Watch: Wolves at Yellowstone National Park bring 'toys' to pups

Scientists at Yellowstone documented adult wolves returning to their dens with "toys" for their pups. 

The moment was captured in a video from the National Park Service provided to the social news platform Storyful. 

Wolves from the Mollie’s Pack are filmed carrying bones to their dens during the spring.

According to the NPS, pups usually wait for food deliveries from successful hunts, but when there’s no food, adult wolves bring "toys." 

"The instinct to bring items back to the den may be reinforced by evolution, and probably helps keep adults from being mobbed by sharp puppy teeth," the NPS said.

Female wolves give birth to an average of five pups in April. The young wolves emerge from the den at 10–14 days while the pack remains at the den for three to 10 weeks, the NPS noted. 

Wolves are highly social animals and live in packs. The NPS explains on its website that the pack size depends on the size and abundance of prey. In Yellowstone, the average pack size is 11.8 individuals.

This story was reported from Washington, D.C.