Extreme Stories: A Whale of a Story
Part of the Extreme Mammals exhibition.
Some 65 million years ago during the Mesozoic Era "age of dinosaurs," the oceans of Earth were home to fish and fierce marine reptiles. Then, along with large, land-based dinosaurs, those marine reptiles went extinct.
About 50 million years ago, some groups of mammals took advantage of the more open oceans and began to invade the sea. Today, about 100 mammal species inhabit the oceans, but one group evolved an amazing array of adaptations for exclusive life in the water--whales.
Over time, the structure and features of the whales were shaped by evolution to make complete and total aquatic living possible. For locomotion, whales developed flippers from what used to be forelimbs. Instead of hair, a whale is encased in blubber that can be as thick as 20 inches (51 cm) to protect against frigid ocean temperatures.
The circulatory system of a whale provides for a slower heart rate and greater blood flow to allow long dives. To survive those long dives and to find food, whales use collapsible lungs to avoid the bends. For navigation they use echolocation to emit high-frequency noises and map out seascapes based on how those sounds bounce back. For feeding, one major group of whales evolved baleen--an extremely efficient filter-feeding device that works like a gigantic sieve to capture tiny crustaceans and other small prey.
Clearly, whales have evolved several adapations to survive in the water. But they retained the basic body plan of all mammals. If you compared your own arm and hand with the flipper of a whale, at first glance they would look very different, but beneath the skin the arrangement of bones is startlingly similar.
Such similarities are called homologies and are the result of descent from a common tetrapod ancestor--one with four limbs.
As mammals evolved, their bones retained the basic pattern of that common ancestor. So no matter how different mammals may look, they share an underlying body-plan. But this is not always obvious in some whales.
The blue whale is the largest living mammal on earth, being able to grow to nearly 200 tons. The heart is the size of a small car and the tongue weighs four tons.
The male narwhal has an eight-foot incisor tusk that can bend about one foot in any direction. The tusks have nerves in them and may be used to locate prey and detect changing ocean conditions. But because the tusks are limited to males, they may use them to attract females.
Some whales have mouths full of baleen plates that hang in opposing pairs from their upper jaws. Baleen grows constantly, renewing itself as the fringe, which faces inward into the mouth, is worn away. That fringe is made from separate "tubules" that act to trap very small animals that feed the whales.
The ties that bind whales with their common mammalian ancestors become much clearer when you scour the details.
Those plates of baleen, which can sometimes grow as long as 13 feet in the mouths of some whales, are made of the same protein found in hooves, horns, claws, and every mammal's hair--even your own.
And if the structural similarities between a human hand and a whale flipper aren't enough, how about a walking whale?
Ambulocetus natans lived about 49 million years ago in the shallow seas of modern-day Pakistan. Ambulocetus had squat legs that splayed from its body, flipperlike hind feet, and weighed about 400 pounds.
Despite its strong limbs, it probably could not walk well on land. Presumably, it waddled and pulled its body with its forelimbs--a bit like a living sea lion. In the water, Ambulocetus swam like a sea otter using its gigantic, probably webbed, hind feet.
Ambulocetus clearly had legs, and, though it may not seem like it, all modern whales retain them. The front legs of whales have evolved into flippers. Tiny remnants of hind legs inherited from their land-dwelling ancestors remain in the skeleton, but they perform no known function.
From this we see that all whales are actually descended from an ancestor that had four legs and walked on land!
So, while whales have undergone some pretty wild adaptations over millions of years, their story--like ours--threads back through evolution to a common ancestor and a time when our differences weren't so extreme.