been shown to increase arousal and alertness as associated with these state changes."

Research also suggests yawning has a brain-cooling function.

"One of the mechanisms that could facilitate changes in state or heightened arousal as a result of yawning is brain cooling," Gallup said.

Brain cooling refers to cooling the temperature of the brain, he said, and this can help us feel calmer, "because stress and anxiety increase brain temperature and also trigger yawning."

Gallup also said he isn't insulted when someone yawns, because it's both contagious and a common, natural mechanism. 

"It helps promote mental awareness and alertness. So when individuals yawn in an academic setting or a boardroom, it could be an indicator that they're actually trying to pay attention," he said.