We know that big baleen whales sing, and so it stands to reason that they can hear. But how? And what did their hearing evolve?
Photograph and 3D reconstruction from CT images of a blue whale (Balaenoptera musculus) fetus (USNM 260581). The specimen was collected in 1936, and is preserved in alcohol. Scale bar is 5 cm. Photo: M. Yamato / Smithsonian Institution.
Today, Maya Yamato and I published a paper, available open-access at PLOS ONE, that gets one step closer to answering those questions. Today's living whales fall into two very ecologically distinct groups: toothed whales, which echolocate; and baleen whales, which filter-feed. These two groups also hear at very different parts of the spectrum, with toothed whales at high frequencies and baleen whales at low frequencies. Since the 1950s, experimental work on captive dolphins has clarified that toothed whales are able to hear the high-frequency sounds (generated from their domed foreheads) using large fat bodies lodged in their jaws, that transmit sound to their ears -- a pathway totally unlike most land mammals, which hear through their ear canal. All whales lack such a canal, but its difficult to trace a hearing pathway in baleen whales for a lot of reasons: their jaws have no such fat bodies that connect to their ear bones, and dissecting them is a rare and unqualified mess.
During her Peter Buck postdoctoral fellowship, Maya turned to a special source of data to approach this problem from another way: could the extensive collections of fetal cetacean specimens at NMNH provide any clues? These collections include rare specimens (many from whaling and commercial fisheries) that are fragile and impossible to dissect by hand in any meaningful way. Instead, Maya did the heavy lifting of CT scanning several dozen specimens (56 in all), across a range of growth stages, and from over 10 different cetacean families, across both toothed whales and baleen whales. Many of these specimens spanned a growth ranges in whale development that have not previously been reported.
We focused our study on a structure known as the "ear trumpet," known to cetacean anatomists for over a hundred years, but difficult to trace through ontogeny unless you know what to look for -- and in the CT datasets, we traced it by way of two ossicles, or small bones of the middle ear: the goniale and malleus (in humans these two bones have complex origins and participate in the middle ear). In all whales the goniale and the malleus begin at a tight V-shaped structure, but soon depart in anatomical orientation depending on where they plot in the whale family tree: the majority have forward-facing ear trumpet, while rorquals and gray whales have side-facing ones. This finding builds on previous work by Maya suggesting that the orientation of the ear trumpet, especially in rorquals, is a solid indicator of the anatomical pathway for hearing in the largest cetaceans ever. Recent modelling studies point to yet more pathways for sound, so the question of how baleen whales (especially rorquals) evolved their unique hearing anatomy, is far from answered! Read our paper for more details!
3D reconstruction of the head of fin whale (Balaenoptera physalus) fetus, based on CT scans of a whole, fluid-preserved specimen (USNM 268884). Incipient ear bones highlighted in yellow, representing the malleus and goniale, become the acoustic funnel. Photo: M. Yamato / Smithsonian Institution.