New study examines how transient killer whales communicate

first published March 2012

For transient killer whales on the hunt, making the slightest sound can mean the difference between a hard-won meal and an empty stomach. Their marine mammal prey—mostly seals and sea lions—have finely tuned hearing that quickly alerts them to danger.

Transient Killer whale
Transient killer whale with Steller sea lions on the rocks.

As a result, transient killer whales have complex rules around the audible clicks, whistles and pulsed calls they make. While echolocation clicks are primarily used for navigation and prey detection, pulsed calls and whistles are important social signals that help members of a group to recognize one another, stay together, and coordinate behaviors.

In a recent study published in the journal Behavioral Ecology and Sociobiology, Consortium scientists conducted the first-ever investigation into how transients use whistles compared to their fish-eating cousins, the resident killer whales.

“Here, we investigated the whistling behavior of mammal-eating killer whales and, based on divergent social structures and social behaviors between residents and transients, we expected to find differences in both whistle usage and whistle parameters,” write co-authors Dr. Volker Deecke of the Marine Mammal Research Unit at the University of British Columbia, and Dr. Rüdiger Riesch of the Department of Biology at North Carolina State University.

Transient whistle
spectogram

Listen to sound sample:
Transient Whistle sample

Riesch and Deecke had hypothesized that transient killer whales should use whistles preferentially over pulsed calls in many contexts because whistles are higher in frequency and therefore not audible to other species over the same distances as pulsed calls. However, they found no support for their hypothesis. Instead of switching from calls to whistles, transient killer whales seem to go completely mum—presumably because even the less detectable whistles can still reduce hunting success by alerting their potential prey to their presence.

The Strong, Silent Type
Riesch and Deecke painstakingly analyzed approximately 60 hours of recordings of West Coast transients ranging from Monterey Bay, California to Glacier Bay, Alaska. Using real-time spectrographic analysis, they searched the recordings for whistles, then analyzed the acoustic profile of each whistle and classified it as stereotypical (frequent and recurring) or variable (infrequent or unique).

Transient whistle
spectogram

Transient Whistle sample 2

Their results showed that West Coast transient killer whales whistled only after making a kill or when they were engaged in social activities, as indicated by tail- and fin-slaps, breaches and spy-hops, and they were almost completely silent during all other activities. All West Coast transients seem to share a population-specific repertoire of stereotypical whistles that is clearly distinct from and less complex than that of resident killer whales.

The acoustic profiles of transient whistles showed properties more consistent with a “public” broadcast communication than a “private” one designed to avoid eavesdroppers. Transient whistles generally have lower dominant frequencies, narrower frequency ranges, a shorter duration and fewer frequency modulations—making them more similar to the public whistles of resident killer whales.

Why Whistle?
Does this mean that transient killer whale whistles serve an entirely different purpose than resident whistles? The authors speculate that while the acoustic profiles of resident and transient whistles were very different, they could share a similar function in instances such as food sharing after a kill. In these types of activities, the rate of transient whistles increased. In instances such as foraging, it appears that transients prefer not to communicate at all than risk alerting their prey.

“Hence, the main strategy of transients to minimize detection by potential prey is to limit vocal communication to certain behavioral contexts, making detection based on whistle recognition by prey impossible during foraging, regardless of a potential receiver’s hearing capabilities,” write Riesch and Deecke. “This in turn seems to have relaxed the selection on making whistles acoustically private.”

In the complex underwater world of transient killer whales, a whistle can be a dead giveaway to potential prey. Whistles are only welcome after a kill is made and there is no longer a need for stealth, which may explain why the behavioral context of resident and transient whistles are so different.

PUBLICATION:

Whistle communication in mammal-eating killer whales (Orcinus orca): further evidence for acoustic divergence between ecotypes.
Riesch, R. and V.B. Deecke. 2011.
Behavioral Ecology and Sociobiology 65:1377-1387.
abstract
Public signaling plays an important role in territorial and sexual displays in animals; however, in certain situations, it is advantageous to keep signaling private to prevent eavesdropping by unintended receivers. In the northeastern Pacific, two populations of killer whales (Orcinus orca), fish-eating resident killer whales and mammal-eating transient killer whales, share the same habitat. Previous studies have shown that residents use whistles as private signals during close-range communication, where they probably serve to coordinate behavioral interactions. Here, we investigated the whistling behavior of mammal-eating killer whales, and, based on divergent social structures and social behaviors between residents and transients, we predicted to find differences in both whistle usage and whistle parameters. Our results show that, like resident killer whales, transients produce both variable and stereotyped whistles. However, clear differences in whistle parameters between ecotypes show that the whistle repertoire of mammal-eating killer whales is clearly distinct from and less complex than that of fish-eating killer whales. Furthermore, mammal-eating killer whales only produce whistles during milling after kill and surface-active behaviors, but are almost completely silent during all other activities. Nonetheless, whistles of transient killer whales may still serve a role similar to that of resident killer whales. Mammal-eating killer whales seem to be under strong selection to keep their communication private from potential prey (whose hearing ranges overlap with that of killer whales), and they appear to accomplish this mainly by restricting vocal activity rather than by changes in whistle parameters.
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