Scientists who fitted heart rate-monitoring tags to Arctic narwhals have discovered a strange paradox in how the animals respond to threats.
When these tusked whales are frightened, their hearts slow, but at the same time they swim quickly to escape.
Scientists say the response could be "highly costly" - because they exert themselves with a limited blood supply.
Historically, narwhals have not come into contact with much human disturbance, because they live mainly hidden among Arctic sea ice. But in recent decades, as the ice has declined, this is changing.
"Shipping and exploration for oil and gas is moving into the narwhals' world," said lead researcher Dr Terrie Williams, from the University of California, Santa Cruz.
Having developed technology to study the physiology of dolphins at her home institute, she explained that her collaborator on this study - Dr Mads Peter Heide-Jorgensen, from the Greenland Institute of Natural Resources - contacted her to see if her tags could be used on wild narwhals.
The tags she developed incorporate a heart monitor with depth and acceleration measurement, as well as a satellite tracking device.
The researchers worked with the hunters to find narwhals already entangled in nets. They released each animal, attaching a tag to its back with a suction cup, before pushing it into the deep water of the East Greenland fjords.
For narwhals and other Arctic marine mammals, the discovery highlights some worrying implications of shipping and mineral exploration moving into increasingly ice-free Arctic seas, reports BBC.