Whales outsmart killer predators by singing too low to be heard

By Stephen Beech

Whales avoid attracting predators by singing too low to be heard, reveals new research.

Orcas, or killer whales, are the only natural predators of baleen whales.

The species, that have “baleen” in their mouths to sieve their plankton diet from the water, face predatory attacks from orcas - especially mother and calf pairs.

They also produce loud underwater songs.

But new research by scientists has shown that some baleen whales call at such deep frequencies that they’re completely undetectable by orcas, who cannot hear sounds below 100 hertz.

These tend to be the whale species that flee in the face of attack - including blue, fin, sei, Bryde’s and minke whales, according to the University of Washington research team.

The findings, published in the journal Marine Mammal Science, show that higher-frequency singing whales that fight back when attacked by orcas also tend to be slower-moving and more maneuverable.

Scientists say the “fight” club includes right, bowhead, gray and humpback whales.

Study leader Trevor Branch, Professor of Aquatic and Fishery Sciences at the University of Washington in Seattle, conducted a review of aquarium experiments on killer whales’ hearing ranges.

He also reviewed the source frequency and source level of populations of all baleen whales and combined them with knowledge of how sounds move through the ocean to predict which whales can be easily heard by orcas.

Branch found that the calls of "flight" species generally can’t be heard more than 1,000 meters away by killer whales, unlike the calls of fight species.

He said the "fight or flight" hypothesis is not new, but research into acoustics is shedding new insights into the adaptations of baleen whales.

Branch wanted to know if the so-called "acoustic crypsis" where whales that call at such deep frequencies that they are acoustically invisible to killer whales - had developed as a defense mechanism from attack.

He explained that killer whales are found in all the world’s oceans, and their prey ranges from small fish to the largest whales on Earth.

Branch said: "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."

He explained that denser gatherings in shallow coastal areas leave less food for fight species, in comparison to the open ocean favored by flight species.

But the opposite is true for finding a mate - it’s easier when in a similar location, compared to spread out over long distances.

Branch said: "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.

"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."

Branch says the "fight versus 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.

He added: “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."