The study reveals new evidence about how whales and dolphins use echolocation

The study reveals new evidence about how whales and dolphins use echolocation

This article has been reviewed in accordance with Science

The restoration shows Xenoropus hunting sea turtles. Credit: Robert Bosenecker

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The restoration shows Xenoropus hunting sea turtles. Credit: Robert Bosenecker

Study published in diversity Provides new insight into how whales and toothed dolphins navigate the underwater world using sound waves.

Whales and dolphins, which lack external ears, rely on a technique called echolocation to navigate and hunt in the dark. Much like screaming and listening to echoes, these animals make high-pitched sounds that bounce off and reflect off objects, allowing them to map their surroundings.

Their skulls and soft tissues near and inside the blowhole are asymmetrical, meaning the structure on one side is larger or differently shaped than its counterpart on the other side. This “imbalance” enables sound to be produced. Meanwhile, the fat-filled lower jaw bone transmits sound waves to the inner ear, allowing animals to locate the source of sounds (directional hearing).

However, how whales and dolphins evolved this sophisticated “built-in sonar” is not fully understood.

Now, research co-authored by Jonathan Geisler, Ph.D., professor and chair of the Department of Anatomy at New York Institute of Technology, and first author Robert Boessenecker, Ph.D., a paleontologist and research associate at the UCLA Museum of Paleontology, provides vital clues.

The fossils show the asymmetry seen in Xenorophus. Credit: Robert Bosenecker

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The fossils show the asymmetry seen in Xenorophus. Credit: Robert Bosenecker

The researchers analyzed a large collection of fossils that included two ancient species of dolphins within the genus Xenoropus, one of which is new to science. These species are some of the primitive members of the Odontoceti, a suborder of marine mammals that includes all living echolocation whales and dolphins.

Xenoropus was a large creature, about three meters long, that swam the waters of eastern North America 25-30 million years ago, likely feeding on fish, sharks, sea turtles and small marine mammals. Externally, it resembled modern dolphins but had several interlocking molar-like teeth, much like the ancestors of land mammals.

Similar to today’s toothed whales, Xenorophus had an asymmetry around its blowhole, although it is not as pronounced as its living relatives. Notably, it also had a pronounced twist and shift of the snout several degrees to the left. Previous studies in other ancient cetaceans (primitive whales) suggest that “snout curvature” may be related to asymmetric placement of fat bodies in the jaw, which increases directional hearing capabilities.

However, Xenorovos took this a step further. The fat bodies in its lower jaws, which function like the outer ears in land mammals, were angled, exaggerating directional hearing. The curvature of the snout and the tilt of the fat bodies were probably similar to the asymmetric ears of owls, which can detect the exact location of prey based on their sounds.

New evidence suggests that Xenorrhus, which has a less pronounced asymmetry near the blowhole, may not have been as adept at producing high-pitched sounds or hearing high frequencies as living odontocetes. However, he was able to locate the sounds. Therefore, Xenorphus likely represents a major shift in the history of how whales and dolphins use echolocation.

“While this asymmetry is seen in other ancient whales, Xenorrhus appears to be the strongest of any whale, dolphin or porpoise, living or extinct,” Bossenecker said. “In addition, although the asymmetry focusing on the blowhole in odontocetes today can be traced back to Xenorophus and its other relatives, the twisting and turning of the snout is no longer visible today. This suggests that Xenorophus is an important puzzle piece in understanding how Whales and dolphins have developed their ability to echolocate.”

Additionally, while many scientists focus on symmetry in nature, Geisler says their new study demonstrates the importance of examining asymmetry as well.

“Biological symmetry, or the mirror image of body parts across anatomical levels, is a key feature in the evolutionary history of animals and humans. However, our research shows the important role of asymmetry in adaptation to different environments, and asymmetry should be more closely examined in fossils.” “Rather than dismissing it as individual variation or assuming it is caused by geological distortion.”

As a next step, the researchers will examine other odontocetes and look for snouts that are curved to one side. These future studies could help determine whether this feature is widespread.

more information:
Robert W. Boessenecker et al., New skeletons of ancient dolphins November. (Mammals, Cetaceans) of the Oligocene of South Carolina, phylogeny, functional anatomy, asymmetry, pathology, and evolution of the earliest odontocetes, diversity (2023). doi: 10.3390/d15111154

Provided by New York Institute of Technology, New York Tech

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