From the 180-metric-ton blue whale—the largest animal that has ever lived—to the 55-kilogram vaquita porpoise, cetaceans have the greatest size range of all mammals. How did this variation evolve? Over the years, scientists have suggested everything from differences in behavior to changes in sea level. But a new study fingers a simpler phenomenon: The size and shape of whales, dolphins, and porpoises results from their ancient dietary preferences.
Cetaceans first appeared about 55 million years ago, but these archaic whales, which resembled giant crocodiles and seals, vanished 20 million years later for unknown reasons. They were replaced by early modern cetaceans, which diversified into a far greater variety of sizes and shapes than was found among the archaic whales. Some researchers suspect that the modern cetaceans quickly diversified into new species because of the ecological void left behind following the demise of the archaic cetaceans, much like mammals did after the extinction of the dinosaurs. But neither the fossil nor the molecular record has given scientists a clear-cut answer about what drove this event—or even if it happened quickly.
So researchers led by Graham Slater, an evolutionary biologist at the University of California, Los Angeles, and Samantha Price, an evolutionary biologist at UC Davis, focused on body size instead of fossils or molecules. The team compiled average body length data from other researchers’ field studies for adult female cetaceans, because females are the larger sex, of nearly all 84 living cetacean species. Then they divided the cetaceans according to their dietary preferences: filter feeders, such as the blue whales, that gulp down huge quantities of krill; cephalopod specialists, such as sperm whales, that dive to great depths to feed on squid; and generalist fish eaters, such as bottlenose dolphins. Finally, the researchers created an evolutionary tree of cetaceans based on the molecular and fossil records and checked to see if the body size differences among cetaceans can be explained by their dietary preferences.
The results showed that whale lineages that evolved first generally kept to their chosen diets and apparently managed to keep out other groups, says Slater. Thus, when the filter-feeding mysticetes (which includes the blue whale) began scooping krill from the water, they essentially excluded other lineages from this niche. It’s thought that filter feeders evolved to be so large because their prey is abundant and easily caught. The fish-eating lineage, including vaquitas and dolphins, have actually decreased in body size over time for unknown reasons. The two oldest and distinct groups of squid eaters, sperm whales and beaked whales, grew large body sizes because of the physiological demands for diving to the great depths where their prey lives—a pattern that pilot whales repeated. They evolved fairly recently from smaller fish eaters yet developed large body sizes like the other squid-eating whales. The new study will appear  online tomorrow in the Proceedings of the Royal Society B.
Daniel Rabosky, an evolutionary biologist at UC Berkeley, calls the work “the most comprehensive look at patterns of species diversification and morphological and ecological trait evolution in whales.” Mark Uhen, a paleontologist at George Mason University in Fairfax, Virginia, adds that the study is the “first real test of the idea that the evolution of large body size in cetaceans is a dietary adaptation.”