The Evolution of Echolocation

a beaked whale surfaces
A rare sighting of a beaked whale. (Courtesy of Dr. Brandon Southall, NMFS/OPR)

Imagine hearing with your jaw — the very same jaw used to catch and eat your food. For the about 75 species of whales, dolphins, and porpoises of the Odonotocete family, also known collectively as the toothed whales, that is their reality. A new study led by Smithsonian Peter Buck Fellow and cetacean evolution and biology expert Dr. Ellen Coombs gives us new insight into the evolutionary history of multi-purpose toothed whale jaws.

Inhabiting a variety of environments from crystal clear off-shore waters to dark, murky coastal estuaries, toothed whales are diverse, widespread, and efficient marine predators. From killer whales to bottlenose dolphins, toothed whales or Odontocetes differ from their filter-feeding cousins, the Mysticetes or baleen whales, based on — you guessed it — their teeth. But some time after the evolutionary split between these two groups, the toothed whales gained another valuable skill.

Similar to bats, all modern toothed whales possess a kind of biological sonar known as echolocation that helps them navigate environments where other senses may not allow. “It’s a hunting tool,” explained Coombs.

“These animals can essentially see with sound. They live in some of the deepest oceans on the planet, they’re hunting in pitch black water, and they’re hunting in cluttered rivers, around icy environments, as well as all over the planet. They’re managing to do that with sound using echolocation.”

    But odontocetes don’t use their ears like we do. Millions of years ago, the openings on the sides of their head all but closed up, leaving small, almost invisible holes behind the eyes. Instead of relying on traditional ear canals, toothed whales adapted to use their lower jaw to pick up noises from the water around them.

Preview an orca whale skull with a yellow background

The recently published study led by Coombs focuses on the lower jaw or mandible and aims to better understand the roles that echolocation and diet play in toothed whale evolution. “We wanted to look at what’s happened [during toothed whales’ evolutionary history]. How they managed to evolve so many different jaw shapes and successfully conquer all of the world’s oceans as apex predators,” she said.

How does echolocation work in whales and dolphins? Echolocation relies on sending and receiving sounds. Toothed whales create noises by blowing air through what is known as their “phonic lips,” a vibrating tissue located along their nasal passage, beneath the blowhole on the top of their head. This outgoing noise, called a “click,” is focused through the fatty organ called the melon, located on their forehead. The click then travels through the water until it hits something like a fish, and its “echo” will bounce back towards the whale. Once it returns, a thin area of bone and ‘fat pad’ at the back of the mandible known as the “acoustic window” absorbs and transfers the returning click to the whale’s inner ear. Measuring the sound and its delay time, toothed whales can discern information like the distance and even identity of the species it bounced off.

Preview a diagram showing the inner anatomy of a toothed whale's skull and how sound is processed by the whale
(Uku Gorter via NOAA)

    In looking at fossils and skeletons of every whale species, from modern day species to the earliest known whale ancestor Pakicetus, Coombs and her collaborators found that toothed whale evolutionary rates and diversity spiked at two points in time: first, in the early to mid Eocene (about  40-45 million years ago) when early whales (archaeocetes) first adapted to life in the water, and later in the mid-Oligocene (25-30 million years ago), a period of time in which toothed whale echolocation was refined and their diets began to specialize. “They’re becoming really specialized in the type of echolocation they can use and the type of diet they can have at the same time,” said Coombs. “If you’re able to echolocate, you can really expand and diversify your diet.” This, in turn, allowed for what the study finds to be some of the fastest rates of jaw evolution in odontocete history, as well the diversity in jaw shape and size we see today.

    But all areas of toothed whale jaws did not evolve at equal rates. In fact, the back of the jaw was found to have evolved more slowly than the front. The back of the jaw, the part used for sound reception, evolved “really early in the whale evolution” when the need for underwater directional hearing first arose — but the study finds that area to stay mostly the same shape after that point.  When echolocation arose in toothed whales about 39 million years ago, that jaw shape began to change again at high rates — but this time, that change was concentrated at the front of the mandible. Coombs explained,“[Toothed whales] took echolocation and ran (swam!) with it, and started to evolve to be able to eat all sorts of animals.” Now able to tailor their hunting towards squid or fish or other marine mammals, the front part of the jaw used to catch prey (as opposed to the back part, used to ‘hear’) diversified among toothed whale species relatively quickly.

Coombs still has more questions. “The mandible itself is really thin. The back part of the mandible, called the panbone, is in some places less than a millimeter thick,” she explained. If a thin mandible is necessary to echolocate, how does evolution balance hearing sensitivity and jaw strength?

“You want to be able to have your jaw receiving sound very clearly and transmitting it to your inner ear so that you can interpret your echolocation,” she said. “But you also want to be able to bite down on the meal you’re finding — especially if you’re a killer whale hunting other marine mammals.” These questions of form and function are where she hopes to bring her research next.

From sperm whales’ hyper-specialized form of echolocation used to hunt squid in deep bathypelagic environments, to “essentially blind” river dolphins relying entirely upon echolocation to hunt fish in dark, muddy water, the unique evolutionary history and anatomy of toothed whales has allowed these animals to survive and thrive in diverse, extreme environments — none of which remain free of human impact.

Preview a dolphin skull on a white background
(Michelle Pemberton, The Children's Museum of Indianapolis)

Studying the relationship between toothed whale diet, behavior, and anatomy may be helpful to understanding the consequences of a changing ocean. Not only are waters warming from growing greenhouse gasses in the atmosphere, but oceans worldwide are getting noisier from busy shipping traffic and marine resource exploration. Understanding the role that sound has played and continues to play in toothed whale ecology and evolution is vital to anticipating problems that might be posed by these new environmental conditions, from disrupting certain modes of feeding to potentially causing physical harm. And as apex predators, toothed whales can serve as key indicators of health for ecosystems as a whole — “canaries in a coal mine,” known to scientists as ‘sentinel species.’ As it turns out, in the jig-saw of preserving the sea, toothed whales with their sound-sensitive jaws are an important piece of the puzzle. Their decline or demise would foreshadow immense loss of all other parts of the ocean world. 

March 2024
Tags: Whales