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LatestPhoto
The hind limbs of ancient whales were replaced with large, powerful tails.
Photo: Iain Kerr

December 17, 2001
The Evolution of Whales
  Real Audio
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Log Transcript

As crewmembers on board the Odyssey, we literally spend our lives with whales. We delight in the sleek, streamlined appearance and effortless fluid movements of all the cetacean (whale) species we encounter at sea. In daylight we catch glimpses of them through the veil of the sea's surface, and at night their sounds permeate our lives.

The earliest mammals appeared about 200 million years ago as air-breathing, land-dwelling creatures that were warm blooded, had hair, and nursed their young. About 50 million years ago, the earliest known true whales appeared, having apparently evolved from large, carnivorous land mammals right after the earliest ancestors of the first whales made the first tentative motions to reinvade the sea.

As they became better adapted to their fluid world, whales lost all but a few of their hairs (some species have hair only during their fetal stage). But even though whales resemble fish, to which they are only distantly related, both groups swim using their tail fins, though the fins of the tails of fish move from side to side while those of whales move up and down.

LatestPhoto
The ocean environment shaped the bodies of these animals. Although from entirely different orders, they have both evolved similiar features to ensure ease of movement with reduced drag.
Illustration: Genevieve Johnson

This is an example of convergent evolution, in which apparently similar structures develop independently in organisms that have entirely different lines of descent. The reason similar external features appear is because selective pressures shape the development of important features of animals, by pushing them towards whatever solution gives the descendants of that animal the greatest advantage. In aquatic animals like whales and fish, the most efficient means of locomotion is apparently achieved by the sweeping motion of a flat, extension of the body—a tail in other words. It apparently makes little difference whether that tail moves from side to side (as in fish) or up and down as in mammals like whales, it comes down merely to the fact that it is a flattened oar, a tail, that makes the most effective means of locomotion underwater.

Hundreds of millions of years before whales appeared, fish had evolved fins and tails for fast, energy efficient movement underwater. But because cetaceans came along after fish, if they were ever to prey on fish in the open ocean they had to be able to swim as well as fish. In achieving this, the bodies of whales became streamlined, reducing turbulence and drag. A thick layer of fat called blubber replaced the fur of their forebears. In Right Whales, this layer may be as much as one and a half feet thick in some places. The main function for this fat layer is not to keep the animal warm in cold water, as is often stated, but to store energy. Fat reserves are like fuel tanks: they act as energy stores during the long periods that whales fast each year while migrating between their low latitude wintering grounds and their high latitude feeding grounds.

As water buoys up a whales' body weight, the primary function of the skeleton became as a framework to stiffen the body and maintain its shape rather than as strong struts to support that massive weight above the ground-something that requires bones more like steel girders. This meant that the bones of whales could afford to be much weaker. And indeed, one finds that the bones of whales are light, porous, and filled with oil, which reduces the relative density of the whale and keeps it from sinking.

The forelimbs of whales took on a different function; they developed into flippers which help the whale stabilize and steer itself while swimming. Interestingly, they still retain identifiable wrist and finger bones. However, the remnant bones of the hind limbs of modern whales are very small. They are not connected to the spine and are often absent altogether.

LatestPhoto
Wrist and finger bones in sperm whale flippers, reveal a sea mammal that once lived on land.
Photo: Chris Johnson

Hind limbs were replaced with a large and powerful boneless tail supporting strong, stiff flukes. It is perhaps the whale's most distinguishing feature. During the course of evolution, the jaws of whales grew forward to prodigious distances and the nostrils shifted from the tip of the snout to the top of the skull, a place wherein they made breathing easier for the whale.

The subject of our research, the Sperm Whale, has evolved into the largest odontocete (or toothed whale) and is the largest predator that has ever lived on the planet. They are among the deepest diving cetaceans, and are found in all the world's oceans. The sperm whale's huge skull encloses the largest brain this planet has ever seen, although its function can still only be guessed at.

Millions of years and the necessity to survive and thrive in the world's oceans has shaped and perfected the body form of the whales we see today. Over a period of time we cannot really comprehend, natural selection has favored attributes that ensure the survival of whales.
This is Genevieve Johnson speaking to you from the Odyssey.

Log by Roger Payne & Genevieve Johnson

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