Can cetaceans go deaf?

For a land mammal, losing hearing simply means living with one sense fewer. For a cetacean, it is potentially a death sentence. Hearing is their principal sense, far more important than sight in an ocean where visibility is limited to a few tens of metres at best. Does deafness really affect cetaceans? The answer is yes, and increasingly so, largely because of human activity.

Sound, the fundamental sense of cetaceans

In the ocean, sound travels five times faster and much further than in air. Light, on the other hand, is quickly absorbed, and beyond 200 metres of depth there is total darkness. For cetaceans, hearing is therefore the dominant sense. They use it for just about everything: communicating with others of their kind over distances that can reach hundreds, even thousands of kilometres for the great whales; orienting themselves on the high seas; hunting by echolocation in the case of the odontocetes; detecting predators; maintaining social cohesion within groups.

Going deaf, for a cetacean, is therefore much more than a simple sensory loss: it means losing the ability to hunt, to navigate, to communicate and, for an animal living in a society, to remain part of the group. It is a direct threat to survival.

Natural causes: ageing and disease

As in land mammals, cetaceans can lose part of their hearing with age. The hair cells of the inner ear, which are responsible for transducing the acoustic signal into a nerve impulse, wear out and progressively die. This cetacean presbycusis has been documented in several long-lived species such as killer whales and sperm whales.

Parasitic infections, in particular by nematodes of the genus Crassicauda which can lodge in the cranial sinuses, can also affect hearing. Certain bacterial or viral diseases can damage the auditory apparatus. Congenital malformations have also been identified in a few stranded individuals.

But these natural causes are today largely eclipsed by a far more massive threat: anthropogenic noise.

The chronic noise of shipping traffic

Anthropogenic noise comes in two main forms. The first is the chronic noise of world shipping traffic. The engines of container ships, tankers, ferries and cruise ships produce a continuous low frequency rumble that saturates the water across every area of dense traffic. In the European shipping lanes, the ambient noise level has at least doubled every ten years since 1960.

For cetaceans, this noise has a masking effect: it covers the biological sounds they depend on. A North Atlantic right whale, whose upcall carried over 10 km in the preindustrial ocean, is now heard only 1-2 km away in the Bay of Fundy. In the long term, this chronic noise causes permanent stress whose effects are beginning to be measured: elevated stress hormones, reduced fertility, acoustic fatigue.

Acute noise: sonar, seismic surveys, pile driving

The second form of threat is more brutal: intense one-off sounds. Mid-frequency military sonars (MFAS) can exceed 230 decibels at source, a level capable of causing direct hearing damage, and even internal haemorrhage in animals exposed at close range. Seismic surveys for oil exploration, which use airguns producing shocks of 250 dB peak, cause comparable effects. Pile driving for offshore wind turbines and other underwater infrastructure likewise generates violent acoustic shocks.

The consequences can be devastating. At short range, exposure to a powerful sonar can cause a permanent threshold shift (PTS), an irreversible partial deafness. At greater range, a temporary threshold shift (TTS) is observed, which can last from a few hours to several days. Several cetaceans stranded alive have shown, at necropsy, hearing lesions characteristic of recent exposure to intense noise.

The consequences in cascade

A partially deaf cetacean can no longer hunt effectively by echolocation, which condemns it to malnutrition. It can no longer locate others of its kind, which progressively separates it from the group and makes it lose the benefits of social cooperation. It no longer perceives predators and becomes vulnerable. It misses breeding opportunities. For a social and intelligent species such as the killer whale or the sperm whale, it is a slow sentence.

The inner ear of a cetacean is not merely a sensitive organ: it is its essential window on the world. To protect the hearing of cetaceans is to protect their very capacity to exist in their environment. That is why the regulation of ocean noise is becoming one of the great issues of marine conservation in the 21^st century, on the same footing as the fight against chemical pollution or overfishing.

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