When you first encounter a group of rhesus monkeys, it is like watching a play with many actors, but no script. The players scurry to and fro', pause to nibble food or groom each other, get into sudden squabbles, have a bit of sex here and there. Throughout the play, you will hear a continuous soundtrack of chatters, coos, grunts and smacks, punctuated with the occasional sharp scream. The activity appears to be random until you learn to recognize the monkeys individually and memorize their genealogies. After that, the social life of the animals takes on the order and complexity of a Victorian novel.
Rhesus monkeys live in the ultimate class-conscious society. The foundation of the group consists of females belonging to different matrilines. These are ranked in a strict dominance hierarchy, so that a monkey's social status depends on who her family is. High class and all the perks that go with it - such as access to the best food - is acquired and maintained through support from kin: relatives come to each other's aid during conflicts. To succeed in their nepotistic society, these monkeys must constantly check on the whereabouts of their most useful allies - their kin. To do this, they must first, just like the naive human observer, be able to recognize all the monkeys in their group and to class them as family or non-family. Scientists have long wondered how animals do this without language: what cues do they use to recognize other individuals? Odor? Appearance? Sound? Behavior?
The subject was tackled recently by Drew Rendall and his colleagues, Peter Rodman and Roger Emond from the Anthropology Department at UC, Davis, by studying a colony of free-ranging rhesus monkeys who were imported to a small island off Puerto Rico in 1938 and have been observed continuously ever since. As a result, the all-important family trees are known in detail for all the females. When a group of 50 - 75 monkeys is spread out in dense vegetation, it's no easy task to keep track of where individuals are. How useful it would be for a monkey to be able to recognize individuals and kin simply from hearing them. Rendall and his colleagues investigated whether or not this was the case.
The researchers tape-recorded 'coos', the quiet sounds (like a high-pitched, melodic moan) that monkeys give frequently while foraging in thick vegetation, or when separated from other group members. To the human ear, one monkey's 'coo' sounds the same as next. The scientists then played back the recorded calls from hidden speakers placed near selected adult females, and video-taped the females' reactions to the calls. The recordings were of two types: either from kin or non-kin, with kin defined as anyone from offspring down to first cousin. While females' responses to the calls were subtle - measured in terms of seconds spent looking in the direction of the speaker - the results were dramatic. Females responded 16 times faster to 'coos' from relatives than from non-relatives, and spent over 3 times as long looking towards the call. And they did not separate them simply on a "in my family/not in my family" distinction, either, since adult females could also distinguish the calls of individual monkeys. Computer analyses of the 'coos' identified subtle acoustic features that were shared by families and that probably enabled monkeys to classify the calls.
This research brings us a step closer to understanding the meaning behind monkeys' chatter. In addition, it offers us an intriguing glimpse into the minds of other animals and the way they classify their social companions. For example, the results suggest that monkeys perceive a lower boundary of relatedness at the level of first cousin, below which individuals are classed as non-kin. Come to think of it, how many of us maintain close relationships with relatives more distant than first cousin?
We cannot ask monkeys to tell us what they know. But UCD researchers
are devising other "interview" methods. Playing vocal
signals to animals and measuring their reactions is one way of
asking them questions about how they see their world.
Department of Anthropology, UCD, Davis, CA 95616
email: kjstewart@ucdavis.edu