The Berkeley Science Review: Articles

The Squid and the Whale
Ocean mammals evolved sonar to chase their prey (view PDF)
by Jacqueline Chretien

As arms races go, the Cold War pales in comparison to the epic struggle pitting squid against whale—or, to be more accurate, cephalopods against cetaceans. Recent work from David Lindberg's group in the integrative biology department suggests that the ancestors of modern whales developed the ability to echolocate (to detect objects by bouncing sound waves off of them) in order to hunt for dinner in the deep ocean.

"Whales weren't always water animals—they made a transition from land to sea," says Nick Pyenson, a graduate student in the Lindberg group. "And when they made this transition, they had to make changes to their sensory and skeletal systems to adapt to the marine environment." The land-based ancestors of whales weren't able to echolocate. But today, carnivorous whales like the orca, sperm whale, and bottlenose dolphin use a finely tuned sonar system to detect prey and other objects in their surroundings.

Whales have a specialized organ for this echolocation. "They use a muscle and soft tissue apparatus right above their blowhole that creates high frequency sound," explains Pyenson. "You want to send this [sound] out through water. One way you can do this is through fat, because it has a similar density to water. A bottlenose dolphin has this big bulb on its head, where the fat is, and this is where the vibrations get focused and transmitted to the environment." Fossil evidence analyzed by Pyenson and others in the Lindberg lab shows that the first whales did not have indentations in their skulls, indicating that they lacked the echolocation organ. Baleen whales that feed on tiny organisms like plankton also lack such an organ, leading Pyenson and Lindberg to theorize that echolocation probably evolved in carnivorous whales because it aided predation.

At the time the whales made the move from land to sea, about 45 million years ago, the oceans were dominated by cephalopods—giant snails, like the nautilus, and their cousins, the squid and the octopus. Because these animals were so abundant, the earliest toothed whales would have been able to hunt them without having to do much hunting at all: "They're really just hunks of meat for the whales to go after," says Pyenson. The impact of this abundant food source is clear today, as 90 percent of carnivorous cetacean species include cephalopods in their diet. However, they weren't always easy pickings. "Cephalopods have a daily cycle, coming to the surface at night and then going down in the depths in the daytime," Pyenson explains. Thus, early whales had to find their dinner by sight (or by bumping into it) in the moonlight.

Over millions of years, as predation made cephalopods scarcer, whales with even the most rudimentary ability to echolocate had a better way to "see" their prey in the shallows in the dark, giving them an edge over their competitors. Whale skulls with room for sound-generating structures appear in the fossil record around 35 million years ago. As whales' sonar improved, they were better able to track their prey, eventually following them deeper into the ocean during the daytime, which is the behavior observed today.

The theory merges nicely with the observed decline in cephalopod diversity and increase in whale diversity over time. "We plotted out the evolutionary success between these two groups, and saw a negative relationship," Pyenson says. In other words, as whales became more and more dominant in the oceans, cephalopod species were dying off.

The biggest victims—both literally and metaphorically—were the giant snails. "Nautiloids are at high risk of getting detected by echolocation. They have a really dense shell that reflects sound well," says Pyenson. "So the expectation is that echolocating whales would really just go to town on shelled ceph-alopods, which is the response we see over evolutionary time." Because they are so easy to detect, modern snails can no longer roam the open oceans. The lone survivors are relatively small species, which have found refuge in shoreline waters or difficult-to-access coral reefs. "[Hard-shelled cephalopods] are completely restricted to really shallow, protected areas now," Pyenson says.

Other than dying off, how did the ceph-alopods fight back? The loss of the shell was obviously helpful; soft-bodied cephalopods like the squid are much harder to detect by sonar and have been successful in the open ocean. Another response might have been to get smarter. Squid and related species are famously intelligent, and may have used increased brainpower to help evade echolocating predators.

The Lindberg group proposes two simpler responses: getting bigger and diving deeper. As Pyenson notes, "A bottlenose dolphin isn't going to go after a giant squid." But, as befits an arms race, the cetaceans have kept pace on both counts. Whale species are incredibly diverse in size, and most whales and dolphins are able to dive deeper than 200 meters, where no light can penetrate the water. Several species of whale can even dive deeper than 1,000 meters (about two-thirds of a mile).

Both strategies are represented in the famed struggle between the sperm whale, which is 50 feet long and dives up to two miles deep, and the giant squid, which is thought to grow to over 40 feet and has been observed at depths of over a mile. As Pyenson puts it, "It's escalation at a grand evolutionary scale."

Jacqueline Chretien is a graduate student in molecular and cell biology.

Want to know more? Check out:
www.ucmp.berkeley.edu/people/davidl/lindberg.html

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