Whales join humans and crows in exclusive club: members with vocal registers only

Breaking new ground 30. apr 2023 5 min Associate Professor Coen P.H. Elemans Written by Morten Busch

The ability to speak and to sing requires precise neural control. This control is limited in most mammals, and vocal registers have been confirmed only in humans – and in crows. In a new study, researchers demonstrate that toothed whales, such as dolphins and killer whales, have evolved a novel distinct nasal structure to produce their echolocation and communication sounds. These sounds span more than four orders of magnitude in their frequency range because whales, like humans, employ different vocal registers. The study proves that humans – at least vocally – are not so special after all, but also raises concerns that noises added to the ocean by ships might disturb communication.

In the animal kingdom, humans are special. Just to mention a few: we have hands and can make fire, we are born naked, so we dress in clothes, we have long childhoods and live on after we stop having children. However, one feature is not unique to us anymore: our extraordinary brains and our ability to speak and sing. In fact, sperm whales have larger brains than humans, and new research shows that toothed whales can produce sounds that are just as complex over a much broader frequency range than humans.

“The voice production system in the nasal passage of toothed whales is strikingly similar to what is known about vocalisation in humans and birds: how their vocal folds create oscillations into the exhalatory airstream, but their voice register is also physically analogous to both the creaky voice in humans and to our chest and falsetto register. This knowledge will help us better understand the extent to which whales can change their voice: for example, due to anthropogenic noise such as cargo vessels that reduce the communicative range of these animals,” explains Coen Elemans, Professor, Department of Biology, University of Southern Denmark, Odense.

Measuring in action

The toothed whales – also called odontocetes – is a biological family of 73 species with well-known members such as sperm whales and beaked whales but also includes dolphins and porpoises. Apart from all possessing teeth – they can capture rapidly moving prey in dark marine environments. This ability critically depends on their echolocating ability by generating powerful ultrasonic clicks. How this is possible has remained unsolved for decades.

“It was a mystery how the air-driven sound source we assumed they possessed could produce both biosonar clicks at depths of more 1000 metres while also communicating socially in a complex way by producing rich vocal repertoires. But for decades the studies have relied on evidence from sound recordings combined with anatomical investigations postmortem. So, finding the mechanism behind the sound production of toothed whales has been extremely challenging,” says Coen Elemans.

Sampling sounds from one specific individual is difficult, and postmortem samples are rare and cannot tell the story about what happens “in action”. But in recent years, techniques have been developed to measure fine-grained dynamic parameters of the human vocal apparatus and map them to the sounds that are produced.

“We decided to try to use the same technique to study toothed whales to show that they blow air through their nasal passage and finely control it to produce diverse sounds. We put sound-recording tags on the whales and dolphins to study sound production in the wild and to test whether the acoustic diversity of their phonations supports different registers. We compiled the acoustic repertoires, and their repertoires indeed comprised clicks, harmonic bursts and whistles that were consistent with the registers seen in humans,” explains Coen Elemans.

Like humans and birds

In humans, the vocal register is described by at least three different laryngeal vibratory mechanisms: M0 – creaky voice, also know as vocal fry, M1 – the head register for men and modal for women, whereas M2 is the head register for women and falsetto for men. All three were found in the whales. Only M3 – the highest pitch in the human voice, corresponding to the human whistling register – was not identified.

“Three human voice registers are physically analogous to those we found in toothed whales. Vocal registers have previously only been confirmed in humans and crows, but finding these sound production mechanisms in toothed whales was unexpected, so we were curious to find out how. So, we examined their phonic lips – both by operating dead-stranded porpoises to look at the anatomy,” says Coen Elemans.

The researcher used video from a thin, tube-like instrument – an endoscope – to image the phonic lips in action of in porpoises and dolphins in captivity. The phonic lips function like the human vocal folds and are tucked away just underneath the blowhole in the nose of the dolphins. By sending pressurised air past these lip-like structures, they vibrate and produce click sounds for echolocation to locate prey through sound waves – but also softer pulses and whistles for communication.

“We show that the toothed whales – including dolphins – possess a sound production system based on air driven through nasal passages. Their phonic lips open for about a millisecond and, when they close, a tissue vibration is created that forms a very loud click in the water in front. So, the lips use the same mechanism, namely air flow-induced self-sustained oscillations, as the human larynx and syrinx in birds,” explains Coen Elemans.

Produce sounds without damaging lungs

Since the sound from whales must travel in water, one critical difference in the sound production systems in whales and humans is that in humans and other land mammals, air is used as the propellant that makes the vocal folds vibrate but also as the medium in which the sounds are propagated. This is not the case in the toothed whales.

“One of the key questions in the study is how they can produce clicks with less than 10% remaining air volume and pressure-collapsed lungs at depths beyond 100 metres,” says Coen Elemans.

The answer is that, during evolution, the toothed whales have simply lost their vocal folds but evolved an entirely new set of sound sources in the nose. With this novel sound source, they can both make clicks to locate, track and capture rapidly moving prey underwater and produce rich vocal repertoires for mediating complex social interactions.

There are obvious evolutionary advantages of a nasal sound source.

“First, a nasal source freed the larynx from sound production, resulting in an effective valve that decoupled the lungs and nasal passages. This allows sounds to be produced at extremely high pressure without damaging lung tissues. Second, the nasal air volume is much smaller than that of the respiratory system, therefore allowing faster pressure control and air recycling,” explains Coen Elemans.

Mitigate noise pollution in the oceans

Based on their findings, the researchers suggest that anatomical adaptations to nasal-source evolution was primarily driven by selection on echolocation signals and only secondarily for social communication. First and foremost, the researchers hope that the findings of the advanced vocal registers will make the world appreciate how unique these animals are.

“I think this is a reminder to humans that we very often toot the horn of how special we are. But here we are dealing with animals with brains that are bigger than ours and that live longer than we do. We have also shown that they produce sounds that are just as complex over a much broader frequency range than us. So, we are not so special, and we have a responsibility to be better neighbours underwater,” says Coen Elemans.

With increasing cargo travel across the world, humans are adding more and more noise to the oceans than toothed whales have ever evolved to cope with.

For 25 million years, they mainly had to cope with ambient noise levels, such as from rain, wind and waves. The question is how much louder they can become to overcome the noise from a cargo vessel passing nearby so they can keep contact with their calf or finding mates or coordinating hunting behaviour,” explains Coen Elemans.

By understanding these limitations, the researchers hope they can also better tell politicians how to mitigate the basic noise pollution in the oceans.

“If the animals cannot produce louder calls than this, it means that the capability to communicate with each other will be limited. And the range at which they can find the fish or squid will also be limited, meaning that the population will be affected,” concludes Coen Elemans.

Toothed whales use distinct vocal registers for echolocation and communication” has been published in Science. In 2017, the Novo Nordisk Foundation awarded a grant to lead author Coen Elemans for the project Give Voice to Your Body: Decoding Vocal Motor Control. In 2020, the Foundation awarded an Interdisciplinary Synergy Grant to Coen Elemans for the project Burst into Song – a Systems Approach to Accurate Predictions of Voice Motor Control.

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