Evolution of Whales

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Sometime between 70 million and 50 million years ago, after the last of the dinosaurs had died and mammals had inherited the land, one or more groups of mammals waded back into the water, presumably to feed on the abundant plant and animal forms there. These pig-sized, four-legged, warm-blooded, placental creatures adapted quickly to their new habitat and soon gave rise to a new branch in the evolutionary tree — the order Cetacea, which today includes all of the world’s whales, dolphins, and porpoises.

Three groups of cetaceans arose from the land-dwelling ancestor or ancestors. The earliest group, the Archaeoceti, or ancient whales, died out about 20 million years ago. Of the surviving groups, one, the Odontoceti, or toothed whales, evolved specialized teeth to grasp fish and other relatively large prey such as squid, while the other living group, the Mysticeti, or baleen whales, lost their teeth and developed very large mouths equipped with filtering fringes or baleen with which they trap large numbers of very small organisms.

Both of these evolutionary paths proved to be successful, and each group has diversified to fill various niches. Although the demands of mobility, heat conservation, and sensory awareness in an aquatic environment have caused both groups to evolve superficially similar body forms, they are quite different animals.

It is not yet clear whether today’s whales have a common ancestor. However, currently favored theory holds that both primitive baleen whales and toothed whales derived from the archaeocetes, and they, in turn, had probably evolved millions of years earlier in the Paleocene or Lower Miocene epoch from a group of small, generalized, carnivorous land mammals called creodonts. The earliest and most primitive cetacean fossil yet found, Pakicetus inachus, dates from the early Eocene epoch (about 60 million years ago) of Pakistan and it seems to have been an amphibious creature preying on fish in the shallow waters of the ancient eastern Tethys Sea.

Evolution of Whales

THE TOOTHED WHALES

The Odontoceti, or toothed whales (from the Greek: odontos — tooth; ketos — whale), is comprised of 68 generally recognized species that include dolphins, beaked and bottlenose whales,  and porpoises as well as the giant sperm whale made famous by Herman Melville’s  Moby Dick. All of these whales are characterized by one external nostril, or blowhole, as well as teeth. Toothed whales typically use their teeth to seize prey such as squid, shrimp, fish, or other creatures which are then swallowed whole. With few exceptions, whales generally do not tear apart or chew their food.

Among the toothed whales, several families — close related groups of species — have evolved, each with distinct behavior and geographic distribution. Each of these families will be discussed at greater length in the chapters you can access on this site, but briefly they are:

•  Physeteridae: Large, deep-diving, gregarious sperm whales — and dwarf and pygmy sperm whales — with a protruding forehead and numerous conical teeth that erupt and are functional only in the lower jaw.

•  Ziphiidae: Medium-sized, deep-diving whales whose snouts are elongated, giving them a “beaked” appearance — having, with one exception, a few peglike teeth (usually two) which erupt in the lower jaw only and are apparently not functional in feeding. The exception, Shepherd’s beaked whale, has numerous small, conical teeth in addition to two large teeth that erupt in males.

•  Delphinidae: True dolphins, small to medium-sized, including some called “whales” with many functional interlocking conical teeth in the upper and lower jaw. These dolphins are adapted to a marine habitat.

•  Monodontidae: Medium-sized whales with relatively few teeth, generally nonfunctional in feeding. There are only two species, both adapted to Arctic marine waters. In one of these, the narwhal, a single highly specialized tooth forms a long spiral tusk.

•  Platanistidae: Small dolphins with a long, slender snout, with many functional interlocking, conical teeth in the upper and lower jaws. For the most part, these dolphins have become adapted to freshwater and estuarine habitats.

•  Phocoenidae: Porpoises with an inflexible neck and small, spade-shaped teeth. Their distribution is entirely marine with a tendency toward inshore rather than pelagic (offshore) waters.

In their habitats, the toothed whales have developed along three principal lines of distribution and prey selection. Some are oceanic deep divers that feed upon bottom-dwelling species. Others are oceanic but feed on prey species living at or near the surface of the sea while still others occupy the productive inshore habitats. The first group includes the giant sperm whale, which is capable of hour-long dives at depths exceeding one mile (1.5 k), and the two smaller species of sperm whale; the Ziphiidae, or beaked whales, which are generally found in areas of water depths greater than one mile (1.5 k), particularly near the edges of major ocean currents and deep sea escarpments; and some species of Delphinidae, notably in the genera Globicephala and Grampus. These oceanic creatures occasionally venture into shallow waters in their search for prey, and they may drift inshore with ocean currents, but are maladapted to cope with shallows and shorelines and may become disoriented and strand themselves.

The oceanic near-surface feeders are characterized by the dolphins such as those in the genera Stenella, Delphinus, and Lagenorhynchus.  These are gregarious animals that often travel in herds of up to several thousand individuals. Their distribution is worldwide in temperate and tropical seas., with Delphinus venturing in large numbers into the Mediterranean Sea, the Black Sea, and the Gulf of California. These oceanic near-surface feeders generally eat squid and fish associated with the deep scattering layers (DSL), often by night as the prey species rises to within the first three hundred-plus feet ( 100 meters) or so of the sea’s surface.

Inshore feeders are represented by the bottlenose dolphin and the harbor porpoise which may be found as close inshore as the surf zone, as well as in many of the busiest harbors on the world. The smaller harbor porpoise is very shy and secretive in its habits, while the friendly and curious bottlenose dolphin frequently announces its presence by splashing and sporting in bow-waves of vessels under way. These inshore species feed upon schooling fish, such as herring and mullet, and a great variety of invertebrates and near-bottom species.
In addition to these offshore/inshore and deep-water/shallow-water distinctions, whole species’ distribution patterns may be dictated by water temperature, most probably as temperature determines the distribution of less mobile prey species. While the vast majority of the toothed whales inhabit the world’s temperate seas — perhaps moving slightly toward colder regions to feed and warmer ones to breed — a few toothed whales have adapted entirely to freezing arctic waters, some others to warm tropical seas.

Wherever they may live and feed, all toothed whales share a sonar-like ability to examine their surroundings and find prey by echolocation. While most terrestrial mammals depend largely on vision for awareness of their environment, marine mammals live with the scarcity of light. The whales’ ancestors, who probably saw quite well in the air, necessarily adapted to the poor visibility typical of underwater environments.

Dolphin Echo-location Graphic   Hearing, however, is not impaired underwater as sight is. On the contrary, it is enhanced because water transmits the pressure waves of sound much more rapidly and effectively than does air, even for extreme distances and depths. Accordingly, the toothed whales, like bats, have developed means by which they emit special sounds that travel out from their heads and reflect off objects around them, producing echoes they can hear and interpret. In this fashion they locate objects, including prey, in their vicinity, and perceive instantaneously the range, bearing, and configuration of each. Some dolphins, tested in aquarium conditions, were able to distinguish a fish they like to eat from one they did not, solely by echolocation, even though the two fish were identical in size and shape. Presumably, the dolphins determined the texture or internal structure of the two fish by echolocation. In other tests, dolphins have been able to distinguish between objects the approximate size of a B-B shot and a kernal of corn at a distance equivalent to 50 paces. Their acoustic discrimination is superb! At ease with their acoustic abilities, blind or blindfolded, dolphins will move about freely and feed normally, but a deaf or deafened dolphin will become frightened, disoriented, and reluctant to move.

We humans usually consider that only we can think conceptually and make use of sophisticated linguistic constructs for communication. Animal communication, if we assume that animals can communicate at all, is generally believed to be inferior. Our assumption of superiority in this regard may be unwarranted. In addition to finding food and resolving exquisite detail in their surroundings by echolocation, many toothed whales have obviously developed the ability to communicate with one another using whistles, clicks, and calls of various sorts. No one really knows if the say anything, much less whether they are capable of abstract thinking. It is certain, however, that they do communicate, and they often coordinate very complex group activities by such communication.

Nevertheless, sophisticated acoustic skills, ability to communicate, and a large and well-developed brain do not prove that cetacean intelligence is equal or superior to that of human beings as some investigators have suggested. Both humans and cetaceans have evolved in different ways, adapting to very different conditions, and it is we who define intelligence in such a way as to exclude other animals. Marvelous cetacean “intelligence” may one day be proven, but that will not be until we can understand their communication. Until then we must be patient. Modern explorations into interspecies communication have just begun and the results of such explorations must be evaluated carefully. After all, human beings have developed a sonar capacity through the use of machines only in the past few decades, while odontocetes have been refining their acoustic apparatus for about 50 million years. It should come as no surprise that they are spectacularly proficient with it — more so then we by far.

Beyond echolocation, the ancestors of the odontocetes needed other adaptations to be successful in the marine environment. They needed mobility and agility in the water, and the capacity to make long, deep dives. The earliest cetaceans quickly became streamlined, losing their drag-producing hair and evolving a spindle-shaped body. To stay warm, they developed a blubber layer and improved their circulatory thermoregulation mechanisms. They also developed great sensitivity and control over their entire skin and their appendages, which evolved from legs, paws, and tails into paddle-shaped fins and flukes. Their nostrils migrated from the front of the face to the top of the head, where breathing could be accomplished with little or no hindrance to forward motion. And their respiratory physiology and internal anatomy changed, ultimately making possible — for some species at least — dives of astonishing depths and duration.
An adaptation no less successful than that of the toothed whales was the accomplishment by the second major group of cetaceans, the Mysticeti (from the Greek: mystax—moustache; ketos — whale). Instead of seizing one or two moderate-sized prey at a time, the Mysticeti, or baleen whales, consume enormous numbers of very small prey with every mouthful. The tremendous blooms of invertebrate organisms and small schooling fish that occur seasonally in various regions of the sea provide an ideal albeit episodic food supply for those creatures that can get to them first and eat the most. In response to these blooms, baleen whale evolution has been characterized by migrations to and from the feeding grounds — often journeys of thousands of miles — and development of sievelike food-gathering structures — baleen plates — in place of teeth.



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