Ancient Whales' Hearing Was Like Hippos And Camels, Fossils Show

Ancient whales were able to hear like their terrestrial ancestors, according to a new study based on analyses of the structures of ancient whale ear bone fossils

Whales rely on their keen sense of hearing to navigate, to communicate and to forage. So it may surprise you to learn that the two major groups of modern whales, the baleen whales (Mysticetes) and the toothed whales (Odontocetes), hear best at opposite ends of the sound spectrum: baleen whales, such as humpback whales, hear infrasonic frequencies that are too low for humans to hear, whereas toothed whales, such as sperm whales, tune in to ultrasonic frequencies that are too high for humans to hear.

The different auditory frequencies that modern whales perceive also affect nearly every aspect of their lives and natural history, from the sort of habitats they live in to what they eat. This raises the question that has generated a lot of controversy: when did these hearing differences first begin evolving?

Currently, there are two competing hypotheses regarding which end of the hearing frequency spectrum evolved first: did the early whales (Protocetidae) first begin hearing low-frequency sounds (i. e; ref) or high-frequency sounds (ref)?

The structure of the cochlea, the headquarters of hearing, is described for two ancient protocetid whales

Paleobiologist Mickaël Mourlam, a graduate student at the Université de Montpellier in France, and his dissertation advisor, paleobiologist Maeva Orliac, a researcher at the the French National Center for Scientific Research, unearthed the fossilized ear bones of early whales from deposits that had accumulated between 46–43 million years ago. These deposits are located in Kpogame, a mining site in the African nation of Togo.

The early protocetid whales whose fossil petrosal bones were examined for this study were transitional forms that spent part of their time on land and part of their time in water. By investigating the structure of their petrosal bones, Mr. Mourlam and Dr. Orliac were traveling back in time. They compared the anatomy of early whales' petrosal bones with those of their relatives, the even-toed ungulates, a group that includes terrestrial and semi-aquatic animals such as hippopotamuses, pigs, and camels. Based on those analyses, they could then decipher the hearing abilities of protocetids.

Mr. Mourlam and Dr. Orliac used Micro x-ray Computed Tomography (micro-CT) to examine the internal structures of the two fossil petrosal bones. Micro-CT scanning designed to probe fossils uses x-rays that are far more intense than those employed by conventional medical micro-CTs used to examine the insides of living beings. These powerful micro-CTs can examine three dimensional structures within a fossil specimen without having to destroy it by cutting it up into a series of thin slices.

“This process was long and difficult because this cavity was filled with sediments and partly recrystallized and because the petrosal bone in cetaceans is particularly thick and dense, which lowers the quality of the images and sometimes impedes analyzing them,” said the study's co-author, Dr. Orliac.

“[T]he extreme density of the bone in protocete cetaceans made it difficult to work with the scanned images,” Dr. Orliac elaborated in email. “The later had a very low contrast because of the density bone. We spent days reconstructing the internal structures of the skull fragment based on scans.”

An individual image is created by placing a sample under the x-ray generator, which emits X-rays that travel through the sample and are documented by an x-ray detector on the opposite side. After the image has been completed, the specimen is rotated slightly to a new position and the process is repeated until the entire specimen has been x-rayed. The resulting series of micro-CT “thin slices” are reassembled using computer software to create a three dimensional image (Figure 1):

“Based on the scans provided by the scanner, we could extract a virtual mold of the hollow cavity that used to contain the hearing organ when the animal was alive,” Dr. Orliac explained.

The resulting three dimensional representations of the interior structure of the cavity within the petrosal bone is known as an endocast. These endocasts allowed the researchers to analyze the internal cavities within the petrosal bone, which contains the cochlea, a spiral bone that is the auditory portion of the inner ear (Figure 1). This bone resembles a coiled snail shell.

Early amphibious whales had hearing capacities close to those of their terrestrial kin

Mr. Mourlam and Dr. Orliac compared nine parameters of the cochlea of terrestrial and semi-aquatic even-toed animals (orange area, Figure 2) to those of the early whales (red area, Figure 2) and to modern whales (Mysticeti: blue area; Odontoceti: pink area, Figure 2):

Further, these structural similarities suggest that early whales could neither echolocate nor communicate underwater using infrasound.

But “they were most probably able to communicate underwater, like seals do,” Dr. Orliac added.

Echolocation and specialization to infrasonic or ultrasonic hearing as seen in modern whales came later, only after whales had returned to the sea.

“[S]ensitivity to high frequency sounds (ultrasounds) and echolocation provide advantage for hunting furtively or with reduced visibility to tooth whales, which are piscivores,” Dr. Orliac said in email. “On the other end of the spectrum, sensitivity to infrasonic noise provides advantages for long distance communication and has been selected within baleen whale which travel far across the ocean.”

Extreme hearing abilities in whales derive from a mid-frequency ancestral ear

Based on their studies, Mr. Mourlam and Dr. Orliac conclude that the extreme hearing abilities of modern whales is derived from a mid-frequency ancestral ear.

“Our findings show that infrasonic and ultrasonic hearing evolved in Neoceti, after the emergence of fully aquatic whales,” write the authors in their paper. Neoceti is a taxonomic group encompassing the major groups of modern whales (Odontoceti and Mvsticeti).

According to Dr. Orliac, she and Mr. Mourlam plan to return to Togo in December to search for more protocetid whale specimens. So far, they have examined two of the three whale species identified in Togo, so they hope to find another specimen that will allow them to explore the ear of the third species.

Source:

Mickaël J. Mourlam and Maeva J. Orliac (2017). Infrasonic and Ultrasonic Hearing Evolved after the Emergence of Modern Whales, Current Biology, published online on 8 June 2017 before print | doi:10.1016/j.cub.2017.04.061

Also mentioned:

Sirpa Nummela, JGM Thewissen, Sunil Bajpai, S Taseer Hussain & Kishor Kumar (2004). Eocene evolution of whale hearing, Nature 430:776-778 | doi:10.1038/nature02720

Morgan Churchill, Manuel Martinez-Caceres, Christian de Muizon, Jessica Mnieckowski, and Jonathan H. Geisler (2016). The Origin of High-Frequency Hearing in Whales, Current Biology 26(16):2144–2149 | doi:10.1016/j.cub.2016.06.004

Ekdale, E.G., and Racicot, R.A. (2015). Anatomical evidence for low frequency sensitivity in an archaeocete whale: comparison of the inner ear of Zygorhiza kochii with that of crown Mysticeti, Journal of Anatomy, 226(1):22-39 | doi:10.1111/joa.12253

More whale science stories by GrrlScientist:

GrrlScientist (2018). “Bowhead Whales: Songbirds Of The Arctic Sea”, Forbes (link).

GrrlScientist (2018). “Extinction Looms for North Atlantic Right Whales”, Forbes (link).

GrrlScientist (2018). “‘Talking’ Orca Learns To Mimic Human Speech”, Forbes (link).

Ancient Whales' Hearing Was Like Their Land-based Relatives, Fossils Show | @GrrlScientist