Like other animals, wild vervets regularly face situations in which efficient communication and representation would help them to survive. About three-quarters of wild vervet deaths are caused by predators. If you are a vervet, it is essential to know the differences between a martial eagle, one of the leading killers of vervets, and a white-backed vulture, an equally large soaring bird that eats carrion and is no danger to live monkeys. It is vital to act appropriately when the eagle appears, and to tell your relatives. If you fail to recognize the eagle, you die; if you fail to tell your relatives, they die, carrying your genes with them; and if you think it is an eagle when it is really just a vulture, you are wasting time on defensive measures while other monkeys are safely out there gathering food.
Besides these problems posed by predators, vervets have complex social relationships with each other. They live in groups and compete for territory with other groups. Hence it is also essential to know the difference between a monkey intruding from another group, an unrelated member of your own group likely to push you, and a close relative in your own group on whose support you can count. Vervets that get into trouble need ways of telling their relatives that they, and not some other donkey, are in trouble. They also need to know and communicate about sources of food: for instance, which of the thousand plant and animal species in the environment are good to eat, which are poisonous, and here and when the edible ones are likely to be found. For all these reasons, vervets would profit from efficient ways of communicating about and representing their world. Despite these reasons, and despite the long and close association between vervets and humans, we had no appreciation of their complex world knowledge and vocalizations until the mid-1960s. Since then, observations of vervet behaviour have revealed that they make finely graded discriminations among types of predators, and among each other. They adopt quite different defensive measures when threatened by leopards, eagles, and snakes. They respond differently to dominant and subordinate members of their own troop, differently again to dominant and subordinate members of rival troops, differently to members of different rival troops, and differently to their mother, maternal grandmother, sibling, and unrelated members of their own troop. They know who is related to whom: if an infant monkey calls, its mother turns towards it, but other vervet mothers turn instead towards that infant's mother to see what she will do. It is as if vervets had names for several predator species and several dozen individual monkeys.
The first clue to how vervets communicate this information came from observations that the biologist Thomas Struhsaker made on vervets in Kenya's Amboseli National Park. He noted that three types of predator triggered different defensive measures by vervets, and also triggered alarm calls sufficiently distinct that Struhsaker could hear the differences even without making any sophisticated electronic analysis. When vervets encounter a leopard or any other species of large wild cat, male monkeys give a loud series of barks, females give a high-pitched chirp, and all monkeys within earshot may run up a tree. The sight of a martial or crowned eagle soaring overhead causes vervets to give a short cough of two syllables, whereupon listening monkeys look up into the air or run into a bush. A monkey who spots a python or other dangerous snake gives a 'chuttering' call, and that stimulates other vervets in the vicinity to stand erect on their hind legs and look down (to see where the snake is). Beginning in 1977, a husband-and-wife team named Robert Seyfarth and Dorothy Cheney proved that these calls really had the different functions suggested by Struhsaker's observations. Their experimental procedure was as follows. Firstly, they made a tape-recording of a monkey jiving a call whose apparent function Struhsaker had observed (say, the'leopard call'). Then, on a later day, after locating the same troop of monkeys, either Cheney or Seyfarth hid the tape and loudspeaker equipment in a bush nearby, while the other started filming the monkeys with a cine or video camera. After fifteen seconds, one of the two scientists broadcast the tape while the other kept filming the monkeys for one minute to see whether the monkeys behaved appropriately for the call's suspected function (for example, whether the monkeys ran up a tree on hearing a broadcast of the supposed 'leopard' call). It turned out that playback of the 'leopard call' really did stimulate the monkey to run up a tree, while the 'eagle call' and 'snake call' similarly stimulated monkeys into behaviour that seemed to be associated with these calls under natural conditions. Thus, the apparent association between the observed behaviour and the calls was not coincidental, and the calls did have the functions suggested by observation. The three calls that I have mentioned by no means exhaust a vervet's vocabulary. Besides those loud and frequently given alarm calls, there appear to be at least three fainter alarms that are given less frequently. One, triggered by baboons, causes listening vervets to become more alert. A second, given in response to mammals like jackals and hyenas that prey on vervets only infrequently, causes the monkeys to watch the animal and perhaps move slowly towards a tree. The third faint alarm call is a response to unfamiliar humans and results in the vervets quietly moving towards a bush or the top of a tree. However, the postulated functions of these three fainter alarm calls remain unproven because they have not yet been tested by playback experiments.
Vervets also utter grunt-like calls when interacting with each other. Even to scientists who have spent years listening to vervets, all these social grunts sound the same. When the grunts are recorded and displayed as a frequency spectrum on the screen of a sound-analysing instrument, they look the same. Only when the spectra were measured in elaborate detail could Cheney and Seyfarth detect (sometimes but not always!) average differences between the grunts given in four social contexts: when a monkey approaches a dominant monkey, when it approaches a subordinate monkey, when it watches another monkey, or when it sees a rival troop. Broadcasts of grunts recorded in these four different contexts caused monkeys to behave in subtly different ways. For example, they looked towards the loudspeaker if the grunt had originally been recorded in the 'approach dominant monkey' context, while they looked in the direction towards which the call was being broadcast if it had originally been recorded in the 'see rival troop' context. Further observations of the monkeys under natural conditions showed that the natural calls had also been eliciting this subtly different behaviour.
Vervets are much more finely attuned than we are to their calls. Merely listening to and watching vervets, without recording and playing back their calls, gave no hint that they had at least four distinct grunts—and may have many more. As Seyfarth writes, 'Watching vervets grunt to £ach other is really very much like watching humans engaged in conversation without being able to hear what they're saying. There aren't any obvious reactions or replies to grunts, so the whole system seems very mysterious—mysterious, that is, until you start doing playbacks. These discoveries illustrate how easy it is to underestimate the size of an animal's vocal repertoire.
The vervets of Amboseli have at least ten putative 'words': their words for 'leopard', 'eagle', 'snake', 'baboon', 'other predatory mammal', 'unfamiliar human', 'dominant monkey', 'subordinate monkey', 'watch other monkey', and 'see rival troop'. However, virtually every claim of animal behaviour suggesting elements of human language is greeted with scepticism by many scientists, who are convinced of the linguistic gulf separating us from animals. Such sceptics consider it simpler to assume that humans are unique, and that the burden of proof should be borne by anyone who thinks otherwise. Any claim of language-like elements for animals is considered a more complicated hypothesis, to be dismissed as unnecessary in the absence of positive proof. Yet the alternative hypotheses by which the sceptics instead attempt to explain animal behaviour sometimes strike me as more complicated than the simple, and often plausible, explanation that humans are not unique.