Imagine you are swimming in the ocean and you see a predator of some kind, like a shark or a killer whale, instantly your heart would start to beat out of your chest. When frightened or under stress most species fight or flight instincts kick in and their heartbeats increase. In case you hadn’t noticed the narwhal is a peculiar animal. Monodon Monoceros, known as the “unicorns of the sea”, is unusual for reasons apart for it appearance. Physiologically they do not respond to stress in the same way that most animals do. When diving to escape a stressful situation they can slow their heartbeats to ten beats per minute as a means to conserve oxygen and hold their breath for extended periods.
This phenomenon was observed by physiologists and recently published in Science. Researchers attached heart monitoring electrodes and cameras via special suction cups to monitor the behavior and physiological response of narwhals. The lead author on the publication Terrie Williams, a comparative wildlife physiologist at the University of California, Santa Cruz, says their international research team called it ‘cardiac freeze’. The narwhals at the center of the work were stranded or had been caught in nets in the Scoresby Sound on the east coast of Greenland, but before they were loosed, they were outfitted with the monitoring devices. The narwhals did their thing, the devices collected the data, fell off, floated to the surface and the process was repeated for a total of nine whales.
Gathering biometric data (Photo by Mads Peter Heide-Jorgensen)
Devices attached to narwhals with suction cups
(Photo by Mads Peter Heide-Jorgensen)
The researchers were shocked by the unexpected results. It could be thought that this is some sort of evolutionary/adaptive trait, but Williams says the response does not “make sense” when you understand that two contradictory signals are being sent to the heart. “You have a set of nerves that are saying ‘beat faster’ and a set of responses are telling you ‘beat slower,’” she says. “And in fact not only beat slower but beat at a very, very slow level. And in other animals and in humans this can lead to fatalities.”
The research Williams, who has studied exercise physiology in a wide range of both marine and terrestrial mammals, and her team has uncovered has lead to a series of questions: How do narwhals get oxygen to their brains? How do they thermoregulate without a properly working cardiovascular system?
They calculated that these escape dives her team monitored required 97 percent of the narwhal's oxygen supply and often exceeded its aerobic dive limit (meaning depletion of oxygen stores in the muscles, lungs, and blood, followed by anaerobic metabolism). For normal non-escape dives of similar duration and depth they found the whales used only about 52 percent of a narwhal's oxygen store.
The narwhal does not have an aorta, a main artery leaving the muscular pumping ventricle which directs blood. “The narwhal doesn't have that… It almost looks like a weather balloon sitting on top of their heart instead of an aorta, and it's enormous.
“It's this elastic balloon that if you imagine the heart beating and it pulses blood into this balloon. It expands out and then over time, we think, that it slowly just lets blood leak out for that period of time in between the peaks.”
Whether this physiological response is dangerous to narwhals, Williams was unsure. However, what has become clear is the vulnerability of narwhals to anthropogenic disturbance. In the past sea ice has protected the arctic from vessel traffic and oil, gas and mineral exploration. But with climate change things are not like they once were. These narwhal which have largely live in relative isolation for millions of years must now face increasing contact.
For this fascinating publication please see:
TM. Williams, SB. Blackwell, B. Richter, MHS. Sinding, MP. Heide-Jørgensen (2017) Paradoxical escape responses by narwhals (Monodon monoceros). Science. 358: 1328-1331. DOI: 10.1126/science.aao2740
