Some species of baleen whales avoid attracting killer whales by singing too low to be heard

But whale species also produce loud underwater songs. What stops killer whales from homing in on their calls and attacking them?

New research from the University of Washington finds some baleen whale species call at such deep frequencies that they're completely undetectable by killer whales, which cannot hear sounds below 100 hertz. These tend to be the whale species that flee in the face of attack. These deep singers in the "flight" club include blue, fin, sei, Bryde's and minke whales.

Meanwhile, their higher-frequency singing brethren that fight back when attacked also tend to be slower-moving and more maneuverable. The "fight" club includes right, bowhead, gray and humpback whales.

The research was published Jan. 31 in Marine Mammal Science.

To conduct the study Trevor Branch, professor of aquatic and fishery sciences at the UW, conducted a review of aquarium experiments on killer whales' hearing ranges, reviewed the source frequency and source level of populations of all baleen whales, and combined these with knowledge of how sounds move through the ocean to predict which whale populations can be easily heard by killer whales. It turns out the calls of flight species generally can't be heard more than 1 kilometer away by killer whales, unlike the calls of fight species.

The fight or flight hypothesis is not new, but research into acoustics is shedding new insights into adaptations of baleen whales. Could this so-called acoustic crypsis, where whales that call at such deep frequencies that they are acoustically invisible to killer whales, have developed as a defense mechanism from attack?

Killer whales are found in all the world's oceans, and killer whales' prey ranges from small fish to the largest whales on Earth. The fight species of baleen whales usually migrate and calve closer to the coast in shallow water, a haven of sorts that provides easier defense against killer whale attacks -- especially for group defense in aggregations. Combined with their slow-swimming and more navigable bodies, their communication with other whales is often at higher frequencies easily heard by killer whales -- above 1,500 hertz. In contrast, flight species have streamlined and slender bodies adapted for speed, and typically disperse across wider open-ocean regions for mating and calving, where they are able to flee in all directions.

These behaviors also have implications for feeding and mating. Denser congregations in shallow coastal areas leave less food for fight species, in comparison to the open ocean favored by flight species. However, the opposite is true for finding a mate -- it's easier when you're all in a similar location, versus spread out over long distances.

Singing is a fundamental part of mate attraction and selection for whales. Males of the flight species sing in a way that maximizes the number of females that hear them, producing simple and repeated songs to attract a potential mate, and singing over prolonged periods to allow females to track them down.

"But these super-loud songs could expose them and their mates to killer whale attack. And this is where acoustic crypsis comes in: singing at low frequencies that are impossible, or very difficult, for killer whales to hear," Branch said.

The research shows that under the sea there is a sound landscape governed by fear, with some whale species choosing to sing their songs to their prospective Valentines at deep levels to avoid attacks; while other whale species compete to sing the most varied and interesting songs, and fight back when attacked. The fight vs. flight differences appear to drive all aspects of the lives of baleen whales, from where they are found, to their communication, to where and when they breed and feed.

"It just never occurred to me that some whales sing low to avoid killer whales, but the more I looked at this, the more I realized that every aspect of their behavior is influenced by the fear of predation," Branch said.