Photo: Thomas Netsch
If you know anyone who is into primates, you know not to call an ape a monkey… or is it the other way around..? First, on behalf of all primatologists, we’re sorry. We experience a physiological reaction when you get it wrong, and can’t help but correct you. Personally, it’s all I can do to narrowly avoid barking gruff corrections at children at the zoo. Here, as an apology, I’m going to give you a sound evolutionary argument to rebut that annoying friend the next time they call you out on this. First, let’s review the commonly accepted usage of “monkey” and “ape.”
Monkeys and apes are both types of primates, as are lemurs, lorises, and tarsiers. Monkeys include a wide range of species, which we group into two categories: Old World monkeys like baboons and macaques, and New World monkeys like capuchins, howler monkeys, and marmosets (the names are self-explanatory–OWM’s are native to Africa and Eurasia, while NWM’s hail from the Americas). A good rule of thumb for distinguishing monkeys from apes is that monkeys don’t really get bigger than dogs. A better rule of thumb–the rule, in fact–is that all monkeys have tails. (So do lemurs, lorises, and tarsiers, so again, this rule is only good for distinguishing monkeys vs. apes.) Apes lack tails, are mostly larger than dogs, and are much less diverse: whereas there are dozens of monkey groups, there are only five groups of living apes: gibbons, orangutans, gorillas, chimpanzees and bonobos (they’re very closely related so they count as one), and humans. Yes, we humans are apes.
The categories delineated by the terms “monkey” and “ape” are defined by more than just superficial similarities like size and whether they have a tail; evolutionary relatedness also plays a role. In the evolutionary tree below, apes are in pink, while monkeys are in blue (Old World) and yellow (New World). We apes, as you can see, comprise what’s called a monophyletic group: a group of species that includes all descendants of a common ancestor. In this case, the ancestor is represented by the node labeled by the number 18, indicating that the apes’ common ancestor lived 18 million years ago.
The special thing about a monophyletic group is that all members are more closely related to each other than they are to any species outside of the group. For that reason, biologists prefer taxonomic terms to refer to monophyletic groups–this is called the principle of monophyly. In this case, because we apes are a monophyletic group, chimps, bonobos, gorillas, orangutans, and gibbons are more closely related to us than they are to any monkey. So calling a chimpanzee a “monkey” (I’m looking at you, small child at the zoo) is not only inaccurate, it can also be seen as perpetuating misconceptions and ignorance about how we are all related. If you’re gonna lump any ape in with the monkeys, then you had better be lumping humans in there too; otherwise it’s a double standard.
But that’s no excuse for rudeness. So, here’s what to say to that annoying friend when they get in your face about this. “Friend,” you say, “by your very logic, apes are monkeys!” Then pull out your copy of the primate evolutionary tree (which, I assure you, it is perfectly normal to keep in your pocket) and point out the relationships between apes, Old World monkeys, and New World monkeys.
If you look again at the evolutionary tree above, you’ll see that “apes,” “Old World monkeys,” and “New World monkeys” are all monophyletic groups, but “monkeys” is not. Apes and Old World monkeys split 25 million years ago–after their common ancestor split from the New World monkey lineage 40 million years ago. In other words, a baboon is more closely related to us apes than to any New World monkey. So your annoying friend is calling you out based on the principle of monophyly, but breaking that principle at the same time! A solution is to redefine “monkeys” to include all descendants of the common ancestor of Old World and New World monkeys (so, apes too), hence your quip “apes are monkeys.”
Quick caveat, savvy primatologists will realize that there’s an alternative option: stop using “monkey” to refer to both Old World and New World monkeys. If only one of these two groups is “monkeys,” then “monkeys” becomes monophyletic and all is well. Accordingly, some primatologists now reserve “monkey” for Old World monkeys, and instead talk about “New World primates.” However, as far as snappy rebuttals go, “by your very logic New World monkeys aren’t monkeys!” doesn’t quite have the same effect.
So now, if that explanation made any sense whatsoever, you have the tools to rebut your annoying primate-enthusiast friend. But first, take a moment to understand where they’re coming from. Part of it is monophyly, sure. But part of it, I suspect, is offense taken on behalf of the besmirched ape. When someone ignores the fact that a chimp is evolutionarily more like us than it is like a monkey, we can’t help but see it as a lack of respect. Now, of course the chimp doesn’t understand or care about that lack of respect. But for those who care about primates, especially given the context of humans’ extreme mistreatment of apes, it can touch a nerve. If we’re being rude, by all means, throw down the rebuttal I’ve provided above. Just please don’t take our reaction as mere semantics. It goes far beyond that. …Most of the time. Sometimes, we’re just being trolls.