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
It seems a modest claim to propose that the different calls which vervets give in response to leopards, eagles, and snakes actually refer to these animals or are intended as communications to other monkeys. However, sceptics were disposed to believe that only humans could emit voluntary signals referring to external objects or events. The sceptics proposed that the vervet alarm calls were merely an involuntary expression of the monkey's emotional state ('I'm scared out of my wits! ) or of its intent ('I'm going to run up a tree'). After all, those explanations apply to some of our own 'calls'. If I saw a leopard coming at me, I too might emit a reflex scream even though there was no one around with whom to communicate. We grunt as a reflex when we throw ourselves into some physical activities, such as lifting a heavy object. Suppose that zoologists from an advanced civilization in outer space observed me to give a trisyllabic scream, 'argh, leopard', and to climb a tree, when I saw a leopard. The zoologists might well doubt that my lowly species could express anything beyond grunts of emotion or intent —certainly 'not symbolic communications. To test their hypothesis, the zoologists would resort to experiments and detailed observations. If I screamed regardless of whether any other human was in earshot, that would support the theory of a mere expression of emotion or intent. If I screamed only in the presence of another person, and only when approached by a leopard but not by a lion, that would suggest a communication with a specific external referent. And if I gave the scream to my son but remained silent when I saw the leopard stalk a man with whom I had frequently been seen to fight, the visiting zoologist would feel certain that a purposeful communication was involved.
Similar observations convinced earthling zoologists of the communicative role of vervet alarm calls. A solitary vervet chased by a leopard for nearly an hour remained silent throughout the whole ordeal. Mother vervets give more alarm calls when accompanied by their own offspring than by unrelated monkeys. Vervets occasionally give the 'leopard call' when no leopard is present but when their troop is fighting with another troop and losing the fight. The fake alarm sends all combatants scrambling for the nearest tree and thereby serves as a deceptive 'time out'. The call is clearly a voluntary communication, not an automatic expression of fear at the sight of a leopard. Nor is the call a mere reflex grunt given in the act of climbing a tree, since a calling monkey may either climb a tree, jump out of a tree, or do nothing, depending on the circumstances.
The supposition that the call has a well-defined external referent is especially well illustrated by the 'eagle call'. Among large, broad-winged, soaring hawks, vervets usually respond with the eagle call to the martial eagle and the crowned eagle, their two most dangerous avian predators. They usually do not respond to the tawny eagle, and almost never to the black-chested snake eagle and white-backed vulture, which do not prey on vervets. Seen from below, black-chested snake eagles look rather similar to martial eagles in their shared pale underparts, banded tail, and black head and throat. Hence vervets rate as good bird-watchers. Their lives depend on it! Vervet alarm calls are not an involuntary expression of either fear or intent. They have an external referent that may be quite exact. They are finely targeted communications which are more likely to be given honestly if the caller cares about the listener, and which may also be given dishonestly to enemies.
Sceptics dispute proposed analogies between animal sounds and human speech on the additional grounds that human speech is learned, but that many animals are born with the instinctive ability to utter the sounds characteristic of their species. However, young vervets appear to learn how to utter and respond to sounds appropriately, just as human infants. The grunts of an infant vervet sound different from those of an adult. 'Pronunciation' gradually improves with age until it becomes virtually adult at about the age of two years, somewhat less than half the age for vervet puberty. That is like human children attaining adult pronunciation at the age of five years; my sons, who are almost four years °ld, are still sometimes difficult to understand. Infant vervets do not learn ro give reliably the correct response to an adult's call until the age of six or seven months. Until then, an adult's snake alarm call may send the infant jumping into a bush, the correct response to an eagle but a suicidal response to a snake. Not until the age of two years does the infant consistently emit each alarm call in the correct context. Before that age, the young vervet may call 'eagle! not only when a martial or crowned eagle goes overhead, but also when any other bird flies over, and even when a leaf flutters down from a tree. Child psychologists refer to such behaviour in our own children as 'overgeneralizing'—as when a child greets not just dogs but also cats and pigeons with 'bow-wow'.
If vervet calls are indeed partly learned rather than entirely instinctive, one might expect vervet populations in different parts of Africa to have developed different 'dialects' for the same reason that different human populations, have. That is, 'word' meanings and pronunciations would gradually change with time, but the changes would develop independently in different areas and would be transmitted by learning, leading first to different dialects and eventually to different languages. This prediction of dialect differences has yet to be tested for vervets, since all the detailed studies of their vocal communication to date have been made in one small area of Kenya. However, song dialects are well developed in some bird species whose young learn the locally correct song from adult birds that they hear around them as they grow up. In a North American songbird called the white- crowned sparrow, such dialect differences are so pronounced that experienced bird-watchers near San Francisco can pinpoint an individual sparrow's home within ten miles.
So far, I have loosely 'applied human concepts such as 'word' and 'language' into vervet vocalization. Let's now compare human vocalizations and those of subhuman primates more closely. In particular, let's ask ourselves three questions. Do vervet sounds really constitute 'words'? How large are animals' 'vocabularies'? Do any animal vocalizations involve 'grammar' and merit the term 'language'?
Firstly, on the question of words, it is clear at least that each vervet alarm call refers to a well-defined class of external dangers. That does not imply, of course, that a vervet's 'leopard call' designates the same animals to a vervet as the word 'leopard' does to a professional zoologist—namely, members of a single animal species, defined as a collection of potentially interbreeding individuals. We already know that vervets give their leopard alarm in response not just to leopards but also to two other medium-sized cat species (caracals and servals). If the 'leopard call' is a word at all, it would not mean 'leopard' but instead 'medium-sized cat that is likely to attack us, hunts in a similar way, and is best avoided by running up a tree'. However, many human words are used in a similar generic sense. For example, most of us other than ichthyologists and ardent fishermen apply the generic word 'fish' to any cold-blooded animal with fins and a backbone that swims in the water and might be worth eating. Instead, the real question is whether the leopard call constitutes a word ('medium-sized cat that… etc. ), a
