When a deer freezes in your car’s headlights it’s because the pupils of their eyes are wide-open from being in the dark and they don’t know what to do but wait until they adjust. Presumably this is a similar phenomenon in rabbits — a terrified state referred to in Watership Down as going tharn. But what narwhals do when scared by humans combines going tharn with the opposite — flight — as they dive deep into the sea at top speed, frozen still except for the tails propelling them downward and away.

A study just published in Science documents this strange response to the terror that is humanity. What’s weird is that, as narwhals’ tails paddle at up to a brisk 25 strokes per minute, their heartbeat drops all the way down into extreme bradycardia and a barely-there three beats per minute. Their normal resting heart rate is 60 beats per minute, and it’s about 20 during regular dives, which allows them to conserve precious oxygen underwater. In fright, they also dive repeatedly to depths of 45 to 473 meters according to the new research.

 

The normal narwhal heartbeat (SCIENCE NEWS)

 

As one of the researchers, biologist Terrie M. Williams, tells Science, “That was astounding to us because 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 have been observed in this fast-moving cardiac stasis for as long as 10 minutes. 

Narwhals respond only to humans in this way. With predators such as Orcas, they slide quietly under ice or gather in waters too shallow for the killer whales to follow. But with humans, it’s something else, and with climate change causing ice to melt, Monodon monoceros are likely to be in more frequent contact with us. Their reaction is so extreme that, “There is a concern from our group that this is just pushing the biology of these animals beyond what they can do,” says Williams.

For the study, Williams and her team worked with indigenous hunters from Greenland. Narwhals were trapped in nets and then released after submersible electrocardiographs, depth, and acceleration recorders were attached to them with suction cups.

sensor array on narwhal

Placement of monitoring devices (T.M. WILLIAMS ET AL/SCIENCE 2017)

The narwhals’ frantic response to people has to be awfully hard on them. The study estimates that they burn three times the energy they consume at rest, and that fleeing with what the report terms “cardiac freeze” tears through the available oxygen in their lungs, blood, and muscles, quickly exhausting up 97% of it.

narwhal energy use in terror

Narwhals’ energy consumption during the initial escape dive and the period afterward is incredible — they get to depths of 45 to 473 meters — and alarming (T.M. WILLIAMS ET AL/SCIENCE 2017)

While understanding narwhals’ response to human interaction — and reducing those intrusions — may be important to their survival as a species, hopefully further catch-and-release research will unnecessary, given its cost to the subject narwhals. Still, the scientists remain curious to know if other human activities, including seismic exploration, noise, and hunting, produce the same or a similar reaction. Let’s hope not.