Migrating Squid Drove Evolution of Sonar in Whales

Sea Technology, Oct 2007

Behind the sailor's lore of fearsome battles between sperm whales and giant squid lies a deep question of evolution: How did these leviathans develop the underwater sonar needed to chase and catch squid in the inky depths?

Now, two evolutionary biologists at the University of California (UC), Berkeley, claim that-just as bats developed sonar to chase flying insects through the darkness-dolphins and other toothed whales also developed sonar to chase schools of squid swimming at night at the surface.

Because squid migrate to deeper, darker waters during the day, however, toothed whales eventually perfected an exquisite echolocation system that allows them to follow the squid down to that "refrigerator in the deep, where food is available day or night, 24/7," said evolutionary biologist David Lindberg, UC Berkeley professor of integrative biology and co-author of a new paper on the evolution of echolocation in toothed whales.

"When the early toothed whales began to cross the open ocean, they found this incredibly rich source of food surfacing around them every night, bumping into them," said Lindberg, former director and now a curator in UC Berkeley's Museum of Paleontology. "This set the stage for the evolution of the more sophisticated biosonar system that their descendents use today to hunt squids at depth."

Lindberg and co-author Nick Pyenson, a graduate student in the UC Berkeley Department of Integrative Biology and the UC Museum of Paleontology, reconstructed this scenario after looking at both whale evolution and the evolution of cephalopods like squid and nautiloids-relatives of today's chambered nautilus-and relating this to the biology of living whales and cephalopods.

The most convincing explanation, that echolocation allowed whales to more efficiently find food in the darkness of the deep ocean, ignores the question of evolution.

Over the millennia, cephalopod species in general fell as the number of whale species boomed, possibly because of predation by whales. Then, about 10 million years ago, the whales seem to have driven the nautiloids out of the open ocean into protected reefs. Lindberg said that the decline in nautiloid diversity would have forced whales to perfect their sonar to hunt soft-bodied, migrating squid, such as the Teuthida, which in the open ocean can grow up to 40 feet long.