Evolution of Whales and Dolphins: From Land to Sea

Introduction to Cetacean Classification

Dolphins, porpoises, and whales belong to one of two cetacean families: the Platanistidae (freshwater dolphins) or the Delphinidae (which includes all other dolphins, porpoises, and the cetaceans commonly called whales). The distinction between porpoises and dolphins has caused considerable confusion, as the names are sometimes used interchangeably. There is, however, a technical difference: true porpoises are limited to a few species within the family Delphinidae that possess spade-shaped teeth; all the rest are properly called dolphins (Green, 1990).

How did whales and dolphins evolve? In his essay "The Social Function of Intellect," Nicholas Humphrey (1976) observed: "The open sea is an environment where technical knowledge can bring little benefit and thus complex societies — and high intelligence — are contraindicated (dolphins and whales provide, maybe, a remarkable and unexplained exception)." This observation helped launch the "Machiavellian intelligence" hypothesis, which proposes that more intelligent animals evolved into social, grouping, and cooperating creatures. The hypothesis is now widely accepted, though it has received relatively little attention in discussions of whale evolution, except among a few cetologists.

Science continues to add to our knowledge of animal social systems and behavior. Of the mammals known collectively as dolphins and whales — which belong to the same order — only about eighty-five species remain. Of these, only four have received the great majority of scholarly attention regarding their evolution: the killer whale (Orcinus orca), the bottlenose dolphin (Tursiops spp.), the humpback whale (Megaptera novaeangliae), and the sperm whale (Physeter macrocephalus). Because the sperm whale possesses the largest brain on earth, has great commercial significance, and maintains a highly social lifestyle, it has received the most intensive study.

Cetacean Intelligence, Social Behavior, and Culture

Studies of animal culture have not historically included whales. However, several field studies have now been undertaken. Observing patterns of behavioral variation in wild populations that cannot be explained by either genetic or environmental factors reveals good evidence for culture in several cetacean species. The bottlenose dolphin (Tursiops) shows the strongest evidence of sophisticated social learning abilities, including vocal and motor imitation, though many other species have yet to be studied in comparable depth. Researchers have also observed imitation and teaching behaviors in killer whales.

Experimental data alone cannot provide the full picture of culture in whales. The complex and stable vocal and behavioral traditions found in groups of killer whales (Orcinus orca) appear to have no parallel except among humans. These animals form an independent group that has evolved cultural faculties. Key factors in their study include the worldwide movements of cetaceans, the great variety of marine environments, large temporal scales relative to those on land, and stable matrilineal social groups. As Rendell and Whitehead (2001) noted, "There have been suggestions of gene-culture coevolution in cetaceans, and culture may be implicated in some unusual behavioural and life-history traits of whales and dolphins."

Sperm whales' interactions with one another are relatively accessible for observation and study. Their daily routines — including birth, predation, and death — have been examined extensively. Some of their behaviors are unusual and unexpected when compared to the sociological and ecological routines of land-dwelling animals. Many elements of sperm whale biology are directly tied to their ecology and behavior.

Evolutionary Origins: From Land to Sea

The sperm whale belongs to the order Cetacea, which encompasses all dolphins and whales. Cetaceans evolved from ungulate-like creatures that returned to the oceans approximately 60 million years ago, during the Eocene epoch. About 25 to 35 million years ago, the mysticetes (baleen whales) separated from the odontocetes (toothed whales).

Unlike fish, whales have bones in their fins that resemble large, jointed hands. Their spines move vertically, a characteristic more typical of a running mammal than of the horizontal movement of fish. Recent fossil discoveries in Pakistan have shed light on this mystery and have made it possible to trace the stages in the transition of cetaceans from land to sea.

Traditionally, scientists assumed that whales were related to mesonychids — carnivorous, hoofed animals resembling wolves — and that these creatures formed a sister group to the artiodactyls. Mesonychids possessed unusual triangular teeth similar to those of whales, leading scientists to conclude that whales evolved from a mesonychid ancestor. However, since 1990, DNA data has indicated that whales should be classified among the artiodactyls, as a sister group to hippopotamids. In their early evolution, the ancestors of dolphins and whales appear to have been slender, long-nosed animals with short legs, somewhat resembling weasels.

About 60 million years ago, as these animals first entered the water, they resembled crocodiles, with long snouts and short legs. Gradually, as they spent more time in the water, their legs grew smaller. They began to resemble sea otters — capable of diving for extended periods, with longer front limbs than hind limbs. The hind limbs shrank over time, though these animals still came onto land periodically. Their long tails began to function as rudders and eventually evolved into broad tail fins with "flukes" that aided steering and increased swimming speed. In certain lineages, the nostrils migrated progressively backward along the skull, eventually reaching the top of the head and becoming what we now call "blowholes." Some species lost their varied teeth and retained only uniform, spade-shaped stumps.

Conclusion: Modern Cetaceans and Their Legacy

Families of both toothed and baleen whales had evolved by the late Miocene and are still represented today. These include the baleen whale families Balaenopteridae (including the blue whale) and Balaenidae (right whales), and the toothed whale families Delphinidae (dolphins and killer whales), Physeteridae (sperm whales), Monodontidae (belugas and narwhals), Phocaenidae (porpoises), and Ziphiidae (beaked whales).

During the last century, whales became highly important subjects of evolutionary study, and whaling came under intense scrutiny and criticism as scientists documented the remarkable intelligence and sociability of these animals. The minke whale, for instance, is known for its unusual ability to recover rapidly from blows to the head and other stunning methods used in preparation for slaughter. In one well-documented incident, a Norwegian whaling vessel in northern waters was pulling in a harpooned minke whale when the animal revived, rammed the ship, broke the mast, and sent two crew members in the crow's nest tumbling into the sea — breaking one sailor's ribs — before escaping. The story spread worldwide under the headline: "Don't Get Mad, Get Even" (Chadwick, 2006). Such accounts underscore the complexity and resilience of cetacean behavior, and remind us how much we still stand to learn from these remarkable survivors of one of evolution's most dramatic transitions.

References

Chadwick, Douglas. Whales, Dolphins and Porpoises, 2nd ed. Checkmark Books / National Geographic, 1999.

Chadwick, Douglas. "Evolution of Whales." National Geographic, November 1, 2001.

Green, John. Whales and Dolphins Coloring Book. Courier Dover Publications, 1990.

Hasegawa, M., Adachi, J., & Milinkovitch, M.C. "Novel Phylogeny of Whales Supported by Total Molecular Evidence." Journal of Molecular Evolution, 44 (Suppl. 1): S117–S120, 1997.

Humphrey, Nicholas. "The Social Function of Intellect." In Bateson, P.P.G. and Hinde, R.A. (Eds.), Growing Points in Ethology, Chapter 9, pp. 303–317. Cambridge University Press, 1976.

Milinkovitch, M.C., & Thewissen, J.G.M. "Even-toed Fingerprints on Whale Ancestry." Nature, 388: 622–624, 1997.

Ogawa, R., and Kamiya, T.A. "Case of the Cachalot [Sperm Whale] With Protruded Rudimentary Hind Limbs." Scientific Reports of the Whales Research Institute, No. 12, pp. 197–208, 1957.

Rendell, Luke, and Whitehead, Hal. Behavioral and Brain Sciences, 24: 309–324. Cambridge University Press, 2001.

Struthers, John. "On the Bones, Articulations, and Muscles of the Rudimentary Hind-Limb of the Greenland Right-Whale (Balaena mysticetus)." Journal of Anatomy and Physiology (London), Vol. 15, pp. 141–321, 1881.

Thewissen, Hans. "Digital Library of Dolphin Development." The Thewissen Lab, November 4, 2006.