On the southern Japanese island of Yakushima, a population of macaques lives alongside a population of sika deer. And they have a unique relationship.

That’s a macaque riding on the back of a deer. Why it happens, no one is quite sure. But it only happens on Yakushima, and they seem to have a somewhat symbiotic relationship; the macaques will groom the deer a bit, and when they go into the trees to forage, the deer will follow and pick up food the macaques drop to the forest floor, food they otherwise wouldn’t have access to.

But the deer seem to have their limits.