Toothed whales target quickly moving prey with a constantly shifting, tightly focused sonar beam, a new study says.
All toothed whales and dolphins echolocate, clicking loudly via special nasal structures and listening for echoes bouncing off objects. This skill is especially crucial in the dark ocean, where the mammals' vision is of little use.
New experiments show that whales can focus their clicks into a type of sonar beam to efficiently track fast-moving prey.
"The bottom line is echolocation is how these animals make their living," said study leader Laura Kloepper, a zoologist at the University of Hawaii in Honululu.
"Not only do they have to locate fish, they have to discriminate fish and figure out what kind of fish it is—it's this constant underwater dance between predator and prey.
"It makes sense [that] of course there has to be focusing going on." (See "Killer Whales Target Favorite Fish With Sonar?")
For their experiments, the team worked with Kina, a false killer whale at the University of Hawaii with decades of training—and a penchant for mischievously splashing Kloepper.
"After a few days I learned to carry an umbrella to protect my equipment," Kloepper said. Kina "got a kick out of watching me scramble."
In the first experiment, a trainer instructed Kina to swim into an underwater hoop up to her pectoral fins. Then, a soundproof door lowered and she echolocated on a target—a hollow cylinder that looks like a toilet paper tube.
Kina had previously been trained to recognize the thickness of this particular cylinder and to signal this by touching a button with her snout, which earned her a fish reward.
The whale was also trained to stay still when shown other cylinders of varying thicknesses.
Kloepper and colleagues then presented Kina with two other types of cylinders to test her echolocation skills: one with much thicker walls, which she could detect easily, and another with only slightly thicker walls, which was tougher for her to pinpoint.
Over a period of weeks, the team also randomly changed the distances from which Kina echolocated the cylinders.
Whale Sonar Has Eye-Like Focus
While Kina was echolocating the various targets, an array of underwater microphones were measuring her constant barrage of sonar waves. From this data, the scientists created a statistical algorithm that recreated Kina's sonar beams.
This revealed that Kina's beam shape had changed depending on the cylinders' distances and differences—much as an eye continually refocuses on an object, explained Kloepper, whose study was published recently in the Journal of Experimental Biology.
"It's remarkable they have this beautiful acoustic lens in their melon," said study co-author Paul Nachtigall, a zoologist at the University of Hawaii in Honululu.
As recently as 2008, "not much attention was paid to the incredible flexibility" of echolocating whales, noted Dorian Houser, director of Biology and Bioacoustic Research at the National Marine Mammal Foundation, a nonprofit based in San Diego.
The new study contributes "to our growing knowledge about the ability of [the whale] to control its echolocation beam by changing its width and frequency content," Houser said via email.
Plenty of echolocation mysteries remain, however—for example, how whales can hear properly even while clicking incredibly loudly (the focus of the study team's next project).
"The more information we obtain on their ability to manipulate the beam," Houser said, "the more complicated the story becomes."