EvolutionFifty million years ago
From biblical tales to more recent accounts, from Jonah to Moby Dick, whales have fed our collective imagination. Even after Aristotle’s enlightened treatise describing them as mammals, they continued to be depicted as sea monsters or water-spouting fish. Far from dispelling the aura of mystery around whales, our growing knowledge of their biology sometimes reveals animals far more surprising than the myths they inspired. Like that of our planet, the history of whales is partially written in stone. The first cetaceans appeared after the extinction of the dinosaurs, approximately 50 million years ago. Their ancestor was an ancient artiodactyl: a four-legged, even-toed, ungulate land mammal well adapted to running. Present-day artiodactyls (such as cows, pigs, camels, hippopotami and giraffes) and cetaceans share a common ancestor. Their relationship is so close that scientists propose including them in the same group: the cetartiodactyls. The trail taken by land mammals when they returned to the sea is a difficult one to trace. On the one hand, paleontologists long believed that a group of extinct ungulates, the mesonychids, were the most likely ancestors of cetaceans. On the other hand, several genetic and molecular clues indicated a close link between cetaceans and artiodactyls, particularly hippopotami. Thus, in 2001, scientists found the "missing link": skeletons of animals with typical artiodactyl anklebones and typical cetacean skulls. The skeletons, which were discovered in Pakistan, are those of Pakicetus, the oldest known cetacean. Contrary to present-day whales, this animal was not aquatic and its anklebones attest to its skill as a runner. This discovery allows us to discard the possibility that mesonychids were the ancestors of cetaceans. However, links between the hippopotamus and cetaceans remain murky. Are hippopotami closer to whales than the other artiodactyls? The specialists are not of a common accord. Modern whales are extremely well adapted to their aquatic lifestyle.
Millions of years of life at sea facilitated favourable transformations
for living in this new environment. Nostrils migrated to the top of the
head and became blowholes.
These adaptations belie the kinship that exists between whales and their closest living relatives. Yet, whales are mammals, like us. They reproduce and nurse their young as we do. Although adapted to life at sea, their behaviour patterns, from mating rituals to parental care, are typical of other mammals. Like other mammals, their social organization is varied and reflects the different ecological niches that they occupy. They have social structures, which are adapted to their feeding behaviour, to how they defend themselves against predators and to their mating strategies. Whales live their lives in rhythm with one inevitable necessity: the need to breathe. As with all mammals, whales have lungs. Whether resting, swimming or feeding they must come to the surface at regular intervals to replenish their supply of oxygen. Yet, they spend less than a quarter of their time at the surface. Certain species, like the sperm whale, will dive over 1000 m for more than an hour in search of food. Others, like the fin whale , will limit their descent to the first 200 or 300 m where they find their preferred prey: small fish or krill. The 22-metre-long fin whale requires but a few strokes of its powerful tail to attain this depth, which is barely 10 times its body length. It would be impossible for a human to dive repeatedly as whales do between the surface and the abyss. To accomplish this feat, whales possess several physiological adaptations that set them apart from most other mammals. Relatively speaking, whale’s lungs are no larger than ours are. Whales thrive underwater mainly by distributing oxygen more efficiently through their blood and muscles and by controlling their heart rate. |
