There are two species of deer in Yellowstone, the mule deer and the somewhat rare white-tailed deer. There are various ways to tell them apart. One way is by the size of the ears. Mule deer have, as you might guess, mule-like or big ears. When white-tails run, at least when fleeing, they raise their tails and wave them like a white flag. A mule deer has a black tip on its tail and it doesn’t raise and wave it when it runs. A mule deer has an unusual gait in that it bounds with all four feet off the ground at the same time. It is believed they evolved this way to make it easier to move quickly through sage and other large shrubs. You can also tell the bucks apart by the way the antlers divide (or don’t divide in the case of the white-tail). Mule deer give birth much later than the other ungulates in the park, including white-tails.