Beaked whales have a killer whale problem.

More formidable whales, of the sperm or pilot variety, have the size and muscle to flee or defend against a killer whale, an ocean superpredator. Smaller prey, like dolphins, can find safety by swimming in large pods. Certain toothed whales even communicate in pitches killer whales can’t hear.

But elephant-sized beaked whales, named for their pointy snouts, have none of these advantages. These extreme divers swim in small groups, are too slow to outswim a killer whale, and rely on audible clicks to echolocate food deep in the ocean. Killer whales (Orcinus orca) should be able to hear them hunting below and easily pick them off as they ascend.

But beaked whales have evolved a sneaky trick.

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An unusual, highly synchronized style of diving helps them silently slip past killer whales when surfacing to breathe, researchers describe February 6 in Scientific Reports. Predation from killer whales has shaped that strange behavior, the scientists say, and also might explain why naval sonar exercises, which can sound like predators to beaked whales, cause mass beaching events (SN: 3/25/11).

“Beaked whales are some of the most mysterious mammals in the world,” says Natacha Aguilar de Soto, a marine biologist at the University of La Laguna in the Canary Islands, Spain. This group of 22 whale species can dive deeper than any mammal, sometimes descending more than 2,000 meters to noisily hunt small fish and squid using echolocation for up to 2½ hours before surfacing.

Previous research has hinted that, when beaked whales return from the deep, they don’t come straight up for air like other whales. Instead, they ascend at a gradual angle, surfacing far from where they dove. “It’s highly unusual for whales to do this,” Aguilar de Soto says. She and her colleagues wondered whether it could help beaked whales slip past predators.

The team suction-cupped sensors that tracked depth, orientation and sound onto 14 Blainville’s beaked whales (Mesoplodon densirostris) and 12 Cuvier’s (Ziphius cavirostris) off the coasts of Spain, Portugal and Italy to better understand diving behavior in these groups. Instead of diving for food whenever an individual whale got hungry, tagged whales in the same group dove together 99 percent of the time.

On their way down, the whales swam in a tight group, remaining totally silent. But once they reached about 450 meters deep, they split up, loudly chirping to echolocate prey hundreds of meters from other group members.

Killer whales cannot hunt mammals this deep. But Aguilar de Soto says that the predators can eavesdrop on beaked whales while they hunt, and could hover above, waiting for them to ascend.

But when the whales finished foraging, they regrouped and began their silent, meandering ascent back to the surface, traveling as far as a kilometer from where they dove.

“That’s the trick to give the skip to killer whales,” Aguilar de Soto says.

The researchers estimate that killer whales, or orcas, can visually explore only 1.2 percent of the potential surfacing area of these beaked whales. Such behavior allows diving groups, which often include young beaked whales, to stay together while also evading detection by predators.

But the unusual diving does have downsides. The beaked whales’ slow and silent ascent cuts foraging time by 35 percent, the study estimates, compared with whales that swim straight up.

“This study is a great achievement; it’s really hard to get good data on these whales,” says Nicola Quick, a behavioral ecologist at Duke University. The work supports the idea that predation has shaped this unusual diving behavior, although the gradual ascent also could be important for avoiding decompression sickness, she says.

Aguilar de Soto says the study helps to explain why beaked whales react so strongly to sonar. Having evolved in a “soundscape of fear,” she says, beaked whales may be hypersensitive to the sounds of predators. Sonar might hijack this response and drive disoriented and scared whales to swim until they’re beached.

While we can’t change this ingrained whale behavior, Aguilar de Soto says, “we can try to push governments to restrict these exercises to places where they’ll have less of an impact.”