Monkeys might not have a very complex language but they do have words for identifying predators. But researchers have now found that they even combine calls into sequences to create different meanings.
Putty-nosed monkeys have two basic predators, leopards and eagles. The defense against them is fundamentally different: they climb up in the trees to escape the leopards and climb down to escape the eagles. In consequence the monkeys have different calls warning against the presence of leopards or eagles. They emit a loud "pyow" for the leopard and a "hack" for the eagle.
According to Klaus Zuberbuhler from the University of St. Andrews in Scotland, who has been studying these monkeys for years, "the visibility in rainforest is terrible, maybe 10 meters, which is why these acoustic signals are so crucial".
But scientists have now noticed that the monkeys also mix the two calls into a "pyow-hack" sequence. They recorded and observed the monkeys for two months to understand the meaning of such sequences. The sequence appears to mean something like "let's go!" or "follow me!". Researchers observed that such a combination of "pyow-hacks," is uttered by males and gets the pack moving a little quicker and further - up to 100 meters in half an hour - than either call on its own.
"They might make a 'pyow-hack' in response to a predator, but the strange thing is they also do it early in the morning while foraging," said Zuberbuhler. "If the male wants to move on, he produces that sequence, which is followed by the group moving."
Sometimes one male makes such a complex call after receiving some input from an elder female in the group, and the rest of the monkeys congregate around the leader to see which way to head next.
"Observationally and experimentally we have demonstrated that this call sequence serves to elicit group movement in both predatory contexts and during normal day-to-day activities such as finding food sources," said Kate Arnold, the other member of the team. "The pyow-hack sequence means something like 'let's go' whereas the pyows by themselves have multiple functions and the hacks are generally used as alarm calls."
"Previously, animal communication systems were considered to lack examples in which call combinations carried meanings that were different to the sum of the meanings of the constituent elements," she added. "This is the first good example of calls being combined in meaningful ways. The implications of this research are that primates, at least, may be able to ignore the usual relationship between an individual call and any meaning that it might convey under certain circumstances."