Hundreds of feet below the ocean's surface, and in total darkness, a solitary sperm whale not-so-silently stalks its prey.
Sperm whales, like other toothed whales, rely on echolocation to navigate and locate food in deep, dark marine environments. The strategy depends on their ability to produce short, rapid ultrasonic echolocation clicks, which for some species exceed a blistering 200 decibels in volume — just shy of some of the loudest human-made sounds ever recorded.
According to a new study, toothed whales or odontocetes produce these powerful clicks, as well as a diverse repertoire of other lower-frequency sounds, using a distinct structure in the nasal passages. They do so in a way that is strikingly similar to the way terrestrial animals use the larynx or syrinx to communicate.
The research, led by Peter Madsen, a biologist at Aarhus University in Denmark, also reveals that these whales use three different vocal registers, analogous to humans' use of vocal fry, chest register or normal speaking voice, and falsetto, to generate the fundamentally different tones used for echolocation and vocal communication.
The findings, which describe and characterize the novel vocal capabilities of odontocetes in unprecedented detail, were published in the March 3 issue of Science. The research team also discussed the study in a media briefing at the 2023 AAAS Annual Meeting.
Finding the Source of a Toothed Whale's Voice
The odontocetes — a group of echolocating toothed whales that includes sperm whales, orcas, dolphins and porpoises — possess exceptional vocal capabilities. Not only do their echolocation clicks enable them to survive in the ocean's dark depths, but these whales are also able to produce an acoustically rich collection of softer, lower-frequency grunts, bursts and whistles used to compose complex vocalizations for social communication.
These sounds are thought to be created through an airflow-driven sound source called the "phonic lips" located in the whale's nose.
How whales can make such complex sounds, particularly their clicks created at depths exceeding 1,000 meters below the surface with pressure-collapsed lungs and minimal respiratory air volume, has remained largely unknown.
In a study that lasted more than a decade, Madsen and the researchers solved this mystery.
Using a combination of high-frame rate endoscopic imaging in live animals and audio recording in wild and trained whales, as well as novel laboratory experiments on animals that had died in the wild, the researchers discovered that when whales dive deep, air from the lungs is compressed into a small muscular sack inside the mouth. To make a series of rapid clicks, short blasts of air — lasting only about a millisecond — pass into the whale's nasal passages and across the phonic lips, causing them to slap together.
This airflow-based sound production is functionally analogous to how the human voice box or larynx works and is similar to the syrinx of birds.
However, where laryngeal sound would be hampered by pressure and a lack of air volume at depths these animals frequent, rapidly releasing pressurized air through the nasal passage and onto the phonic lips allows them to generate powerful clicking sounds while also enabling a wide array of other sounds for communication.
From Vocal Fry to Falsetto
Like humans, the researchers discovered that toothed whales use distinct vocal registers to create their diverse sounds.
In humans, a vocal register is a range of tones produced by a certain vibratory pattern of the vocal folds in the larynx. These include the chest register (normal speaking voice), vocal fry (a low creaky voice) and falsetto.
The researchers show that whales use similar registers when making sounds for different functions. For example, the vocal fry register is used to produce echolocation clicks, while the falsetto is used in higher-pitched whistles for social communication.
Until now, vocal registers have only been confirmed in humans and crows.
"By understanding how [whales] make these sounds in the nose, we hope in the future to also understand what the limitations are for their sound production, which in turn will allow us to understand to what degree the animals can call louder or differently in the face of increasing anthropogenic noise at sea," said Coen Elemans, a researcher at the University of Southern Denmark and study co-author.
"Ultimately, that will hopefully allow us to be better neighbors underwater."