Pilot Whales the 'Cheetahs of the Deep Sea'

Researchers reveal first glimpse of whales' high-speed, deep-diving hunts for squid

Source: Oceanus Magazine

Until now, what happened in the depths, where many whales hunt, stayed there.

A new study has revealed that pilot whales are “the cheetahs of the deep sea,” making 15-minute, high-speed, all-or-nothing dives up to 3,280 feet (1,000 meters) deep to chase and catch large squid, before surfacing to catch their breath.

"They make colossal dives and must come back exhausted," said Natacha Aguilar de Soto, the study's lead author. "They have to spend time at the surface catching their breath before undertaking a new sprint to catch prey."

The pilot whales’ hunting tactics contrast with beaked whales, another family of similar-sized, deep-diving squid-eaters that ply nearby waters in the Canary Islands, a research team involving scientists in three countries reported online in the Journal of Animal Ecology. Beaked whales dive slowly, conserving their breath so that they can remain at depth for 20 to 30 minutes hunting for smaller squid. Their dives can last up to 50 to 90 minutes.

“We didn't expect such differences in tactics between deep-diving squid-eaters,” said Mark Johnson, an engineer at Woods Hole Oceanographic Institution (WHOI) who designed innovative tagging devices that record whale sounds and movements in the deep and allowed this first glimpse of their hunting behavior. Perhaps the different species can coexist in the same territory because they have evolved different hunting styles, just as cheetahs have done to carve a niche on the same Serengeti Plains where other big cats hunt, he said.

Rather than expending energy in a sprint for a big, one-time payoff as pilot whales do, beaked whales are more like endurance runners. Moving more slowly, they gather more, though smaller, prey. “It’s more like picking berries,” said Aguilar de Soto of the University of La Laguna in Tenerife, Canary Islands.

The new research also helps explain and reinterpret thinking about the whales’ behavior at the surface. Beaked whales are seldom seen because—moving more slowly and expending less energy—they can spend most of their time in the depths. Meanwhile, pilot whales often congregate in groups at the ocean surface and have a reputation for being calm, approachable, and not bothered by boat activity. Instead, they may be too tired to react.

The new understanding about whale diving and feeding behaviors should also inform policies to better manage whale-watching and shipping movements in areas with whales, said the researchers, who also included Peter Madsen of the University of Aarhus in Denmark, Francisca Diaz, Iván Dominguez, and Alberto Brito, all from the University of La Laguna, and Peter Tyack of WHOI.

Technology reveals the hunt

Unlike scientists studying predators on land or in the air, whale biologists could not observe their subjects or attach radio-transmitting collars to track them. To overcome these barriers, Johnson developed a non-invasive tag, called a D-tag, which temporarily attaches to whales. It digitally records their movements and the sounds the whales make and hear on their dives, from start to finish.

In the dark depths, sound takes the place of light. To perceive their surroundings and locate prey, whales send out sound signals and receive sound reflected back from objects.

Pilot whales, for example, produce regular “clicks” after reaching a certain depth, and sense what’s near them from sound echoes. When the whales get close to prey, the clicks become more rapid, turning into a “buzz” of more than 300 clicks per second.

The whales can “look” further with the more powerful clicks, spaced farther apart, but they get that information more slowly. The rapid buzzes provide less range, but can give the whale fast-paced updates about whether its prey is taking evasive action, for example.

Over the last few years, the research team has pioneered a method using D-tags to capture  whale sounds echoing off objects that the whales encounter. The team has been able to follow prey as whales chase them, sometimes trying to escape at the last minute or schooling together to gain strength in numbers.  So far the method only works for beaked whales, Johnson said. “Pilot whales have a square head that gets in the way of the returning sound.”

“The D-tag was the first instrument to capture an echo from prey in the wild, one of the great challenges in studying echolocation,” said co-author Peter Madsen, a professor at the University of Aarhus inDenmark. “It is a really unprecedented chance to study how whales find and capture their dinner.”

Putting it all together, Aguilar de Soto and Johnson believe that the whale buzzes, followed sometimes by the sound of a bump or crash, indicate the moment that whales are capturing prey. The D-tags also records the sounds of the whales “fluking”— pumping their tail flukes to power forward, as well as the sound of water flowing fast over tag during the pilot whales’ chases (“it sounds like a train approaching,” Johnson said). By analyzing the recorded “clicks,” “buzzes,” and other whale sounds together with the whales’ recorded speeds, orientations, and depths, scientists can reconstruct a picture of the whales’ dives and hunts.

“What’s cool,” Johnson said, “is to ‘see’ how whales hunt at depth using their own sounds, —without the intrusion of lights and a camera.”

High-speed chases are high-cost

The picture that emerges is this: Pilot whales dive down to about 1,640 feet (500 meters) and then level out to select a target—“sort of like acoustic window shopping,” Johnson said. Once they have picked their prey, a chase starts that can take the whales down to around 3,000 feet (1,000 meters) and reach speed of 20 miles per hour (9 meters per second). (Their terrestrial counterparts, cheetahs, run up to 60 miles per hours, but only for 20 seconds, before they have to rest for long periods—“and, of course, they can breathe all the time, whereas the pilot whales are working on just one lung-full of air,” Aguilar de Soto said.) Most deep-diving whales swim at only 1 to 2 meters per second, allowing them to conserve energy and oxygen and maximize hunting time in the depths where their prey dwell. "The sprints of these short-finned pilot whales open a new perspective into optimal foraging theories of deep-diving breath-holding mammals," said Aguilar de Soto.

Running is harder than walking, even in air, and water exerts a much higher drag than air. Factor in the limited amount of oxygen whales can carry on the dive, and the whales must balance their needs for food and air.

So sprint-diving poses a calculated risk versus reward. The pilot whales’ speed offers them advantages to catch prey, but they expend much more energy and oxygen doing it.  “The power needed to swim goes up with the cube of the speed,” Aguilar de Soto calculated. That means that when pilot whales swim at four times their normal speed in a chase, they could be using 64 times as much energy per second.

Working with the echolocation sounds made by the tagged pilot whales, the team concluded that the whales don't always get a meal. For all their effort, pilot whales hunt unsuccessfully about 40 percent of the time, Aguilar de Soto estimated. Given the high energy cost of their sprints, they can afford only one or two capture attempts per dive. “A missed catch,” Aguilar de Soto said, "could cost them the whole dive.”

The food, then, has to be worth the risk and the energy spent to get it. What are pilot whales catching at 3,000 feet?

Pilot whales are known to eat several species of large deep-water squid, some of which get big enough to be worth chasing. Squid tehtacles four feet long have been seen trailing from pilot whales’ mouths after they have resurfaced. But even larger prey may be the whales’ quarry.

“We’ve found pieces of the arms of giant squid, Architeuthis dux, floating where the whales are coming up,” Johnson said. “We have always imagined sperm whales hunting giant squid but maybe these tough pilot whales, one third the size of a sperm whale, are the ones showing up for the fight.”

Rest and risks

Finding food is not the only challenge pilot and beaked whales face. Boats and sonar are two others.

A large whale-watching industry spotlights seemingly calm pilot whales that congregate at the surface off the Canary Islands.

“Because they don’t react, people think boat traffic doesn’t bother them,” Johnson said, but the pilot whales’ survival could depend on their ability to rest and recover, undisturbed, from sprint dives. “They’re not just relaxing there,” Aguilar de Soto said.

The more elusive beaked whales are considered sensitive to sonar, and several species have experienced mass strandings correlated with the use of sonar in naval exercises. Living in deep waters that are not usually considered coastal, they may not be subject to regulations, but Aguilar de Soto and Johnson believe there’s an urgent need to protect them.

“They fall into a hole in policy,” Johnson said. “They are so difficult to find that no one really knows how many there are, and under most regulations, if you can’t count them, you can’t protect them.”

D-tags may help scientists learn more about the mysterious deep diving species, and Johnson and colleagues are now working on similar acoustic recording devices installed on long-term moorings that could acoustically identify whale species, numbers, and locations, and help conserve whale populations.

—Kate Madin

This work was supported by the Strategic Environmental Research and Development program of the U.S. Department of Defense, the U.S. National Oceanographic Partnership Program, and the Canary Islands Government-Spanish Ministry of Defense. Research was conducted under a permit to La Laguna University from the Government of the Canary Islands and was approved by the Woods Hole Oceanographic Institution Animal Care and Use Committee.