Whale “sonar” an ancient invention!

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Whale “sonar” an ancient invention!

A new species of whale, described in Nature this week, shows the best early evidence for echolocation — the production of sounds to detect objects. The “new” ancient whale, Cotylocara macei, is known from a fairly complete skull and a few vertebrae found about a decade ago.

artist rendering of whale floating in turquoise green water
An artistic reconstruction of the 28 million-year-old Cotylocara macei as it patrolled the shallows near present-day Charleston, South Carolina.
Credit: Carl Buell

The skull carries the evidence for using reflected sound to understand the nearby environment, says first author Jonathan Geisler, an expert on whale anatomy at the New York Institute of Technology College of Osteopathic Medicine. The new species has a fairly complete skull, together with a few vertebrae, but little else.

Echolocation — or sonar — helps animals see in dark or murky conditions. Bats use it to hunt insects at night, and dolphins, porpoises and some whales use it to hunt in the black ocean depths.

The two major groups of whales use contrasting life strategies. Toothed whales — odontocetes — hunt fish and squid with the help of sonar. Baleen whales — mysticetes — are filter feeders that trap tiny, shrimplike organisms called krill. Although filter-feeders use no sonar, they do make those eerie whale songs that let them communicate across long distances.

Lines of evidence

yellowish brown skull of whale shows long thin upper jaw and slightly wider bottom jaw. Lots of teet.
The skull of the 28-million-year-old Cotylocara macei. Its anatomy and density variation indicate that this early toothed whale found prey with echolocation. Bottom: Air-filled sinuses help direct the outgoing sonar signal toward the front. The skull is at the Mace Brown Natural History Museum at the College of Charleston.
Credit: James Carew and Mitchell Colgan
grey computer-generated model of whale skull fossil. Colored areas along side of snout/mouth highlight 'air-filled sinuses'

The two major categories of odontocetes, the beaked and sperm whales, both echolocate, Geisler says. “They can dive more than a thousand meters down, where there is absolutely no sunlight.” Although studying echolocation in the deep ocean is gnarly, the closely related dolphins and porpoises certainly do it.

The new species, like the living whales and dolphins, have bones near the blowhole that create zones of contrasting density, with air-filled sinuses adjacent to dense bones.

The density contrast reflects and shapes the sound emerging from below the blowhole, Geisler says. “The sound emanates as a spherical wave, but when that impacts the density difference, the sound is reflected forward” to detect objects ahead of the whale.

The high-pitched hunting noise used by modern toothed whales, dolphins and bats does not travel far, but it does confer great accuracy about location. In contrast, low-pitched whale songs travel much further but have much less locational accuracy.

Listen to the sound of evolution

The new species, Cotylocara macei, died about 28 million years ago, and was fossilized in the area of present-day Charleston, S.C. A slightly older whale, from 32 million years ago, Simocetus rayi, also shows a limited amount of evidence for echolocation, Geisler says.

If echolocation began to evolve in whales about 32 million years ago (just after the filter-feeding whales split away from the toothed whales), that is more than 20 million years after the whales’ four-legged ancestors left land and took up residence in the ocean.

Although a few other animals make noises and listen for the echoes, Geisler says, “the complexity of the anatomy and the sounds does not match what you see in bats and whales.”

Other echolocators

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Scientists figure that bats and whales evolved sonar separately, but have recently found intriguing genetic similarities in their auditory systems, Geisler says. How could this happen, given their rather remote relationship? “They are doing the same thing, using sound to get as detailed an image of the surrounding environment as possible,” Geisler says, “and they are both mammals with a somewhat similar anatomy and genes.”

Ultimately, the new report may shed light on a “chicken-and-egg” question, Geisler says. “Did high-frequency hearing or the production of high frequency sound evolve first, or did they evolve together? We have one good data point, but we need to get several more fossils and understand their capability. That’s a key question that needs to be answered.”

– David J. Tenenbaum

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Terry Devitt, editor; S.V. Medaris, designer/illustrator; Yilang Peng, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive

Bibliography

  1. A new fossil species supports an early origin for toothed whale echolocation, Jonathan H. Geisler et al, Nature, Mar. 14, 2014.
  2. Homing In on Mammalian Echolocation
  3. Even Motionless Prey Isn’t Safe From This Bat’s Unique Echolocation Ability
  4. Dolphin Hear, Dolphin Do: Imitation by Echolocation