He Said, She Said

One of the most distinctive behavioural features of the human species is our ability to communicate. So far as we know, no other animal can convey even remotely as much information as we can by speech alone, never mind the advantages that writing brings. But we didn't come out of nowhere, and other mammals can also communicate vocally, something that may have particular relevance to the development of our own species when we look at other primates.

A number of primate species are highly vocal, such as gibbons or the aptly named howler monkeys. Another example, which may be less obvious. is the common marmoset (Callithrix jacchus). This is a species I've talked about before, a small monkey native to South America with a number of unusual biological and anatomical adaptations related to its reproduction and diet. (Unusual for primates in general that is, not compared with other marmosets). But it is behaviourally interesting as well.

Analysis of the vocal repertoire of common marmosets has revealed 13 different types of call, made under varying types of situation. These include alarm calls, alerting the group of a potential predator, calls by infants to attention from their caregivers (marmosets share childcare duties among a group of females) and so on. Among these are two main types of contact call, used by the monkeys to indicate their presence to each other: the 'phee' and the 'trill'.

The phee is a relatively loud call, used when the monkey cannot see its neighbours, and perhaps fears that it might have become lost or accidentally abandoned. The use of this call is clearly important in keeping the group together, something that matters a lot to a small, somewhat vulnerable, communally living monkey. Because it is loud and relatively easy to monitor - the whole point is that you can hear it from a distance - it has been fairly well studied.

We know, for instance, that the exact form of the phee call changes as a young marmoset reaches adulthood, apparently influenced by feedback from its caregivers. We see something similar in the way birds develop the ability to sing and, perhaps more significantly, in the babbling of human infants. Marmosets can also identify one another by the specific sound of their phees.

While it is more common when the marmoset is completely isolated or has been taken away from an established partner and placed with a stranger, this clearly isn't its only purpose. That's because it is also used when the monkey is part of an established group; in this case, it's probably marking territory, letting other groups of marmosets, out of visual range, know that it's there and willing to defend its food supply (or other resource) if it has to. In the latter situation, however, there are subtle differences in the pitch and structure of the call which has, to human ears, less of an urgent or panicked sound to it.

A particularly key feature of the phee call, however, is that it is what is technically referred to as an antiphonal call. This means that the marmosets take it in turns to issue the call; one calls, and then another replies. This is particularly key to the evolution of human speech because it's one of the absolute core features of conversation. Deliberately speaking over one another is not conducive to discourse, and this is, as we might expect, found in all human cultures and something that develops very early in infancy. Such turn-taking has also been observed in other primate calls including, for example, those of Japanese macaques.

So much for the phee call. But what about the trill, the other, quite distinct, type of contact call made by common marmosets? These are used between individual marmosets that can see one another, perhaps even in immediate proximity and, in this respect, are closer to the way that humans use conversation. Obviously, they don't have the complexity of human speech, but that doesn't mean that they convey no information at all since they clearly serve some purpose and it can't be simply announcing one's presence.

Because they're quieter, trill calls are harder to study than phee calls, and much less work has been done on them. The little that has has largely had to be done on captive animals, given the difficulty of getting close enough to those in the wild without spooking them. This limits its utility in terms of knowing how the animals use these calls under natural conditions, and one has to interpret what we learn in light of that. Even so, we do know that trills, like phee calls, are used antiphonally, with the marmosets taking it in turns to make the sounds, just as a human would in a conversation. Indeed, there seem to be subtle rules as to who 'speaks' when, which we haven't yet gotten to the bottom of.

The primary function of the trill in marmosets is likely to be affiliative, which is to say that it promotes social bonding. In effect, it's a signal of friendship and reassurance that helps to keep the group together socially as well as physically. This is a common feature among many primates, having been shown not only in other species of marmoset but in other, more distantly related, species of monkey.

A recent study of trill calls in captive common marmosets showed that they are more commonly employed between individuals that regularly share a cage, and presumably have a good social connection with one another. Not every trill received a direct reply but, when they did, the gap between the first call and the reply was fairly consistent, at just under two-thirds of a second; this is something we also see in human conversation, although the exact timing does vary between different cultures.

Trills were also more common, and higher pitched, when the animals were separated, but still able to see one another. How much this would apply to a wild situation, where a tree may have plenty of obscuring foliage, but the animals aren't physically in separate cages (less than 30 cm apart, but still...) is another matter. But something similar has been observed in wild-living Diana monkeys in Africa, so it's plausible enough as a general rule even if the circumstances are unnatural.

To human ears, a higher-pitched call sounds as if it's signalling excitement and not necessarily in a good way. Human voices do rise in pitch when we are frightened or exasperated, as well as when we're particularly thrilled by something. And it turns out that that's pretty much a universal feature of mammalian communication; a higher pitch indicates a greater degree of emotional arousal in just about every mammal species we've ever studied, from cats and guinea pigs to bats and dolphins.

Is this something that's deliberate on the part of the marmoset, or just a side product of their emotional state? And can the listener infer the emotional state of the marmoset making the trill based on its pitch, or other features? This is, perhaps, harder to prove with certainty, but it's difficult to imagine that they can't tell the difference and that it doesn't mean anything to them at all.

If nothing else, they probably want to make sure that they're heard.

[Photo by Evaldo Resende, from Wikimedia Commons.]