When an animal senses danger, it might freeze to try to avoid being seen. Or it might flee, to try to get away. But when narwhals encounter people, they do a bit of both, a new study finds.
After escaping from a net, these whales dove deep underwater. That part of their reaction looked like fleeing. At the same time, their heart rates and breathing slowed, as if they were trying to hide.
It’s an unusual combo. Normally the heart races during intense physical activity. That helps pump more blood, oxygen and nutrients to working muscles. And the narwhals’ escape dives definitely qualify as intense activity. During those dives, they pump their tails at rates of up to 25 strokes per minute. So this drop in heart rate was surprising — especially as it was so dramatic.
Indeed, says Terrie Williams, “That was astounding to us.” As an ecophysiologist (EE-koh-fih-zee-OL-oh-gizt) at the University of California, Santa Cruz, she studies how animals’ body functions affect how they interact with their environment. “There are other marine mammals that can have heart rates that low, but not typically for that long a period of time. And especially not while they’re swimming as hard as they can.”
Narwhals are well-adapted to diving. Their main food is deep-dwelling ocean fish. During their main feeding season, these whales can dive 800 to 1,500 meters (2,400 to 4,500 feet) below the Arctic sea ice for 25 minutes at a time. But like other mammals, they need to breathe air. To conserve oxygen while foraging for food, they can slow their body functions. The normal heart rate for a narwhal is about 60 beats per minute. That’s similar to what’s seen in people. But during a long underwater feeding session, a narwhal can drop its heart rate to just 10 or 20 beats per minute.
To evade natural predators such as killer whales, narwhals don’t usually dive away. Instead, they tend to hide. They may sneak under ice sheets or huddle in spots too shallow for their pursuers, Williams notes.
But in response to people, they react very differently. “When narwhals detect humans, they often dive quickly and disappear from sight,” observes Kristin Laidre. She is an ecologist at the University of Washington in Seattle. Although she studies Arctic narwhals, she did not take part in this study.
Williams worked with research colleagues from Denmark. During these escape dives, they wanted to know what happens to the whales’ bodies. To do this, they partnered with indigenous hunters in East Greenland. The hunters knew how to capture narwhals in nets. The researchers used suction cups to stick monitoring equipment to the narwhals’ backs. After releasing the creatures, this team tracked tail strokes and heart rates in the whales. They used these data to calculate how much energy the narwhals used during their deep escape dives.
Dives to feed were different from those that came after being entangled in a net for an hour or more. During escape dives, Williams explains, “the heart rates were going down to levels of three and four beats per minute!” And they stayed “at that level for 10 minutes at a time.”
Her team observed the narwhals making multiple dives in the hours following their escapes. And they dove deeply — 45 to 473 meters (about 150 to 1,500 feet) down. Such dives burned about twice as much energy as normal feeding dives. Those post-escape dives burned up three to six times more energy than when the animals were at rest.
Frantic getaways, combined with super-low heart rates, take a steep toll. Escape dives gobbled up 97 percent of the available oxygen in the narwhals’ lungs, blood and muscles, the authors calculated. That’s almost twice the 52 percent used up during normal dives of similar depth and duration.
So far, Williams says, these costly escapes have been observed only after a prolonged interaction with people. It’s not yet known whether such an extreme physical response could weaken the animals over time.
The international team of researchers shared its findings December 8, 2017 in Science.
This new study highlights how the behavior of these whales makes them especially vulnerable to interactions with people, Laidre says. And, she adds, such human contact is “likely to increase in the Arctic with sea ice loss.” Less ice means more boats and more encounters with people. Seismic exploration, hunting and noise from large vessels and fishing boats also can disturb wildlife. Today, narwhals already are a species that is nearly threatened with extinction. So conservation efforts must account for human impacts, Laidre says.
Williams’ group plans to investigate whether narwhals show the same flee-and-freeze reaction to other disruptions. They also want to track any long-term health effects of such extreme escapes.
“There is a concern from our group that this is just pushing the biology of these animals beyond what they can do,” Williams says.
Arctic A region that falls within the Arctic Circle. The edge of that circle is defined as the northernmost point at which the sun is visible on the northern winter solstice and the southernmost point at which the midnight sun can be seen on the northern summer solstice. The high Arctic is that most northerly third of this region. It’s a region dominated by snow cover much of the year.
behavior The way something, often a person or other organism, acts towards others, or conducts itself.
biology The study of living things. The scientists who study them are known as biologists.
colleague Someone who works with another; a co-worker or team member.
conservation The act of preserving or protecting something. The focus of this work can range from art objects to endangered species and other aspects of the natural environment.
ecology A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).
extinction The permanent loss of a species, family or larger group of organisms.
function A relationship between two or more variables in which one variable (the dependent one) is exactly determined by the value of the other variables.
Greenland The world’s largest island, Greenland sits between the Arctic Ocean and North Atlantic. Although it is technically part of North America (sitting just east of Northern Canada), Greenland has been linked more politically to Europe. Although this is the 12th biggest nation (based on surface area), Greenland averages the fewest people per square kilometer of its surface area.
heart rate Heart beat; the number of times per minute that the heart — a pump — contracts, moving blood throughout the body.
ice sheet A broad blanket of ice, often kilometers deep. Ice sheets currently cover most of Antarctica. An ice sheet also blankets most of Greenland. During the last glaciation, ice sheets also covered much of North America and Europe.
killer whale A dolphin species (Orcinus orca) whose name means whale killer. These animals belong to the order of marine mammals known as Cetacea (or cetaceans).
marine mammal Any of many types of mammals that spend most of its life in the ocean environment. These include whales and dolphins, walruses and sea lions, seals and sea otters, manatees and dugongs — even polar bears.
muscle A type of tissue used to produce movement by contracting its cells, known as muscle fibers. Muscle is rich in protein, which is why predatory species seek prey containing lots of this tissue.
nutrient A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.
oxygen A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).
predator (adjective: predatory) A creature that preys on other animals for most or all of its food.
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
threatened (in conservation biology) A designation given to species that are at high risk of going extinct. These species are not as imperiled however, as those considered “endangered.”
whale A common, but fairly imprecise, term for a class of large mammals that lives in the ocean. This group includes dolphins and porpoises.