Call of the whales

Tuesday 14 December 2004

Scientists studying complex underwater communication in killer whales have discovered that their vocal behaviour is shaped depending on the hearing abilities of their prey.

The study, conducted by biologists at the Universities of St Andrews and British Columbia, presents some of the first evidence that predators shape their communication because of the potential to lose out at feeding time to quick-witted and sound- savvy prey.

Whales and dolphins rely extensively on underwater sound to orientate and to stay in touch with one another, and killer whales have a particularly complex system of underwater communication.

But the team based in St Andrews and Canada have discovered that the cost for such communication is high for one type of killer whale: while talk is cheap for fish- eating groups, mammal-eating killer whales pay a stiff price for underwater calls.

The simple reason for this is because fish are hard of hearing, they cannot detect the calls at any distance. Mammal-eating killer whales on the other hand hunt prey with excellent underwater hearing who eavesdrop on their calls and use them as a cue to escape. The scientists found that killer whales that prey on marine mammals restrict their sound communication accordingly.

The research was undertaken on the coast of British Columbia and Southeast Alaska by biologist Dr Volker Deecke of the University of St Andrews and the Marine Mammal Research Unit in Vancouver, Canada, in collaboration with Professor Peter Slater at St Andrews and Dr John Ford of Fisheries and Oceans Canada. The work was funded in part by the Vancouver Aquarium Marine Science Centre.

The team worked from small boats, scanning the waters using high- powered binoculars for hours at a time. The researchers studied two distinct forms of killer whale – residents and transients – that feed on different types of prey. The two forms do not interbreed and rarely interact, even though they inhabit the same waters. Residents are exclusive fish- eaters and feed particularly on Pacific salmon. Transients, on the other hand, prey only on warm- blooded sea mammals – mainly harbour seals, sea lions, porpoises and Pacific white-sided dolphins. Occasionally, transients will attack larger prey such as minke whales or grey whales.

It is thought that killer whales produce underwater calls to coordinate behaviours and to keep track of each other, since vision is limited underwater. Calls carry for long distances. The researchers monitored the communication of both types of killer whales and found that fish- eating residents called almost continuously and in all behaviour states. Only rarely did groups of residents travel in complete silence. By contrast, for the mammal-eating transients, silent travel was the norm: these whales did not produce any calls for the vast majority of the time and only interrupted their stealthy silence for brief bouts of vocal communication after they made a kill or when exhibiting surface- active behaviours such as flipper slaps or dolphin leaps.

Dr Deecke explained: “When animals communicate, they risk passing on information to unintended listeners, which intercept their communication signals. In our case such eavesdropping wasn’t a problem for the fish-eating resident killer whales because their primary prey, Pacific salmon, are very hard of hearing. Transient killer whales on the other hand feed on marine mammals which have excellent underwater hearing so that they risk warning their prey every time they call.

“The reason why transients call after a kill is likely to be because the animals interact a great deal while feeding and because, having just captured prey, the cost of calling is low since the whales are fed and will not therefore need to hunt again for some time.

“Because calling behaviour warns marine mammal prey and so greatly reduces their hunting success, sound communication is far more costly for transients compared to residents. Our finding that mammal-eating killer whales call much less often than fish-eaters demonstrates that the calling behaviour of killer whales is shaped by the hearing ability of the prey that they eat.”

The study will be published early next year by the scientific journal Animal Behaviour.




VOLKER DEECKE (NOTE CANADIAN TIME DIFFERENCE = – 8 HOURS GMT APPROX): (+1) 604 822 9150 OR (+1) 604 659 3429 / 3430.




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