Are apes monkeys?

Martin Robbins wrote a fun piece in The Lay Scientist the other day on the incorrect use of the term monkeys to describe apes (triggered by an article by Graham Smith for the Daily Mail). In Martin’s words:

A great crime against pedantry is in progress and it’s time for someone to draw a line

So as a pedant with a professional interest in this issue I am taking my stand, to help ensure that a miscarriage of pedantic justice doesn’t occur.

Nested hierarchies

Apes are monkeys in the same way that monkeys are primates, humans are apes and I am a human – it’s called a nested hierarchy.

This means that all apes are monkeys, but not all monkeys are apes. Just as all humans are apes, but not all apes are human. By the same token, humans are all ape, contrary to the title of the otherwise rather good book 99% Ape: How Evolution Adds Up.

This nested hierarchical system is the mainstay of biological taxonomy – each individual fits in its species (with the possible exception of hybrids), each species in its genus, each genus in its family and so on. It’s how Linnaeus organised things back in the 1750’s and it works remarkably well. [EDIT it’s actually a bit more complex than that]

Why do nested hierarchies work in biology?

Linnaeus’ system works well in biology because species share varying degrees of similarity depending on when they branched off from a common ancestor. The groups with most shared characteristics can be clumped together. That’s a bit tricky to explain in a few words, so here’s a simple diagram:

The groups that form after a branching event are called ‘clades’ and the members of the group can correctly be referred to by the name of any of the clades that they are part of. This is known as monophyly (which means one leaf).

If a name is given to a group of species that are not all related in this way it will either be a polyphyletic group (many leaves) or a paraphyletic group (excluding leaves). Again, this can be confusing to describe, so here’s another diagram:

Polyphyletic and paraphyletic groups are not particularly scientifically informative, since they include or exclude members of clades with no evolutionary justification. This means that scientists prefer to base names for groups on clear monophyletic clades.

That’s a scientific argument for considering apes to be monkeys.

But ‘Monkey’ isn’t a scientific term

Aha! I must make it clear that I have been using the term monkey as a direct match for the term Simiiformes (which includes the Old and New World monkeys, the lesser apes and the great apes) in the discussion above, but is that valid?

Obviously I argue that it is, since I’ve been using it – the question should be why wouldn’t it be valid? The only logically robust answer (that I can think of) is that it wouldn’t be valid if people don’t commonly use the two terms to refer to the same things. So do people use the term monkey to refer to the apes?

The simple fact is that this whole discussion has been raised because Graham Smith published an article in which he refer to apes as monkeys, which demonstrates that monkey is indeed used as a generic term to refer to apes.

Another example is Terry Pratchett’s Discworld, where the Librarian (who happens to be an Orang-utan) dislikes being referred to as a monkey, which happens regularly – it may be fiction, but it seems reasonable to suggest that it reflects the use of terminology of real people.

In addition, I would suggest that it’s helpful for common terms for biological groups to directly reflect the scientific terminology as much as possible, since this improves the ability of scientists to communicate with a non-specialist audience. So even if monkey didn’t mean the same as Simiiformes (which it seems to) it probably should. Otherwise monkey has no scientifically meaningful analogue, since it would refer to a paraphyletic (and therefore arbitrary) grouping.

Case closed?

I think that my argument is pretty robust and I’ll stand by it until I hear something convincing enough to change my mind.

That said, I will add an important caveat – monkey is a generic term and when referring to Chimpanzees, Gorillas, Gibbons, Orang-utans and Humans the more specific term of ape should be used for clarity. After all, you don’t refer to your pet cat as a pet carnivore, or a budgie as a theropod, because generic terms omit a lot of additional information.

So although apes are monkeys, they are still apes – and that means something.

[N.B. this discourse continues in more detail here]