How do you capture a raven's heart? Arrest its attention by showing it a twig or stone. Ravens use referential gestures – one of the foundations of human language – to initiate relationships.
From an early age we learn to use referential gestures such as pointing to direct another's attention.
"People think that this pointing forms the basis of language," says Simone Pika at the Max Planck Institute for Ornithology in Seewiesen, Germany.
"It has also been linked with mental-state attribution – the idea that you understand what I am pointing out."
Apes raised in captivity can learn to use referential gestures to communicate with their human caregivers.
Now Pika and Thomas Bugnyar at the University of Vienna, Austria, have recorded common ravens (Corvus corax) using them for the first time.
The researchers observed seven pairs of wild ravens showing and offering stones, twigs and moss to each other – by holding the object in their beaks – in an apparent attempt to grab the attention of another bird and initiate a relationship.
Importantly, the ravens made these gestures only when another bird was watching, and the items they show and offer are not food. They usually gesture only to members of the opposite sex.
Like humans, ravens form monogamous pairs that will defend a territory and raise their young together. They even develop a repertoire of vocalisations that are exclusive to the couple.
This high degree of cooperation may be what prompted the evolution of referential gestures in both humans and ravens, Pika says.
"If communication is governed by cooperation, then this could be what prompted the evolution of language."
Rachel Shaw of the University of Cambridge says that the conclusions, although fascinating, should be viewed with caution.
Although it might look like the birds are attempting to redirect the attention of another bird, the behaviour might simply be a mating or nesting ritual triggered by a peak in hormones, she says.
Alex Kacelnik at the University of Oxford would like to know whether the ravens have as much flexibility as humans in their range of gestures and responses.
"If both sender and receiver use a small, rigid set of targets, and fixed actions for responding, then the interactions could have more in common with classic avian communication systems than with human attention-sharing," he says.
