Complex calls for alarm among monkeys

A few weeks ago I picked up a slew of old anthropology books, many of which were at least minimally concerned with figuring out what makes our species human (or what defines "Man," to put things in their historical context). Bipedalism, making tools, and language were the classic examples of features that separate us from the "beasts,"* but prior to any of these books Charles Darwin noticed that it would be practically impossible to point to any one point in our evolutionary history and claim that specific point as the time that our ancestors became human. Even in a time when transitional fossils were not known (the first Neanderthal fossils were only found a few years earlier and much confusion surrounded them), the habits of living animals were enough to prove to Darwin that our species is perhaps not as intellectually unique as we would like to suppose;

Nevertheless the difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind. We have seen that the senses and intuitions, the various emotions and faculties, such as love, memory, attention, curiosity, imitation, reason, &c., of which man boasts, may be found in an incipient, or even sometimes in a well-developed condition, in the lower animals.

*Jokingly, I've sometimes remarked that air-conditioning is what "separates us from the animals."

Presently language seems to be one of the last bastions of humanity. The research of Jane Goodall, showing that chimpanzees not only use tools but make them, stripped us of the title "Man the tool-maker," but (to borrow the antiquated terminology that marks so many of the dusty books I picked up) we still consider our species "Man the Orator." Even our close relatives, like the Neanderthals, are often denied the ability of language, but some new research of living primates may reveal that the beginnings of language are far older than might be assumed.

One of the most interesting areas of current primatology involves the study of alarm calls, noises made by monkeys to effectively warn other members of a group about danger. In some of the species studied thus far, there is not just one general "Run away!" signal that is used over and over again; there are specific calls to specific types of danger, and whether an alarm call is sounded depends on the type of predator in the area. Such trends were most famously studied among vervet monkeys (Chlorocebus pygerythrus) by Dorothy Cheney and Robert Seyfarth (1980, 1980a, 1981), their studies showing that there were distinct alarm calls for leopards, eagles, and snakes. Young individuals took some time to get the calls just right (i.e. they would use the bird call for any bird, not just a dangerous one), but there was a definitive connection between a particular predator and a particular call. Diana monkeys (Cercopithecus diana) in the Tai forest also give alarm calls when danger is near, but they seem to discriminate between ambush predators like leopards (where a long-distance alarm will let others know and also tell the predator that it's been spotted) and predators that can chase them, like chimpanzees. If a monkey makes an alarm call when a chimpanzee is near they are essentially calling attention to themselves and could very well end up being prey, so it is more to their benefit to refrain from calling in such circumstances.

New research published in Current Biology reveals a much more complex system in putty-nosed monkeys (Cercopithecus nictitans), however. In this species, males combine two kinds of alarm calls, deemed "hacks" and "pyows," into sequences depending on the situation. Leopards (or the inferred presence of a leopard) elicits a string of "pyows," and crowned eagles often cause a series of "hacks" or "hacks" followed by "pyows" to be emitted. In addition to these usual responses, the males sometimes produce a "pyow-hack" (P-H) sequence of 1-4 "pyows" and 1-4 "hacks," which can occur alone or at the beginning of another call sequence. Whenever they occur, though, P-H sequences are directly linked to the group of monkeys moving elsewhere, and the group will only move when it hears a P-H sequence from one of the males in the group (neighbors were ignored).

What if the females in the group were responding to subtleties in "pyows" and "hacks" rather than the sequence, though? In order to test this hypothesis, the researchers composed a P-H sequence from recorded alarm calls they had previously recorded and played them back and saw no difference in the response between their "artificial" sequence and a "natural" sequence as vocalized by a male. Acoustically there did not seem to be any slight variations in the sounds that would suggest that the females were reacting to the character of the sound rather than the sequence, so such a hypothesis does not appear to be supported.

The sequence for combining two calls into sequences may be because vocal repertoires have limits. A greater amount of information can be transmitted if combinations of sounds are used rather than one sound per concept, and the calls of the putty-nosed monkeys seem to convey three types of information to members of a group. A P-H sequence identifies the event or danger witnessed by a male, the identity of the male, and whether that male is going to remain in the area or move elsewhere. That's a fair amount of information to get out of an alarm call repertoire of just two sounds, and such a system may provide an alternate model for the early evolution of language. In a species where the variety of sounds that can be produced is slim, the sequence of sounds can open up the ability to communicate a greater number of ideas, although further research is required to determine if other animals have been adapted to use such systems and how they make use of them.

Of further interest, in the same issue of Current Biology, there is another paper entitled "Communicative Signaling Activates 'Broca's' Homolog in Chimpanzees." In humans, Broca's area is active during "language-related tasks" and "the execution of language." Previously it had been unknown whether primates have a homologous are in their brains that is active during communication, but the research presented in the Current Biology paper suggests that the same region that contains Broca's area in humans is active during communication in chimpanzees. This could be due to convergence, but it also could be inferred to mean that the last common ancestor of chimpanzees and humans had the basic neurological wiring for language production. Research on other primates is going to be required before the latter hypothesis can be strengthened, however, especially if other primates (perhaps like putty-nosed monkeys) show similar patterns in their brain activity.

Arnold, K., Zuberbühler, K. (2008). Meaningful call combinations in a non-human primate. Current Biology, 18(5), R202-R203.

TAGLIALATELA, J., RUSSELL, J., SCHAEFFER, J., HOPKINS, W. (2008). Communicative Signaling Activates ââ¬ËBroca'sââ¬â¢ Homolog in Chimpanzees. Current Biology, 18(5), 343-348. DOI: 10.1016/j.cub.2008.01.049