Read: Why whales got so big

Goldbogen has spent decades studying blue whales by sticking data loggers on their back. The device, held in place by suction cups, can record a whale’s position, speed, and acceleration as it swims, dives, and forages. Goldbogen began to wonder whether, by adding electrodes to the suction cups, he could also capture a heartbeat. It would be just like the way a doctor takes an electrocardiogram from a human patient—except that the electrodes would have to record through inches of blubber, which meant they would need to be placed in just the right spot. “To be honest, I thought it wasn’t going to work,” Goldbogen says. (Ba-bum.)

He tried anyway. Between dives, blue whales surface for about 10 consecutive breaths, producing distinctive waterspouts. When Goldbogen and his team spotted one of these in Monterey Bay, California, they maneuvered their small inflatable boat to the animal’s left flank, and used a 20-foot pole to stick the heart monitor next to a flipper. The tag stayed. The whale descended. Several hours later, the tag floated back to the surface, and the team members retrieved it. When they downloaded the data and saw the traces of a beating heart, “we did a victory lap around the lab," Goldbogen says. “You have long days at sea and in front of a computer, but those are the moments you get into this business for.” (Ba-bum.)

Based on equations that apply across mammals of different sizes, a 220-ton blue whale (the largest animal on record) should have a resting heart rate of 11 beats a minute. For comparison, humans have a resting rate of 60 to 100 beats a minute. But Goldbogen found that the very concept of a resting rate doesn’t apply to a blue whale. It’s constantly diving to depths of 150 to 200 meters, feasting on shrimplike crustaceans called krill for 15 minutes at a time, and then resurfacing to reload on oxygen. As it sinks and rises, its heart oscillates between two extremes—very slow or very fast, with nothing in between. (Ba-bum.)

During dives, the whale’s heart rate plummeted to between four and eight beats a minute, and sometimes as low as two. How did the enormous ticker keep blood flowing during those long pauses between beats? The secret lies in the whale’s incredibly elastic aortic arch—the part of the major artery just outside the heart. “Think of it as a balloon,” Goldbogen says. It expands to take in most of the blood ejected by a heartbeat, and then slowly deflates to release that blood into the rest of the circulatory system. This adaptation, which was discovered in 1994, allows a diving whale to continuously send blood to its organs, even in the long breaks between heartbeats. (Ba-bum.)

When the whale surfaced, its heart sped up and rapidly reached 30 to 37 beats a minute. Because each beat can take about 1.8 seconds, for a blue whale, those rates are positively frenetic. “I’m not all that surprised,” says Sascha Hooker, a physiologist at the University of St. Andrews who studies diving mammals. Wild gray seals, for example, have similarly shown heart rates as low as two beats a minute during a dive, but as high as 135 at the surface. “This shows the quite extraordinary level of flexibility and control that these diving mammals have over their heart rate and blood flow,” Hooker adds. (Ba-bum.)