Monkey Vocab Richer Than Thought

March 10, 2008 -- "Hack hack hack pyow hack hack" might not mean much to human ears, but to a putty-nosed monkey it means, "I'm adult male X; I have just seen an eagle; I will now move away," according to a new study showing that primates can combine individual calls to express different meanings.

While such syntax-like behavior has been described in other species, such as whales and dolphins, the new findings are the first to clearly demonstrate the skill in a non-human primate.

"What our research shows is that individual calls do not carry any specific meanings, but different call sequences do," co-author Klaus Zuberbuhler told Discovery News.

"So, for example, a series of hacks almost certainly indicates the presence of a crowned eagle, whereas a series of hacks preceded by 1 to 2 pyows reliably indicates that the caller is about to start traveling away," added Zuberbuhler, who is a researcher in the School of Psychology at the University of St. Andrews in Scotland.

For the study, published in this week's Current Biology, Zuberbuhler and colleague Kate Arnold focused on alarm calls emitted by free-ranging male putty-noses at Gashaka Gumti National Park, Nigeria. These monkeys live in groups with one male, six to nine females and their offspring.

The scientists first observed that when males emitted a mostly "pyow"-based call series, a leopard was around. "Hack" sounds followed by "pyows," in turn, were linked to the presence of an eagle. The males also assembled these two alarm calls into unique "pyow-hack" sequences.

Using a GPS unit, the researchers tracked how females would move in response to hearing certain recorded calls. As predicted, the females moved whenever the sounds indicated the caller was traveling and that an eagle was present.

Since the monkeys usually just hide up in the forest canopy in the presence of a leopard, both the calling male and the listening females stayed relatively put in response to leopard-linked calls.