By Victoria Gill
Science and nature reporter, BBC News
Once orcas catch their prey, the noise begins again
Orcas avoid being overheard by their prey by hunting in "stealth mode", according to researchers.
The scientists wanted to know how orcas, commonly known as killer whales, communicate when hunting mammals, which can hear their distinctive calls.
The researchers thought the predators might switch to very high frequency whistles to co-ordinate the hunt.
But the orcas actually go completely silent and are somehow still able to form organised hunting groups.
Volker Deecke from the University of St Andrews in Scotland and Rüdiger Riesch from North Carolina State University in Raleigh, US, carried out the study, which was published in the journal Behavioural Ecology and Sociobiology.
They used hydrophones - underwater microphones - to listen to and record orcas communicating with each other. The team could even hear crunching sounds when the animals were eating their prey.
They communicate while they eat then gradually wander off and go quiet again
Volker Deecke St Andrews University
The researchers focused on transient orcas, in the Pacific Ocean off the coast of Canada and Alaska. These tend to live in smaller social groups and to move around more than resident orcas.
Some scientists believe that the two are distinct sub-species.
"The most striking difference between the two is their diet," explained Dr Deecke.
Residents eat fish, whereas transients hunt and eat marine mammals, including seals and porpoises.
Dr Deecke added: "In the 40 years that these animals have been studied, scientists have never seen a resident eat a mammal and never seen a transient eat a fish."
Resident orcas hunt for salmon using echolocation. The orcas click, producing waves of sound that travel through the water and bounce off the fish, allowing the predator to sense its location.
"But all marine mammals have excellent underwater hearing," explained Dr Deecke.
"If if a killer whale swam along clicking like mad, all the seals and porpoises would think - here comes a predators, let's get away."
But the transient orcas' solution surprised the researchers.
"They go into stealth mode - completely silent," said Dr Deecke. "This raises the question: how are they communicating?"
It seems that orcas can carry out complex, co-ordinated mammal-hunting trips without "talking to each other" at all.
"To cover a wider area, they fan out occasionally - travelling hundreds of metres, even kilometres apart, and they come back together again," said Dr Deecke.
Only once they catch their prey, does the noise - whistling and pulsing calls - begin.
"It's a bit like us at a dinner party," said Dr Deecke. "They communicate while they eat then gradually wander off and go quiet again."
The orcas are unlikely to be able to see each other from these distances. Glaciers that descend into the sea on the Alaskan coast give the ocean the consistency of milk.
Dr Deecke thinks that the orcas might "rehearse" their hunting routines, to learn the position of each group member.
"They tend to be very predictable," he said. "I often know exactly where they are going to surface."
How they manage this level of co-ordination is not clear. And the scientists plan to continue their research by fitting sound recording and satellite tracking tags to individual orcas to follow their behaviour much more closely.
Dr Deecke said: "It seems like there's no way for them to communicate without their prey being able to eavesdrop."
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