Ancient Whales Could Hear 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
by GrrlScientist for Forbes | @GrrlScientist
Mourlam and Orliac describe the inner ear of two partly terrestrial early whales (Protocetidae). Comparisons with other fossil and modern whales and terrestrial relatives reveal that earliest whalesβ hearing capacities were similar to those of their terrestrial kin and that infrasonic/ultrasonic hearing evolved after the emergence of modern whales. (Credit: Current Biology (2017) / doi:10.1016/j.cub.2017.04.061)
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 β and of course, what they eat affected whether they are baleen or toothed whales. This raises the question that has generated a lot of controversy: when did these hearing differences first begin evolving?
(Credit: U.S. National Oceanic and Atmospheric Administration (NOAA) / Public domain)
(Credit: Gabriel Barathieu / CC BY-SA 2.0)
The structure of the cochlea, the headquarters of hearing in the ear, is described for two ancient protocetid whales
Paleobiologist MickaΓ«l Mourlam, a graduate student at the UniversitΓ© 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 46β43 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 whalesβ 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 that is 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.
βThis 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,β said the studyβs co-author, Dr. Orliac.
β[T]he extreme density of the bone in protocete cetaceans made it difficult to work with the scanned images,β Dr. Orliac elaborated in email. βThe 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.β
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 βthin slicesβ are reassembled using computer software to create a three dimensional image (Figure 1):
(DβF) Protocetidae indet. UM-KPG-M73.
(A and D) In situ cochlea through a translucent rendering of the petrosal bone presented in ventral view.
(B, C, E, and F) Detail of the cochlea in medial (B and E), ventral Β©, and antero-ventral (F) views. Abbreviations: ca, cochlear aqueduct; cc, cochlear canal; fc, fenestra cochleae; fv, fenestra vestibuli; pb, petrosal bone; sbl, secondary bony lamina (imprint of basal ridge).
βBased 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,β 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):
This dataset was compiled based on the PCA of Churchill et al. [ref], augmented by the two protocetids from Kpogame, ten land artiodactyls, and data from the literature [ref]. Red stars represent the protocetids; red circles represent the other archaeocetes. Triangles represent extant (orange) and Paleogene (yellow) land artiodactyls. Squares represent Oligocene (cyan), Miocene (light blue), and more recent (black) mysticetes. Crosses represent Oligocene (pink), Miocene (magenta), and more recent (purple) odontocetes. The large symbols correspond to the centroid of the three main morphospaces. Abbreviations: IF, infrasonic frequencies; MF, midfrequencies; UF, ultrasonic frequencies.
βWe found that the cochlea of protocetes was distinct from that of extant whales and dolphins and that they had hearing capacities close to those of their terrestrial relatives,β Dr. Orliac said.
Further, these structural similarities suggest that early whales could neither echolocate nor communicate underwater using infrasound.
But βthey were most probably able to communicate underwater, like seals do,β 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.
β[S]ensitivity to high frequency sounds (ultrasounds) and echolocation provide advantage for hunting furtively or with reduced visibility to tooth whales, which are piscivores,β Dr. Orliac said in email. βOn 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.β
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 that their relatives living on land also have.
βOur findings show that infrasonic and ultrasonic hearing evolved in Neoceti, after the emergence of fully aquatic whales,β write the authors in their paper. Neoceti is a taxonomic group encompassing the major groups of modern whales, the toothed whales (Odontoceti) and the baleen whales (Mysticeti).
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:
MickaΓ«l 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 430:776β778 | 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):2144β2149 | 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):22β39 | doi:10.1111/joa.12253
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Originally published at Forbes on 8 June 2017.
