Large whales are notoriously hard to study. Except when rising to breathe, they swim beneath the ocean's surface out of human sight, which makes it difficult to find and track them. They often live far from land, beyond human reach, and can be quite shy if people do approach. Even if scientists could catch up with them, large whales are too big to capture for study at sea or in captivity.
Modern technologies, like GPS trackers, recordings of whale sounds and DNA testing, have helped us to learn more about whales in the wild. New technologies can also help us learn from museum collections of whales, some of which have been preserved for more than 100 years. In a new study, scientists at the Smithsonian’s National Museum of Natural History did just that. Using a CT scanner, they studied the tiny fetal ear bones of 15 whale species (56 museum specimens total) to better understand how whale hearing evolved over millions of years.
Sound is the dominant sense for whales, and they use their ears like we use our eyes. Underwater clicks, songs and echolocation allow them to communicate with one another, "see" and navigate their worlds, and find food. To perceive and make sense of these complex auditory signals, they have evolved a complex auditory system. But it didn't happen overnight. Whales evolved from animals on land (early relatives of hippos) over a period of 50 million years, slowly gaining their ability to hear sound underwater. At some point during their evolution, the whales split into two groups (toothed and baleen whales), gaining different traits and specializations.
Hearing is important for both baleen and toothed whales, but the mechanisms they use to take in and interpret sound may be different. Toothed whales (including dolphins, sperm whales and beaked whales) receive high-frequency sounds through specialized “acoustic fats” that lead to their internal ears. They use echolocation, or biological sonar, to navigate and “see” objects. Baleen whales (including humpback whales, blue whales and right whales) specialize in hearing low-frequency sounds for long-distance communication. They also have fatty tissue that seems important to their hearing, but the specific mechanisms are not well understood. So, how did these two groups come to have such different ways of hearing?
Maya Yamato, the lead author of the study and a Peter Buck postdoctoral fellow in the Museum’s Departments of Vertebrate Zoology and Paleobiology, along with Nick Pyenson, the Museum Curator of fossil marine mammals, dove into hundreds of years of whale collections to better understand the evolution of whale hearing. Instead of looking at the ears of adult whales, they focused on unborn fetal whales. This is because, early in development, all whales have approximately the same ear structures, shared among all mammals. As the whale ears develop further in the womb, the ears of each species begin to develop differently. Analyzing these early differences can reveal the major changes that led to different whale ear structures and ways of hearing.
To avoid any damage to the specimens, Maya and Nick generated detailed 3-D images of 56 whale fetuses using the museum’s CT scanner, a machine typically used in hospitals to see inside patients' bodies. The specimens, featuring 15 species of baleen and toothed whales, were collected by the Smithsonian over many years. Some were salvaged from whaling operations during the early to mid-1900s, while others came from more recent cases of whale strandings and bycatch from fisheries. Combining Maya's expertise on modern whale ear structure with Nick's expertise on fossil whales, they compared the ear development of these unborn modern whales to known changes in fossilized ears of extinct whales over millions of years.
They found that the developmental changes in fetal ear bones paralleled changes in ear structure throughout whale evolution. The youngest fetal ears of all whale species had features that are found in fetal land mammals, just as the oldest fossil whale ears shared many features with land mammals. As the whale fetuses developed further, both groups developed a structure called the acoustic funnel, but the position of the funnel differed between the toothed and baleen whales. The acoustic funnel in fossils is more difficult to observe, but because Maya and Nick found the structure in both toothed and baleen whale fetuses, it likely evolved in early whales before the two groups split—around 34 million years ago. The detailed mechanisms of hearing in whales is still an area of active research, so the new information may help scientists better understand how the different groups hear.
“This research provides a window into evolutionary processes that took place millions of years ago and helps explain how whales evolved to hear after they moved from living on land to thriving in today's oceans," Maya says. It may also help whales living today. Their underwater world is becoming increasing noisy from increased boat traffic, seismic surveys and sonar. Learning more about how whales hear is an important step forward in determining how our noise affects them so that we can better protect them in the future.