In their hunt for squids

Sep 11, 2007 11:23 GMT  ·  By

For a long time now, humans have been fascinated by the spectacular fights between the huge sperm whales and giant squids, that triggered the myth of the kraken, the island big sea monster. But how do the whales manage to detect the squids in the complete darkness of the deep ocean? It's all in their ultrasound sonar.

A team at the University of California, Berkeley discovered how dolphins and other toothed whales develop their sonar tool to chase schools of squids. As squids migrate during the day to deeper, darker waters, toothed whales need a complex echolocation system to allow them to detect the squids in the "refrigerator in the deep, where food is available day or night, 24/7", says co-author David Lindberg, UC Berkeley evolutionary biologist and professor of integrative biology.

"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. This set the stage for the evolution of the more sophisticated biosonar system that their descendants use today to hunt squids at depth," said Lindberg.

The team rebuilt the way whales evolved the sonar in parallel with the evolution of cephalopods like squid and nautiloids, relatives of modern shelled chambered nautilus. All toothed whales (odontocetes), echolocate; the baleen whales, which sieve krill and small fish from the water with the baleens (they are toothless), do not.

The sperm whale, up to 60 feet (20 m) long, is the biggest toothed whale and dives to 3,000 meters (1.8 mi) in search of squid (they eat exclusively squid); so do the deep ocean beaked whales.

Belugas and narwhals dive beyond 1,000 meters, while typical dolphins, porpoises, killer whales and pilot whales, can reach depths below 200 meters, where complete darkness 'reings'. "The first whales entered the ocean from land about 45 million years ago, and apparently did not echolocate. Their fossil skeletons do not have the scooped forehead of today's echolocating whales, which cups a fatty melon-shaped ball that is thought to act as a lens to focus clicking noises," said co-author Nick Pyenson, a graduate student in the UC Berkeley Department of Integrative Biology, who focuses on the evolution of the whales.

"Skulls with the first hints of a concave forehead and potential sound-generating bone structures arose about 32 million years ago, by which time whales presumably had spread throughout the oceans. Whales had developed underwater hearing by about 40 million years ago," said Pyenson.

"Whale biologists had various theories about echolocation, including that whales developed this biosonar soon after entering the water as a way to find food in turbid rivers and estuaries. The evolution of toothed whales, however, indicates otherwise. Whales first occupied the ocean, and only later invaded rivers," said Lindberg.

But the most realistic explanation claims echolocation enabled whales to more efficiently detect food in the darkness of the deep ocean. Cephalopods represent the most abundant and high-energy food resource in the ocean, food base for 90 % of all toothed whales. "Cephalopods have migrated up and down on a daily "diel" cycle for at least 150 million years," said Lindberg, a specialist in the evolution of sea mollusks.

When whales developed biosonar, nautiloids were the dominating ocean mollusks. Whales could prey easily on these hard-shelled, slow moving mollusks in surface waters at night by using a primitive sonar, an advantage over cetaceans that detected the prey only based on moonlight or starlight. Whales could thus follow the cephalopods in their downward migration into the darkness for daytime.

Most modern squid species spend the day at about 500 meters (1,500 ft) below the surface, and some even at 1,000 m (3,000 ft) deep, while during the night, 50 % of the squid climb to about 150 meters of the surface.

Over the millennia, cephalopod species in general - and especially shelled cephalopod species - fell as the whales thrived and their species number increased based on the nautiloid species; but this predation wiped out the nautiloid species, and about 10 million years ago, only 6 species of nautiloids survived out of the open ocean into protected Indo-Pacific reefs.

"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 are typically two feet (0.6 m) long or bigger and range up to the 40-foot (13 m)-long giant squid. Whales didn't need to have a very sophisticated sonar system to follow the nautiloids, they could just home in on the hard part. Only later, did they develop a complex system with finer resolution to detect and capture soft-bodied squid." said Lindberg.

"Whales, like bats, developed a sensory system for seeing with sound, and every single toothed whale echolocates in a different way, just like how different bat species echolocate in different ways," said Pyenson. Whales even specialized for different segments and depths of the water column, just like bats hunting at different heights for insects did. "Whales and bats are strong examples of convergent evolution to take advantage of unexploited food resources: nocturnal insects, in the case of non-migrating insectivorous bats, and nocturnal cephalopods, in the case of whales." said Lindberg.

Just like fruit bats, which do not need echolocation to detect their immobile food, have not developed a sonar, so filter-feeding baleen whales - which do not require a biosonar for feeding - did not evolve one.

The team based their conclusions on data coming from tectonics, paleontology, physiology, ecology, anatomy and biophysics. "Thinking from an evolutionary perspective about existing data from biology, paleontology and ecology could answer questions about the origin of echolocation in bats, shrews and other animals," said Lindberg.