A Blog by Ed Yong

Boom-boom-krak-oo – Campbell’s monkeys combine just six ‘words’ into rich vocabulary

Many human languages achieve great diversity by combining basic words into compound ones – German is a classic example of this. We’re not the only species that does this. Campbell’s monkeys have just six basic types of calls but they have combined them into one of the richest and most sophisticated of animal vocabularies.

By chaining calls together in ways that drastically alter their meaning, they can communicate to each other about other falling trees, rival groups, harmless animals and potential threats. They can signal the presence of an unspecified threat, a leopard or an eagle, and even how imminent the danger is. It’s a front-runner for the most complex example of animal “proto-grammar” so far discovered.

Many studies have shown that the chirps and shrieks of monkeys are rich in information, ever since Dorothy Cheney and Robert Seyfarth’s seminal research on vervet monkeys. They showed that vervets have specific calls for different predators – eagles, leopards and snakes – and they’ll take specific evasive manoeuvres when they hear each alarm.

Campbell’s monkeys have been equally well-studied. Scientists used to think that they made two basic calls – booms and hacks – and that the latter were predator alarms. Others then discovered that the order of the calls matters, so adding a boom before a hack cancels out the predator message. It also turned out that there were five distinct types of hack, including some that were modified with an -oo suffix. So Campbell’s monkeys not only have a wider repertoire of calls than previously thought, but they can also combine them in meaningful ways.

Now, we know that the males make six different types of calls, comically described as boom (B), krak (K), krak-oo (K+), hok (H), hok-oo (H+) and wak-oo (W+). To decipher their meaning,  Karim Ouattara spent 20 months in the Ivory Coast’s Tai National Park studying the wild Campbell’s monkeys from six different groups. Each consists of a single adult male together with several females and youngsters. And it’s the males he focused on.

With no danger in sight, males make three call sequences. The first – a pair of booms – is made when the monkey is far away from the group and can’t see them. It’s a summons that draws the rest of the group towards him. Adding a krak-oo to the end of the boom pair changes its meaning. Rather than “Come here”, the signal now means “Watch out for that branch”. Whenever the males cried “Boom-boom-krak-oo”, other monkeys knew that there were falling trees or branches around (or fighting monkeys overhead that could easily lead to falling vegetation). 

Interspersing the booms and krak-oos with some hok-oos changes the meaning yet again. This call means “Prepare for battle”, and it’s used when rival groups or strange males have showed up. In line with this translation, the hok-oo calls are used far more often towards the edge of the monkeys’ territories than they are in the centre. The most important thing about this is that hok-oo is essentially meaningless. The monkeys never say it in isolation – they only use it to change the meaning of another call.

But the most complex calls are reserved for threats. When males know that danger is afoot but don’t have a visual sighting (usually because they’ve heard a suspicious growl or an alarm from other monkeys), they make a few krak-oos. 

If they know it’s a crowned eagle that endangers the group, they combine krak-oo and wak-oo calls. And if they can actually see the bird, they add hoks and hok-oos into the mix – these extra components tell other monkeys that the peril is real and very urgent.  Leopard alarms were always composed of kraks, and sometimes krak-oos. Here, it’s the proportion of kraks that signals the imminence of danger – the males don’t make any if they’ve just heard leopard noises, but they krak away if they actually see the cat. 

The most important part of these results is the fact that calls are ordered in very specific ways. So boom-boom-krak-oo means a falling branch, but boom-krak-oo-boom means nothing. Some sequences act as units that can be chained together to more complicated ones – just as humans use words, clauses and sentences. They can change meaning by adding meaningless calls onto meaningful ones (BBK+ for falling wood but BBK+H+ for neighbours) or by chaining meaningful sequences together (K+K+ means leopard but W+K+ means eagle).

It’s tempting to think that monkeys have hidden linguistic depths to rival those of humans but as Ouattara says, “This system pales in contrast to the communicative power of grammar.” They monkeys’ repertoire may be rich, but it’s still relatively limited and they don’t take full advantage of their vocabulary. They can create new meanings by chaining calls together, but never by inverting their order (e.g. KB rather than BK).  Our language is also symbolic. I can tell you about monkeys even though none are currently scampering about my living room, but Ouattara only found that Campbell’s monkeys “talk” about things that they actually see.

Nonetheless, you have to start somewhere, and the complexities of human syntax probably have their evolutionary origins in these sorts of call combinations. So far, the vocabulary of Campbell’s monkeys far outstrips those of other species, but this may simply reflect differences in research efforts. Other studies have started to find complex vocabularies in other forest-dwellers like Diana monkeys and putty-nosed monkeys. Ouattara thinks that forest life, with many predators and low visibility, may have provided strong evolutionary pressures for monkeys to develop particularly sophisticated vocal skills.

And there are probably hidden depths to the sequences of monkey calls that we haven’t even begun to peer into yet. For instance, what calls do female Campbell’s monkeys make? Even for the males, the meanings in this study only become apparent after months of intensive field work and detailed statistical analysis. The variations that happen on a call-by-call basis still remain a mystery to us. The effect would be like looking at Jane Austen’s oeuvre and concluding, “It appears that these sentences signify the presence of posh people”.

Reference: PNAS doi:10.1073/pnas.0908118106

More on monkey business (clearly, I need more headline variation):

7 thoughts on “Boom-boom-krak-oo – Campbell’s monkeys combine just six ‘words’ into rich vocabulary

  1. Could we get more reference info please? A DOI search for 10.1073/pnas.0908118106 does not seem to yield results in the PNAS database. Thanks.

  2. Great post! Yes, it makes you think how we are able to learn each others languages (with difficulty, despite being equipped to do so), but knowing what the others are talking about helps. We have little idea of what monkeys might ‘talk about’ other than predators. A Spanish listening to two British guys talking about cricket might take an eternity to learn English…

  3. The calls are not analogous to human language words, but rather to human language syllables. Syllables can be combined into words, but don’t necessarily themselves have a fixed meaning (though it may appear so in some cases, where a word consists of a single syllable, for example). Syllables also are subject to fixed order constraints, like the calls above. But combining syllables (and calls) doesn’t involve syntax: the meaning of the combinations are *not* compositionally derivable from the meaning of the parts. (Just as the meaning of “timber”, consisting of two syllables, “tim” and “ber”, is not a function of the meaning of the name “Tim” [a homophone] and “ber” [which is a well-formed syllable of English, but not an independent word].)
    I’m somewhat hesitant to comment on this without having been able to see the PNAS article itself (my university hasn’t made the issue available yet), but the chart included here looks like it is from the paper itself. The chart gives us no reason to believe that these calls and their combinations involve anything like syntax; rather, they seem to involve phonology. I only wish the authors had consulted with a linguist before trying to draw parallels to human language structures.

  4. @A professor of linguistics: Since the concept of “syllable” and “word” is pretty much that same in some languages (isolating languages), that doesn’t seem like that big a problem. It seems to me that the bigger problem would be the lack of a tree-like structure — the fact that you can’t parse these into nested segments.
    (Then again, I’m not a professor of linguistics.)

  5. The same authors recently publised the work on the element combinations (e.g. the “kak” versus “kak”-“oo”)
    Outtatara et al 2009 Campbell’s monkeys use affixation to alter call meaning. PLoS ONE. Something about both these papers is missing, although I haven’t really been able to pin point it down. For one it is strange that the results of the papers weren’t combined into 1 paper, because the PNAS paper here certainly takes away some of the issues of the PLOS paper. In general playbacks are missing. Now you can always aske the question if it is really only the calls that lead to the differences obsereved. Yes the make different calls in different situations, but I would like to know if playbacks of the various call combinations would lead to differences in behavioral responses without any other simuli that could explan the response

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