Footage of giraffes in the wild often shows a gentle herbivore feeding on leaves hanging from the tops of trees. But a new video shows a giraffe in South Africa feeding on something entirely different—the skull of a buffalo.

While the act might look gruesome, feeding on bones provides giraffes with the calcium and phosphorous they need for their own skeletons. A 2013 study published in the Journal of Archaeological Science found that it's not just bones giraffes feed on. The tall mammal also regularly consumes antlers, horns, and ivory.

The behavior is known as osteophagia and varies by individual. Especially tall giraffes, for instance, may be more prone to feeding on bones than those that are shorter. (See why Africa may have more giraffe species.)

Jarod Hutson, a post-doc researcher from the University of Nevada, has studied the skeletal remains bearing the marks of giraffe teeth.

"There are bones scattered everywhere [in their environments]," said Hutson. "They do resort to eating them."

One past study theorized that giraffes more frequently consume bones as a result of nutritional stress. Seasonal changes that resulted in less foliage likely prompted the behavior, the study concluded.

Giraffes will rarely swallow much of the bones directly. Instead, they chew and suck on them using their saliva to dissolve nutrients, usually dropping the material once they're finished.

"Based on the marks, they're not crunching off big pieces. They sort of scrape their teeth along the bones," Hutson added.

He put forth another theory for why giraffes might be seen with a bone protruding from their lips: "I have a feeling that it also could be due to boredom. They come across bones, sniff around and chew."

It's not just giraffes that supplement their diets with bones. Other herbivores, such as camels and cattle, also frequently scavenge the skeletal remains of animals.