The apes were more likely to make warning calls when they spotted a
venomous snake if others in their troop had not seen the danger,
researchers found. As chimps in the know arrived at the scene, they
passed the warning on to others who lagged behind but were still within
earshot.
Video footage of wild chimps foraging in Uganda's
Budongo forest show apes at the front of their groups jumping with
surprise on spotting a model snake lying camouflaged in the undergrowth.
When the chimps regained their composure, they called out with repeated
"hoos" to alert those behind them that a threat lay ahead. They made
calls less often when other chimps already knew of the danger.
The behaviour suggests the animals knew what their fellow apes knew and
made decisions over what warnings to give based on the information.
"Lots of animals give alarm calls and are more likely to do so if
there's an audience, but these chimps are more likely to call if the
audience doesn't know about the danger. It's as if they're picking up on
differences in ignorance and knowledge in others," said Dr Catherine
Crockford, who studies ape communication and social interaction at the
University of St Andrews.
"The chimps would sometimes jump when
they saw the snake, but they didn't call then. They would only call
after going back for a second look. So there's a dissociation between
their emotional reaction and the vocalisation. The call is not a knee
jerk reaction to the snake, it's intelligent behaviour," she said.
Crockford teamed up with researchers at the Max Planck Institute for
Evolutionary Anthropology in Leipzig and the Budongo Conservation Field
Station in Uganda to investigate how 33 wild chimps reacted to model
vipers placed on paths use by apes in the forest.
When
confronted with danger, chimps scream or bark if the threat is serious,
such as an ambush by a leopard or rival chimp troop. But they respond to
less serious threats, such as snakes or fresh faeces from predators,
with gentle, repeated "hoo" sounds. Though vipers do not prey on
chimpanzees, they can attack with a lethal bite if trodden on or
disturbed.
The scientists filmed 111 encounters between the
chimps and the model vipers and recorded alarm calls the apes made on
spotting the snakes. Most chimps reacted with repeated hoos, and only
rarely made more serious barking sounds.
The chimps hoo-ed most
when they caught sight of companions who had been too far away to see
the snake or hear earlier warning calls. They hoo-ed least when other
chimps had seen the viper themselves, according to a report in Current
Biology.
On hearing the hoos, some chimps stopped in their
tracks and waited. Others picked their way through the forest on detours
that took them around the snake.
Crockford said the findings
suggest humans are not alone in knowing the minds of others, an ability
that may have been pivotal in the evolution of language as it allowed
humans to share information and boost collective knowledge.
"More of the essential ingredients needed to kickstart complex
communication are evident in chimpanzees than we thought," Crockford
said.
www.guardian.co.uk/science/2011/dec/29/chimpanzee-chimps-minds