Do whales escape killer whales by singing below their hearing threshold?

Killer whales are the only natural predators of baleen whales. These giants of the sea, which filter plankton through their baleen, have to live with that constant threat, particularly females accompanied by a calf. Two survival strategies emerge: flee or fight back.

And this is not confined to physical behaviour. Their underwater songs, spectacular as they are, raise an intriguing question: why do killer whales, with their excellent hearing, not use them systematically to locate their prey?

A frequency too low for killer whale ears

Researchers at the University of Washington, led by Trevor Branch, analysed the range of frequencies audible to killer whales. The result: these predators do not perceive sounds below 100 hertz. Yet some whales sing precisely within that band, which is inaudible to them.

These species, described as the “flight” type, comprise blue, fin, sei and Bryde’s whales and common minke whales. Their deep song acts as acoustic camouflage: killer whales cannot hear them beyond one kilometre.

By contrast, the whales that choose to defend themselves, right whales, bowhead whales, grey whales and humpback whales, vocalise at high frequencies, sometimes above 1,500 hertz. Their signals are far more detectable.

An acoustic strategy correlated with defensive behaviour

The “flee or fight” dichotomy is not new. What is new is its acoustic expression. The study brings together data from hearing experiments in aquaria, the acoustic characteristics of each species, and the real propagation of sound underwater.

Whales in the “flight” group have streamlined bodies, built for speed. They move away from the coast to give birth in pelagic areas where dispersal makes flight possible in every direction.

Whales in the “fight” group, for their part, migrate and calve close to coastlines, in shallower water. These areas allow a more effective collective defence against killer whale attacks, in particular through groupings of individuals.

An acoustics dictated by fear

All of these strategies appear to be dictated by one factor: fear of predation. From morphology to communication, by way of breeding and feeding areas, everything is organised around the presence of killer whales.

The songs of males, which serve to attract females, are also modulated by that pressure. The “fleeing” species sing for longer and more simply, with maximum range in order to reach scattered females. But singing too much is exposing oneself.

“These extremely powerful songs could attract killer whales, putting the male and his partner at risk.”

Hence the use of a little-known weapon: acoustic crypsis. By singing below the hearing threshold of killer whales, whales drastically reduce the risk of being located.

Major ecological consequences

This decoupling between song, breeding and predation alters the ecology of the species. Coastal whales (the fight group) breed more easily, because they concentrate in restricted areas. But they face stronger competition for food.

Conversely, oceanic whales (the flight group) benefit from vast feeding areas, but their chances of a breeding encounter are statistically lower.

The paper, published in Marine Mammal Science, sheds new light on these evolutionary choices, by showing that even courtship songs are never entirely disinterested, nor disconnected from predators. In singing, whales are not only courting: they are negotiating their survival.

Source of the article: http://dx.doi.org/10.1111/mms.13228