When is a donkey an ass?

It often gets a giggle when a donkey's called an "ass" in the United States. (Not so much in Britain, where a slightly different word is used to refer to the human behind).

I was just asked which word was correct, "donkey" or "ass." (I actually know somebody who has a shirt with mules on it that reads "I'm surrounded by half-asses").

Here's the situation.

The word donkey only refers to the domestic donkey, Equus africanus asinus.

All other "Asinus" are asses. The Asiatic wild ass is sometimes called the Onager. The Tibetan wild ass is more correctly called the Kiang. But they are all asses.

In other words, "donkey" is a specific species term. Ass refers to an entire group of species. Unlike in horses, where only one wild species remains, there are a whole bunch of wild asses.

And don't laugh too hard, or they might kick you.

A Turkmenian kulan, a kind of wild ass, at the Tierpark Pforzheim. Source: Hans-Peter Scholz via Wikimedia Commons.