I was reading the ‘Things people do that irritate you when you go to the zoo?’ thread, and came across that oft-made complaint of guests calling apes ‘monkeys’. To ease some minds and hopefully enlighten a few, I would like to say that apes are indeed monkeys. It is true that, traditionally, the word ‘monkey’ has referred to any of the ‘higher’ or simian primates (formally called the Simiiformes) that was also not part of the apes (formally called the Hominoidea). In other words, a monkey is traditionally a non-hominoid simiiform. However there have been some major objections to this system. Firstly, the term ‘monkey’ has always been informal. Unlike the word ‘mammal’ or ‘passerine’, which were coined and defined by scientists, ‘monkey’ is an informal word which does not have a official, scientific definition. One could argue that, for ease of communication, it would be good idea to assign a scientific meaning to the term. However, even then, one runs into problems. Almost all scientists today classify living things cladistically. This means that they are grouped by their evolutionary ancestry. To use an example from before, a passerine is defined as any descendant of one common ancestor (in this case the first perching bird) - it forms a clade (the word comes from Greek for ‘branch’). Anything not descending from that ancestor cannot be included no matter how similar it is. Nor can any bird that evolved from that ancestor be excluded from the Clare for being to dissimilar. With the word ‘monkey’ we run into problems. Apes evolved directly from monkeys. Cladistically, they are descended from the same common ancestor as monkeys, indeed, old world monkeys are closer to apes evolutionarily than American monkeys. This means that ‘monkey’ in the traditional sense does not equal the formal definition of Simiiformes as a clade. We say that the traditional definition of ‘monkey’ is paraphyletic. Here is a useful inforgraphic by Dr. Darren Naish on the topic: As you can see, apes are just another branch midst a mess of monkey evolutionary branches, not a distinct, outside group. This means one of two things. We could either: 1) we admit ‘monkey’ is an informal term and can be used somewhat variably, or 2) that we consider apes as monkeys such that the whole of clade Simiiformes become ‘monkeys’. The former proposition can become problematic, since people will become unsure what another person considers a ‘monkey’, and communication can become faulty Therefore I suggest what others have suggested: we use the latter suggestion, and start considering apes ‘monkeys’.
Apes are indeed monkeys, but because clades are nested, squirrels are also fish and finches are also dinosaurs, so, depending on the context, we use different terms. To refer to a tailed member of the Primates in the same clade as humans but not as lemurs, we can say "non-hominoid simiiform primate" or just "monkey", depending on whether we're talking to a scientist or a lay-person, or talking about zoo exhibits or evolutionary biology. Same applies to "dinosaurs" vs. "non-avian dinosaurs", to refer to extinct Mesozoic dinosaurs not closely-related to modern birds, or "reptile" vs. "sauropsid." In other words, it's fine to use the term "monkey" to refer to only tailed simiiforms, or to all simiiforms, but you may need to clarify depending on the context.
