Transient Killer Whales: Hunting With the Strong, Silent Type

A research team set out to study how mammal-eating killer whales use sound to communicate – and in the process, they found a powerful tool to study killer whale hunting behavior. How did they know when the elusive whales had made a kill? They listened, and the whales let them know.

Sound is an essential communication tool for killer whales, but it can also mean the difference between a successful hunt and an empty stomach. A recent paper by Volker Deecke and colleagues in the journal Animal Behaviour explores how mammal-hunting killer whales off the coast of British Columbia and Southeast Alaska have adapted their use of sound to compensate for their prey’s excellent hearing.

The northeastern Pacific Ocean is home to two distinct forms of killer whales: fish-eating ‘resident’ killer whales, which appear predictably in large groups from Washington to Alaska, and marine mammal-eating ‘transient’ killer whales, which appear infrequently and in smaller groups. While resident killer whales are well researched, the scarcity and unpredictability of transients makes them difficult to study. Consequently, relatively little is known about their behavior, movement patterns, and prey preference.

All killer whales use underwater calls to communicate over long and short distances, but scientists have found that residents and transients use sound in very different ways. Resident killer whales can afford to be noisy because their prey — almost exclusively salmon — have poor hearing. But how do transients, whose marine mammal prey have excellent hearing, communicate during a hunt without giving themselves away?

To answer this question, a team of researchers followed groups of killer whales, noting their behavior and recording their calls with an underwater microphone (hydrophone). They found that transients call much less frequently than residents — often only when socializing, and almost always after a successful attack. The researchers believe that these post-kill communications are part necessity — the whales interact a great deal while feeding — and part luxury, since well-fed whales will not need to hunt again for some time.

Spectogram (a visual representation of sound) of resident killer whale calls.

Spectogram of transient killer whale calls.

The discovery that these whales typically hunt silently and vocalize only after a kill suggests that anyone who is trying to find out what transients eat should take along a hydrophone.

“Acoustic monitoring is a powerful research tool,” said Volker Deecke, a Researcher at the University of British Columbia’s Marine Mammal Research Unit and the Vancouver Aquarium Marine Science Centre. Deecke worked in collaboration with John Ford at the Pacific Biological Station in Nanaimo and Peter Slater at the University of St Andrews in Scotland.

“Since we began listening for vocal behavior, we have been able to document many predation events that we would have otherwise missed.” Transient kills can be extremely subtle, Deecke explained, with much of the action occurring underwater and out of sight. Kills of smaller prey such as harbour seals and harbour porpoises are particularly difficult to detect since they generate little surface activity.

“Because of this, we often underestimate the importance of these species as prey for transients,” said Deecke. “Now, we look more closely when we hear calls, and we are often able to recover prey remains for identification.”

This new information shows that it pays to listen and not just look when studying animals that rely primarily on sound to navigate and communicate. This ‘sound’ approach is providing significant new insights into the behavior of transient killer whales, and may prove to be the most effective means for evaluating the effects that killer whales are having on seals and sea lions.

21 January 2005

Publication:
The vocal behaviour of mammal-eating killer whales (Orcinus orca): Communicating with costly calls. Deecke, V.B., Ford, J.K.B., Slater, P.J.B. 2005. Animal Behaviour 69:395-405. abstract The cost of vocal behaviour is usually expressed in energetic terms; however, many animals pay additional costs arising from predators or potential prey eavesdropping on their vocal communication. The northeastern Pacific is home to two distinct ecotypes of killer whales (Orcinus orca): resident killer whales feed on fish, a prey with poor hearing abilities, whereas transient killer whales hunt marine mammals, which 5 have sensitive underwater hearing at the frequencies of killer whale vocal communication. In this study, we investigated how the superior hearing ability of their prey has shaped the vocal behaviour of the transient ecotype. We recorded pulsed calls and the associated behavioural context of groups of transient and resident killer whales in British Columbia and southeastern Alaska. Transient killer whales emitted pulsed calls significantly less frequently than residents. Transient killer whales only exhibited significant amounts of vocal 10 behaviour after a marine mammal kill or when the whales where displaying surface-active behaviour. Vocal activity of transients increased after a successful attack on a marine mammal. Since marine mammals are able to detect killer whale pulsed calls and respond with anti-predator behaviour, the reduced vocal activity of transients is probably due to a greater cost for calling in this ecotype resulting from eavesdropping by potential prey. The increase in vocal behaviour after a successful attack may represent food calling (informing other animals in the area about the presence of food), but is more likely to reflect an increase in social interactions during feeding and/or the fact that the cost for vocal behaviour is comparatively low after a successful attack.