Saturday, August 8

Beaked whales may evade killer whales by silently diving in sync

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.

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.”