As a male blue whale briefly surfaced in California’s Monterey Bay, a team of scientists in an inflatable boat deftly placed a harmless suction-cup on his back using a carbon-fiber pole. When it was recovered a day later, the sensor carried the first-ever recording of a wild whale’s heart rate.
The experiment, which is described in a study published on Monday in Proceedings of the National Academy of Sciences, offers an unprecedented window into the extreme cardiac adaptations of the largest known animal in Earth’s history. As you can see in the below footage, the researchers had a short window during a bumpy boat ride to put the tag on the whale. Co-author David Cade, who placed the tag, almost looks like Quint from Jaws as he times the contact at the bow of the boat:
“Our study presents a major advancement in our ability to measure a vital rate (heart rate) in an economically and ecologically important species such as the endangered blue whale,” said lead author Jeremy Goldbogen, an assistant professor of biology at Stanford University, in an email.
“The blue whale is the largest animal of all-time and has long fascinated biologists,” he continued. “What is life like and what is the pace of life at such a large scale? These are among the many questions that motivated this study.”
While the heart rates of some captive whales have been measured, retrieving this vital sign from wild whales presents a much bigger challenge. These whales aren’t trained to obediently expose the target area under the whale’s left flipper, where the heart beat can be mostly clearly recorded, so Goldbogen’s team had to focus on getting the tag’s placement just right.
Though the team originally stuck the tag onto the whale’s back, it fortunately slipped into the right position over time, enabling an electrocardiogram (ECG) to be recorded.
The ECG was taken on August 27, 2018, and revealed that the blue whale’s heart rate slowed to four to eight beats per minute during his deep dives, far lower than the predicted rate of 15 beats. In contrast, the whale’s heart beat an astonishing 25 to 37 times per minute once he reached the surface.
That high heart rate approaches the upper limit of what scientists had projected was possible for these ocean behemoths, suggesting that cardiac constraints “may have limited the evolution of maximum body size” in blue whales, the authors said.
In other words, while these huge hearts powered the unrivaled evolutionary enlargement of blue whales, they may have also capped their growth at the current size. Whales cannot physically pump blood much faster than the rates observed in the new ECG recordings, according to the study, which may explain why we have never observed animals that are bigger than blue whales.
Goldbogen and his colleagues were able to retrieve 8.5 hours of cardiac data from the blue whale, which is an adult that is at least 15 years old and weighs about 150,000 pounds. The whale embarked on several dives with the tag, including one trip that lasted 16.5 minutes and took him 184 meters (600 feet) under the surface.
During these dives, the whale’s heart rate dropped in order to preserve oxygen and energy, though it rose by a factor of about 2.5 whenever the animal lunged, open-mouthed, toward schools of small crustaceans that blue whales rely on for food.
“Blue whales are interesting because have such an energetically demanding foraging strategy, called lunge feeding, whereby huge volumes of water and prey are engulfed repeatedly during foraging dives,” Goldbogen said.
While the whale had a short-term elevated heart rate during its lunge-feeds, the beats really started pumping overtime when he reached the surface again. As the whale breathed in air after dives, his heart rate shot up to ensure that his body and tissues would be efficiently reoxygenated. The deeper the dive, the higher the heart rate at the surface, the team found.
The successful collection of this heart rate data represents “a major advancement in our ability to bring the physiological laboratory to the open ocean,” the team said. Goldbogen’s team plan to try the technique out on other types of wild whales, such as sperm whales and bowheads, which dive for much longer periods than blue whales.
“We are interested in how animals differ in these types of vital rates,” Goldbogen said. “We will focus future efforts on other large whale species to see if they exhibit similar types of heart rate dynamics.”