Early on, whale hearing may have been ho-hum.
Unlike today’s whales that specialize in making — and hearing — very high- or low-pitched sounds, early whales’ ears probably picked up noises somewhere in the middle, paleontologists Mickaël Mourlam and Maeva Orliac report June 8 in Current Biology.
Looking at CT scans of ancient whale ear bones allowed the researchers, from the University of Montpellier in France, to weigh in on a long-standing debate over the evolution of whale hearing. At the heart of the controversy is when whales got their super-hearing abilities. Those abilities allow today’s whales to chitchat long distance and use sound to locate prey with calls that fall under the radar of human ears.
Modern toothy whale species such as orcas specialize in high-frequency sounds, while humpbacks and other baleen species are great at detecting low-pitched noises. Toothed and baleen whales split around 35 million years ago, and paleontologists have speculated that the early whales already had some form of extreme hearing — either high or low, depending whom one asks (SN Online: 08/05/16). Both camps might be wrong, the new finding suggests.
The team used a CT scanner to see details in the remains from two fossilized whales unearthed at a phosphate mine in Kpogamé, Togo, sometime between 1973 and 1985. At 43 million to 46 million years old, these whales appear to be a mix between modern whales and their land-mammal ancestors. They had legs that allowed them the best of both worlds, going back and forth from water to land in a manner similar to seals.
“Their teeth tell us that they were hunting for fish, but the shape of their skulls and our study of their ears indicates that they could not echolocate like modern toothed whales,” says Orliac.
Inner ear bone scans show that the whales have the start of some features similar to modern whales. But some parts — particularly the diameter and coiling of the cochlear canal — looked more like those of their land-loving, average-hearing relatives. The shape of the inner ear determines the frequencies it picks up. These “proto” whales had neither ultrasonic (high frequency) nor infrasonic (low frequency) hearing, the researchers conclude.
“Midrange hearing in the earliest whales isn’t too surprising,” says Eric Ekdale, a paleobiologist at San Diego State University. These animals split time between land and water, and it makes sense that they’d adapt to communicate in both.
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Using a CT scanner that detect small details in bones, the researchers created a virtual model of a whale’s ear bone (left) and used that to determine the shape, size and orientation of the soft tissue that once comprised the ear (right). Whales hear low-frequency sounds at the top of their ear canal, and tiny hairs near the base of the ear canal pick up high-frequency sounds.
Lack of specialized hearing means that echolocation was a later innovation for whales. However, Orliac notes, just because the Togo animals didn’t broadcast as modern whales, doesn’t mean they couldn’t communicate underwater. Seals, for example, make sounds that can be heard above and below water.
The new analysis also suggests that the inner ear features needed for either form of extreme hearing didn’t emerge until after toothed whales and baleen whales parted ways. But, Ekdale thinks the possibility of a common ancestor with exceptional ears still exists. The direct relatives of modern whales would have been fully aquatic and would need some hearing abilities more akin to modern whales to signal and hunt exclusively underwater.
Fossilized whale ears are hard to come by, but finding and studying more samples could fill in holes in the timeline. “With inclusion of more species, the picture of whale hearing evolution will become clear, and this study is a great step in that direction,” says Ekdale.