What is the difference between monkeys and apes?

January 20, 2015, 1 p.m.

How do you know if a primate is a monkey or an ape?

"Quit monkeying around."

"She's just aping his movements."

"He made a monkey out of me."

There are so many ways in which monkeys and apes have made their way into our language that when it comes to choosing the right term to describe a certain primate, it's no wonder we often select the wrong term, using "ape" when we're pointing at a monkey or "monkey" when we're pointing at an ape. But really, apes and monkeys live on different branches of the evolutionary tree. So how exactly do you know which you're looking at?

The most basic way to tell the difference is to see whether or not the animal has a tail. Most monkeys have tails, while no apes have tails. But this isn't a foolproof method, since most but not all monkeys have tails. For instance there are a couple species of macaque that do not have tails, but macaques are monkeys. So to know for sure, there are other features to look for. Apes are more reliant on vision than smell and their noses are shorter and broader than monkeys. They also tend to be larger (and in the case of gorillas, much larger!) than monkeys. Though gibbons, one of the lesser apes, can be smaller than some monkey species.

How Stuff Works gets even more specific: "Monkeys are much more like other mammals than apes and humans are. For example, most monkeys cannot swing from branch to branch, as apes and humans can, because their shoulder bones have a different structure. Instead, monkeys run along the tops of branches. Their skeletal structure is similar to a cat, dog or other four-footed animal, and they move in the same sort of way."

There are far fewer species of ape than monkey so if you learn your ape species, it's much easier to sort out what is not an ape and therefore is a monkey. So, if it is not a gorilla, chimpanzee, orangutan, bonobo, gibbon, siamang or human, it's a monkey. Easy as that!

So, is the animal in our Photo of the Day a monkey or an ape? (Answer: it is a Bengal Hanuman langur, a species of Old World monkey.)

Related content on MNN: