Chimpanzees' 66 gestures revealed
Wild chimpanzees use at least 66 distinct gestures to communicate with each other, according to scientists.
A team of researchers from the University of St Andrews in Scotland filmed a group of the animals in order to decipher this "gestural repertoire".
The team then studied 120 hours of footage of the chimps interacting, looking for signs that the animals were intentionally signalling to each other.
The findings are published in the journal Animal Cognition.
Previous studies on captive chimps have suggested the animals have about 30 different gestures.
"So this [result] shows quite a large repertoire," lead researcher Dr Catherine Hobaiter told BBC News.
"We think people previously were only seeing fractions of this, because when you study the animals in captivity you don't see all their behaviour.
"You wouldn't see them hunting for monkeys, taking females away on 'courtships', or encountering neighbouring groups of chimpanzees."
Dr Hobaiter spent 266 days observing and filming a group of chimpanzees in Budongo Conservation Field Station, Uganda.
"I've spent two years studying these animals, so they know me," she said. "I follow them through the forest and they just ignore me completely and get on with their daily lives."
She and her colleague, Professor Richard Byrne, scrutinised the footage and categorised each distinct gesture.
They looked for clear signs that the animals were making deliberate movements that were intended to generate a response from another animal.
"We looked to see if the gesturer was looking at their audience," explained Professor Byrne.
"And we looked for persistence; if their action did not produce a result, they would repeat it."
The team is still studying the footage for the next stage of their project - to figure out what each gesture means.
For some of these gestures, the meaning seems obvious to us, perhaps because - as great apes- we make similar movements. A chimp will often beckon to another group member, or a youngster will hand shake at another juvenile to entice it to play.
In one piece of footage captured by Dr Hobaiter, a mother reaches with her left arm towards her daughter.
"The mother wants to move away and is gesturing to request that her daughter 'climbs on' her," Dr Hobaiter explained.
"She could just grab her daughter, but she doesn't. She reaches and holds the gesture while waiting for a response."
When the youngster starts to approach, the mother repeats the gesture and adds a facial expression - a "bare-teeth grin", at which point the daughter climbs on and they move away.
"But actions often have effects that their maker did not intend," said Professor Byrne.
"So to understand the intended meaning, it's no good just discovering a gesture's typical effect. We have to look for what effect makes the signaller stop gesturing and appear satisfied and content with the outcome, to be sure that that was what they intended."
The results have provided clues about the origins of chimps' gestures, suggesting that they are a common system of communication across the species, rather than each movement being a learned custom or ritual within one social group.
In fact, by comparing these observations with those of gestures made by gorillas and orangutans, the researchers showed there was significant overlap in the signals used throughout the family of great apes.
Dr Hobaiter said: This supports our belief that the gestures that apes use (and maybe some human gestures too) are derived from ancient shared ancestry of all the great ape species alive today."