The heart of the Etruscan shrew, one of the world’s smallest mammals, beats incredibly fast — up to 1,500 times per minute, or 25 times per second. The human heart, in comparison, is sluggish, beating only 60 to 100 times a minute.
Then there’s the heart of the blue whale, the largest animal ever to have lived. These marine giants can be longer than two school buses, and their hearts — which are roughly the size of a loveseat and weigh over 1,000 pounds — beat as few as two times per minute.
If you were to put an enormous stethoscope up to a blue whale’s chest underwater, it might sound something like this.
That clip was produced using real data that scientists collected a few years ago from a blue whale in Monterey Bay, California. The heart beat slowly when the animal dived, but when it came to the surface to breathe, the rate picked up dramatically, reaching as fast as 37 beats per minute.
In the last few years, scientists have figured out how to listen to the heartbeats of wild whales. They’re not interested in checking these animals’ vitals, per se, but trying to answer one of biology’s most fundamental questions: How large can an animal on Earth get?
Measuring the heart rate of blue whales — which are larger than dinosaurs — reveals that body size might be constrained by heart size. And with more advanced monitoring tools, it could also help scientists protect these marine giants from one of the ocean’s most mysterious threats.
The short answer: food. Several million years ago, blue whales evolved to binge-eat tiny crustaceans called krill, which are super abundant in some coastal regions during part of the year. All of that food can fuel a big body, and being big allowed these animals to take larger gulps of krill and swim efficiently from one krill buffet to the next.
But what’s interesting is that there are actually enough krill and other aquatic critters in the ocean for the whales to be even bigger. Food, alone, does not seem to limit the potential body size of whales, said Max Czapanskiy, a doctoral researcher at Stanford who studies marine mammals. “There has to be something about their bodies that’s keeping them from getting even bigger,” he said.
The answer, scientists suspect, may lie in the heart.
Whales hold their breath as they’re feeding on krill, which tend to aggregate hundreds of feet underwater. That causes carbon dioxide to build up in their blood. When these marine mammals return to the surface to breathe, their hearts beat fast to rid their bodies of CO2 and replace it with fresh oxygen, so they can dive back down and continue foraging.
Larger hearts beat more slowly and take longer to replenish oxygen in the body. That means whales have to spend more time on the surface, catching their breath, which eats into precious time they have to feed on a seasonal resource like krill. Too big a heart and these behemoths might not have enough time to eat.
If heart size is limiting whales in some way, these organs should, theoretically, be maxing out their speed when the animals come up for air. That’s one thing that scientists wanted to figure out when they set out to measure a blue whale’s heartbeat in 2018.
There are plenty of ways to measure our own heart rates, from simple stethoscopes to wearables like Fitbits and Apple Watches. It’s much harder to measure a whale’s.
These animals are covered in a thick coat of blubber and dive down several hundred feet, where the pressure is immense. Even if a heart rate monitor works under these conditions, scientists then have to find a whale, attach the device, and retrieve it.
It wasn’t until 2018 that scientists were able to do this successfully. In late summer, California-based researchers were out on the water in Monterey Bay studying a large group of blue whales.
On an inflatable research boat, the researchers approached one of them and, using a 20-foot pole, stuck a specialized EKG sensor behind its left flipper. The whale descended, and several hours later, the device floated back up to the surface, where the researchers were able to retrieve it.
The EKG sensor, which measures electrical signals, recorded the animal’s heartbeat for several hours. That’s where the video clip above comes from: Jessica Kendall-Bar, a marine scientist and artist who was not affiliated with the study, turned a segment of the heartbeat data into an audio file, which she shared with Vox.
But this approach has some serious drawbacks, said Czapanskiy, who was a co-author of a 2019 paper based on the EKG research. “The failure rate is really high,” he said, mentioning that saltwater often interferes with electrical sensors.
That’s why scientists have been looking for other approaches. In a paper Czapanskiy published in May, he showed that a device called an accelerometer — which measures an animal’s movement — can actually detect the pulse of a heart, as well.
Each time a whale’s heart beats, it sends out a wave of blood that causes its body to shake ever so slightly (not unlike how a hose kicks back when you turn on the faucet). When a whale is otherwise still, accelerometers can pick up those subtle movements.
Like an EKG sensor, accelerometers only work if they’re strapped to a whale. But these devices offer a big benefit: Scientists have been putting them on whales for roughly 20 years to measure other things, Czapanskiy said, which means there’s already plenty of potential heart rate data out there that just needs to be analyzed.
Heart rate data from the blue whale shows that these animals essentially have two different heart rates. One of them is slow, like the clip you heard above; that’s when the whale is diving and trying to conserve oxygen. The other is fast, when the whale is back at the surface and its heart is racing to replenish oxygen.
As researchers suspected, it’s there, on the surface, where big-bodied-ness could become a problem.
The EKG data shows that a single beat of the blue whale’s heart takes about 1.8 seconds, which means its heart can only beat roughly 33 times per minute. But as the whale was catching its breath, its heart was maxing out slightly above that number. This suggests something critical: The blue whale’s heart is working at “peak performance,” Czapanskiy said, and it literally can’t beat any faster.
But what does that have to do with the limits of body size? If the whale were any bigger, it would need a bigger heart and more food. But, again, a bigger heart would beat slower and require the animal to spend more time at the surface, giving the whale less time to forage for krill. So basically, any bigger, and these animals likely wouldn’t be able to consume enough food to sustain their hulking figures.
That’s why Czapanskiy has a hard time imagining even a hypothetical animal evolving to be larger than a blue whale. These animals live in an environment with an enormous amount of food, yet their bodies limit how quickly they can consume it. Unless a new and massive source of nutrient-rich food appears — or an animal evolves a highly novel and efficient physiology — the blue whale may not only be the largest animal to have lived but the largest animal to live, period.
That’s a theory, anyway.
It’s worth noting that there may be a handful of other factors that limit body size, such as the distribution and seasonal abundance of krill, said Jeremy Goldbogen, an associate professor at Stanford and lead author of the 2019 EKG study. There are also open questions about the ecology of blue whales, such as how much time they spend feeding. And that’s where additional research — and Czapanskiy’s accelerometer data — will likely come in.
Equipping whales with heart rate monitors could also benefit the animals. Just as Apple Watches detect an elevated heart rate when we’re nervous or scared, sensors on whales could reveal when these animals are under duress.
These devices could even help solve the enduring mystery of whale strandings, said Dave Haas, a marine scientist and co-founder of FaunaLabs, a company that develops Fitbit-like devices for whales, dolphins, and other animals.
Thousands of whales get beached every year, and yet scientists don’t really know why. At least in some cases, strandings appear to be linked to naval activity, leading some scientists to suspect that sonar might interfere with the navigation of some whales and dolphins.
“If we’re able to measure their physiology, we’re going to be able to see in real time what those signals are doing to their heart rate,” Haas said.
With heart rate monitors, scientists may be able to determine what stresses out whales and even test potential solutions. In the best-case scenario, Haas said, a subtle change in the frequency or intensity of sound emitted by ships might prove to be less harmful to whales. Maritime groups like the Navy — which has funded research into how whales respond to sonar, Haas said — might be amenable to those adjustments.
“That could have some big conservation outcomes,” Haas said.
In this way, eavesdropping on the heartbeats of whales completes a loop: It teaches us about how unique these animals are — how singular their anatomy is in the animal kingdom. But it may also help us preserve these marine giants — the largest animals on Earth, ever — for many years to come.