Although whales are mammals that breathe air, they spend most of their time roaming the depths of the oceans. There, sound travels faster underwater and farther than it does on the world’s surface, at about 1500 meters per second compared to just 340 meters per second in air. So, a whale’s world is replete with sound — it’s a key element to its survival, touching everything from socializing and breeding to navigation and feeding.
But if whales don’t have any sign of external ears, how do whales hear sounds? Scientists use anatomical data, mathematical models, and behavioral experimental data to test echolocation and other hearing methods out in the wild.
Do Whales Hear Sound?
All whales rely heavily on sound to understand information about the world around them. Researchers at the Smithsonian Museum created detailed 3-D images of 56 whale fetuses from 15 different species of baleen and toothed whales from the museum’s specimen collection and compared and contrasted them with fossilized ears of extinct whales from millions of years back.
They discovered that both the oldest whale fossil ears and the youngest fetal ears share many features with land mammals — in fact, whale ears have been “acoustically isolated” for the past 45 million years or so, according to Nick Pyenson, the curator of fossil marine mammals at Smithsonian’s National Museum of Natural History.
But since there are two main types of whales — toothed whales and baleen whales — experts think that different whales have long been using sound in slightly different ways.
What Are Toothed Whales?
Toothed whales are the category of whales including beaked whales, sperm whales, and even dolphins. They’re called toothed whales because they literally have teeth. These marine mammals are specialized in high-frequency sounds, or ‘ultrasonic’ sounds.
How Does Echolocation Work for Whales?
Toothed whales are famous for their ability to produce high-pitched clicks and bounce sound off of elements in their environment to understand where they are and what they’re like. They use this biological sonar technique, called echolocation, to get around, find food, and ‘see’ their surroundings. This is an ancient ability that is thought to have developed independently several times throughout evolution.
They can hear sounds we cannot even remotely perceive, such as frequencies exceeding 150 kilohertz in some species.
What Are Baleen Whales?
Humpback whales, blue whales, and right whales are categorized as baleen whales because they feed by filtering plankton through a fringe-like structure in their mouth called the baleen. They are some of the largest creatures on Earth.
What Are Whale Songs?
The sounds Baleen whales make tend to be low-frequency — known to science as ‘infrasonic’ — and can travel long distances. These far-reaching, low-frequency sounds are often woven together into eerie, melancholic songs that whales use to communicate with each other.
Rebecca Dunlop, a professor at the University of Queensland in Australia, is an expert in carrying out hearing tests on humpback whales: releasing a signal out into the water once whales on migration pass by, and observing whether they react. Her work has shown that they can hear from 63 hertz up to 22 kilohertz.
“But past those two points, we don't know, probably a bit lower, probably a bit higher, that's all we know, at this point,” says Dunlop. Her work also is starting to suggest that humpback whales are most sensitive to sound at approximately 4 kilohertz she says — actually where humans are quite sensitive as well.
Do Whales Have Ears?
Humans have a fleshy outer structure to the ear that captures sounds and directs them into the internal bones of the ear, which is tasked with deciphering them for the brain to understand. This is the same for most land mammals: cats, dogs, monkeys, horses, cows, and hippos. This fleshy part is called a “pinna.” In water though, it doesn’t work as well.
Instead of having a pinna, toothed whales have developed a literal pad of fat in the mandible to capture sound with — they look like gigantic fat bunny ears along the jaw, says Darlene Ketten, a scientist at the Woods Hole Oceanographic Institution who specializes in dissecting whale ears to understand how they work.
These fatty pads are detailed convolutions that are specifically designed to entrap the frequencies they need to, just like the cartilage around our ears. Then, sound travels into the bony labyrinth of the ear and to the cochlea.
Read More: AI Might Help Us Decode Whale Language
How Do Whales Hear?
Much of how baleen whales hear, compared to toothed whales, is still a mystery. This is mainly because it’s difficult to study baleen whales: they’re just too big. A baleen whale’s ear is as big as a human head and as dense as a bowling ball, according to Ketten.
Baleen whales have fat pads close to the ears too, and their ear bones are homed inside a conch-like bone the size of a bulldog’s head bulging on the side of their skull.
By CT scanning the head of a small fin whale, researchers at San Diego State University were able to reconstruct an accurate picture of what happens when sounds interact with the geometry of a whale’s skull. Their findings suggest that baleen whales have specialized skulls they use for hearing, and they use them in two different ways.
If the sounds are longer than the whale’s body, the whale uses their skull to capture the sound and conduct it, amplifying it as it vibrates through their skull, before making it to the tympano-periotic complex. This is almost four times more sensitive than just letting sound pressure reverberate through tissue, so this is likely the mechanism that baleen whales use the most.
Ketten doesn’t agree with this theory, though. Using the skull to conduct sound to the inner ear wouldn’t let the gigantic whales hear where the sound is coming from, argues Ketten — something these animals seem to be quite good at.
Read More: Understanding How Whales Communicate
How Humans Interfere With Whale Communication
The issue with whales being so sensitive to underwater sound and having such sophisticated methods to make sense of it, likely means they’re also highly affected by all the raucous we input into the ocean: ships, sonars, and military tests all add noise pollution to the world’s water basins. This is, again, difficult to test though, according to Dunlop, because of how different all of these sounds can be.
Dunlop’s team has already found that air guns used in marine exploration by military teams are loud enough to be heard by humpback whales as far as three kilometers out — it’s still unclear whether this impedes them from communicating with each other, though, because air guns go off at intervals. It's likely that constant vessel noises, on the other hand, will mask their communication signals, says Dunlop, preventing humpback whales from communicating not within a group, but probably between groups.
More research is needed to figure out whether these human-made sounds could affect their migrating patterns, their ability to communicate, feed, and breed, and how this data can inform mitigation strategies for noise in the ocean.
Read More: How Many Whales Are Left In the World?