Boom-boom-krak-oo ā Campbellās monkeys combine just six āwordsā into rich vocabulary
BYED YONG
PUBLISHED DECEMBER 7, 2009
⢠7 MIN READ
Many human languages achieve great diversity by combining basic words into compound ones ā German is a classic example of this. Weāre not the only species that does this. Campbellās 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. Itās a front-runner for the most complex example of animal āproto-grammarā so far discovered.
Many studies have shown that the chirps and shrieks of monkeys are rich in information, ever since
Dorothy Cheney and Robert Seyfarthās seminal research on vervet monkeys. They showed that vervets have specific calls for different predators ā eagles, leopards and snakes ā and theyāll take specific evasive manoeuvres when they hear each alarm.
Campbellās monkeys have been equally well-studied. Scientists used to think that they made two basic calls ā booms and hacks ā 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 Campbellās 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 Coastās Tai National Park studying the wild Campbellās monkeys from six different groups. Each consists of a single adult male together with several females and youngsters. And itās the males he focused on.
With no danger in sight, males make three call sequences. The first ā a pair of booms ā is made when the monkey is far away from the group and canāt see them. Itās 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 āCome hereā, the signal now means āWatch out for that branchā. Whenever the males cried āBoom-boom-krak-ooā, 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 āPrepare for battleā, and itās 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 monkeysā 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 ā 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 donāt have a visual sighting (usually because theyāve heard a suspicious growl or an alarm from other monkeys), they make a few krak-oos.
If they know itās 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 ā 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, itās the proportion of kraks that signals the imminence of danger ā the males donāt make any if theyāve 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 ā 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).
I
tās tempting to think that monkeys have hidden linguistic depths to rival those of humans but as Ouattara says, āThis system pales in contrast to the communicative power of grammar.ā They monkeysā repertoire may be rich, but itās still relatively limited and they donāt 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 Campbellās monkeys ātalkā 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 Campbellās 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 havenāt even begun to peer into yet. For instance, what calls do female Campbellās 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 Austenās oeuvre and concluding, āIt appears that these sentences signify the presence of posh peopleā.
Reference: PNAS doi:10.1073/pnas.0908118106