Researchers prepare to attach a heart monitor to a surfacing blue whale.M. S. Savoca

Ba-bum.

The heart of a blue whale, diving off the coast of California, has just contracted. The beat took about two seconds to finish, and pushed dozens of gallons of blood through the arteries of the largest animal that lives or has ever lived. According to Jeremy Goldbogen of Stanford University, the first person to attach a heart monitor to a blue whale at sea, the creature’s organ constantly swings between extremes of speed. During a dive, it can conserve oxygen by slowing down to just two beats a minute. If you’re reading this piece at an average speed, that’s roughly one beat at the end of every paragraph. (Ba-bum.)

David Attenborough once claimed that the blue whale’s heart “is the size of a car, and that some of its blood vessels are so wide that you could swim down them.” Neither factoid is true. In 2015, when a dead blue whale washed ashore in Newfoundland, Canada, a dissection team from the Royal Ontario Museum managed to extract and measure its heart. At 400 pounds, it was undoubtedly and impressively big. But the main artery was barely big enough for a human head, and the whole organ was more like “a small golf cart or circus bumper car for two,” said Jacqueline Miller, a mammalogy technician, to the BBC. Goldbogen compares it to “an easy chair or a single-person sofa.” (Ba-bum.)

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.)

Goldbogen knows that other divers show a wider range of rates, but he thinks that the blue whale is special for two reasons. First, its feeding style is uniquely demanding. It lunges at swarms of krill, accelerating to eight miles an hour in less than a minute. (For comparison, Michael Phelps’s top speed is six miles an hour, and he doesn’t weigh 220 tons.) Its mouth balloons outward, engulfing a volume of water larger than the whale itself, and capturing half a million calories’ worth of krill. It might do this several times a dive, expending phenomenal amounts of energy and soaking up huge amounts of oxygen. And yet, during such lunges, the heart of Goldbogen’s whale beat just 2.5 times faster than its mid-dive minimum. (Ba-bum.)

Second, the surface heart rate of 37 beats a minute is likely as much as the whale can possibly manage. “It’s beating as hard as it can possibly go, and the animal wasn’t even sprinting,” Goldbogen says. “This was routine foraging behavior. They do this all day long.” For that reason, he wonders whether blue whales have hit the largest possible size that an animal can reach. The heart of a bigger creature couldn’t possibly beat fast enough at the surface to repay the oxygen debt that it accrued by beating slowly at depth. And without beating slowly, it couldn’t sustain dives long enough to capture enough krill. “They’re approaching their physical limits,” Goldbogen says.

Ba-bum.

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