Blue whales have a flair for paradox. They live in water but breathe air. They’re enormous — the biggest creatures that have ever lived, as far as anyone knows — but subsist almost entirely on tiny krill.
And as new research reveals, even the animal’s dunk tank-size heart jumps between extremes. In a study published Monday in the Proceedings of the National Academy of Sciences, a team of researchers for the first time attached an electrocardiogram tag to a free-diving blue whale to trace its heart rate.
They found that the rate ranged as low as two beats per minute and as high as 37. Such numbers paint a picture of an animal frequently pushing its own limits, and suggest that the whale is not only the largest animal ever, but perhaps as large as an animal with a circulatory system can possibly be.
To learn the secrets of the blue whale’s heart, Jeremy Goldbogen, a marine biologist at Stanford University, led a team that attached a noninvasive suction-cup tag to a blue whale in Monterey Bay, California. The tag contains electrodes, a depth measurement sensor and a GPS tracker. It stayed on for eight and a half hours, and recorded data from a number of dives before it detached and floated to the surface. The researchers then tracked it down and reviewed its readings.
Such tags are part of a suite of technologies that make it easier for researchers to study the lives of whales without disturbing them too much. When the tags are attached, a whale likely just feels something akin to a tap on the shoulder, Dr. Goldbogen has said.
The data showed that when this whale descended, its heart rate plummeted, too. At the bottom of the dives, the whale’s heart rate hovered around four to eight beats per minute. At times, it fell all the way down to two.
In general, sticking to a slow rhythm while diving lets marine mammals conserve oxygen, so they can stay underwater for longer. But this is “just amazingly low” — far lower than the 11 beats per minute scientists had predicted, said Dr. Goldbogen. And it stays down around 8.5 beats per minute even when the whales are feeding, an energy-intensive process that involves lunging into clouds of krill.
Dr. Goldbogen credits the blue whale’s flexible aortic arch, which is able to hold about 90 percent of the animals’ blood and slowly release it even when the heart isn’t actively beating.
As a whale climbs back to the surface, its heart rate rises again. By the time it reaches the surface, its blood is moving much more quickly, reoxygenizing in preparation for the next dive. This whale’s heart rate reached a peak of 37 beats per minute.
“That’s about as fast as that heart can physically beat,” Dr. Goldbogen said.
These rapid shifts happen quite quickly — sometimes within the span of a minute or two — and repeat often. It’s sort of like if you tried moving from a comfortable recliner straight into a set of wind sprints over and over again. “They’re doing this all day long,” said Dr. Goldbogen.
The blue whale’s heart still keeps some secrets.
“It would be interesting to know how the whale nervous system controls such rapid changes in heart rate,” said Travis Horton, an associate professor at the University of Canterbury in Christchurch, New Zealand, who was not involved in the study but has undertaken his own investigations of humpback whale heart rates using drones and infrared light.
And Dr. Goldbogen and his team hope to plumb other unfathomed depths. “Sperm whales, beaked whales — some of those species can dive for an hour or more,” he said. “We would like to understand what their hearts are doing.”