Parapontoporia, an extinct genus of long-snouted dolphins that lived off the Pacific coast of North America from the Late Miocene epoch until the Pliocene, was likely to have been able to hear within narrow-band high frequency ranges, according to a new analysis of Parapontoporia’s bony labyrinth.
Qiqi, the last confirmed Chinese river dolphin. Image credit: Roland Seitre / CC BY-SA 3.0.
“Whales, dolphins, and porpoises (cetaceans) embody one of the most dramatic transitions in mammalian history: the return to water from land,” said Vanderbilt University researcher Dr. Joyce Sanks and Senckenberg Research Institute and Natural History Museum’s Dr. Rachel Racicot.
“Consequently, this group acquired a series of aquatic adaptations, including movement of the nostrils to the top of the head, and streamlining of the body.”
“Echolocation evolved early in the evolutionary history of toothed whales (Odontoceti), in the Oligocene, and all modern toothed whales echolocate.”
“The biosonar clicks of most living toothed whales typically encompass a broad frequency spectrum, ranging from tens of kilohertz up to 150-170 kHz.”
“Conversely, certain toothed whales exhibit distinctive biosonar clicks that are narrow in bandwidth but have a high centroid frequency.”
“These unique clicks have peak frequencies ranging from 125 to 140 kHz and bandwidths of 11-20 kHz.”
Using high-resolution X-ray CT scans, the authors examined the inner ears of three Parapontoporia specimens from two species, Parapontoporia sternbergi and Parapontoporia pacifica, from the collections of the San Diego Museum of Natural History.
With the help of 3D models, they were able to prove that these creatures already possessed narrow-band high-frequency hearing during the Miocene, around 5.3 million years ago.
“The echolocation used by the animals also developed quite early in their evolutionary history,” Dr. Racicot said.
“The animals emit a sound wave that bounces off an object and returns an echo, which provides information about the distance and size of the object. Today, all toothed whales use this natural sonar system.”
“Echolocation is a logical hunting and communication strategy, especially in the sea, where sound travels five times faster than in the air, while at the same time visibility is often impaired.”
“What is particularly exciting is that these dolphins changed their habitat once again and left the marine environment to colonize rivers.”
Even today, there are still a few dolphins living in rivers. All six species are currently very rare and threatened with extinction.
As a relative of the Chinese river dolphin (Lipotes vexillifer), which was last documented in 2002, Parapontoporia offers insights into the transition from a marine habitat to a freshwater environment.
“We assume that selective pressure and/or ecological advantages caused this early and widespread evolution of echolocation in the dolphins we studied,” Dr. Racicot said.
“River systems are spatially complex habitats in which this form of orientation and communication was probably advantageous for the long-snouted dolphins.”
“Further research into the sensory systems of toothed whales can be an important tool to study the influence of habitat on cetacean hearing and to understand the evolutionary dynamics of marine mammals.”
The results were published this month in the journal Anatomical Record.
_____
Joyce Sanks & Rachel Racicot. Predicting ecology and hearing sensitivities in Parapontoporia – An extinct long-snouted dolphin. Anatomical Record, published online July 15, 2024; doi: 10.1002/ar.25538
Source : Breaking Science News