The Etruscan shrew is one of the smallest mammals in the world, and its heart beats very fast – up to 1,500 times per minute, or 25 times per second. In contrast, the human heart is slow, beating only 60 to 100 times per minute.
Then there’s the heart of the blue whale, the largest animal ever built. These ocean giants can be longer than two school buses, and their hearts—about the size of a loveseat and weighing more than 1,000 pounds—beat only twice a minute.
It might sound like this if you put a giant stethoscope on the chest of an underwater blue whale.
The clip was made using real-world data scientists collected several years ago from a blue whale in Monterey Bay, California. The heart beats slowly as the animal descends, but when it comes to the surface to breathe, the heart rate rises sharply to 37 beats per minute.
Over the past few years, scientists have figured out how to listen to the heartbeat of wild whales. They weren’t interested in examining the animals’ vital signs per se, but were trying to answer one of biology’s most fundamental questions: How big can animals on Earth grow?
Measuring the heart rate of blue whales larger than dinosaurs suggests that body size may be limited by the size of the heart. With more advanced monitoring tools, it could also help scientists protect these marine behemoths from one of the ocean’s most mysterious threats.
How did the blue whale get so big?
Short answer: food. Millions of years ago, blue whales evolved to binge eat small crustaceans called krill, which are abundant in some coastal areas for part of the year. All of these foods can fuel a massive body, and the large size allows these animals to gulk krill and swim efficiently from one krill buffet to the next.
But interestingly, there are actually enough krill and other aquatic life in the ocean for whales to eat Big. Food alone doesn’t seem to limit the whales’ potential size, says Max Czapanskiy, a doctoral researcher who studies marine mammals at Stanford University. “There has to be something in their body that keeps them from getting bigger,” he said.
Scientists suspect that the answer may be in mind.
Whales hold their breath as they feed on krill, which tend to congregate hundreds of feet underwater. This causes carbon dioxide to build up in their blood. When these marine mammals return to the surface to breathe, their hearts beat fast to expel carbon dioxide from their bodies and replace them with fresh oxygen so they can dive back down to continue foraging.
A larger heart beats more slowly and takes longer to replenish oxygen in the body. This means that whales have to spend more time on the surface gasping for breath, which takes up valuable time when they feed on seasonal resources such as krill. With a heart too big, these behemoths may not have enough time to eat.
If heart size limits whales in some way, then these organs should theoretically maximize the speed of the animals when they come up to breathe. That’s one thing scientists were trying to figure out when they started measuring blue whales’ heartbeats in 2018.
There are many ways to measure our own heart rate, from simple stethoscopes to wearables like Fitbits and the Apple Watch. Measuring whales is much more difficult.
These animals are covered in a thick layer of blubber and can dive hundreds of feet, where the pressure is enormous. Even if the heart rate monitor worked under these conditions, scientists would have to find the whale, connect the device and retrieve it.
It wasn’t until 2018 that scientists were able to do this successfully. In late summer, California researchers study a swarm of blue whales above the waters of Monterey Bay.
On an inflatable research boat, the researchers approached one and used a 20-foot pole to attach a specialized EKG sensor behind its left flipper. The whale descended, and a few hours later, the device floated back to the surface, where researchers were able to retrieve it.
An electrocardiogram sensor, which measures electrical signals, recorded the animals’ heartbeats for hours. That’s where the video clip above comes from: Jessica Kendall-Ba, a marine scientist and artist unrelated to the study, turned a piece of heartbeat data into an audio file, which she shared with Vox.
But this approach has some serious drawbacks, said Czapanskiy, co-author of a 2019 paper based on the EKG study. “The failure rate is really high,” he said, noting that salt water often interferes with electronic sensors.
That’s why scientists have been looking for other ways. In a paper published in May, Czapanskiy showed that a device called an accelerometer — which measures an animal’s movement — can actually detect the pulse of the heart as well.
Every time the whale’s heart beats, it sends a wave of blood that causes its body to shudder slightly (unlike the way the hose bounces when the faucet is turned on). When the whale is stationary, the accelerometer can capture these subtle movements.
Like the ECG sensor, the accelerometer only works when strapped to the whale. But the devices offer one big benefit: Scientists have put them on whales for about 20 years to measure other things, Czapanskiy said, which means there’s already a lot of potential heart rate data to analyze.
What you can learn from the beating heart of a whale
Heart rate data from blue whales shows that these animals basically have two different heart rates. One of them is slow, like the clip you heard above; that’s when the whale dives and tries to conserve oxygen. The other is fast, and when the whale returns to the surface, its heart is revving up to replenish oxygen.
On the face of it, large body size could be a problem, as the researchers suspected.
The electrocardiogram data showed that the blue whale’s heart takes about 1.8 seconds to beat in a single beat, which means its heart can only beat about 33 times per minute. But just as the whale held its breath, its heart was slightly higher than that. This suggests something key: The blue whale’s heart is working at “peak performance,” Czapanskiy said, and it can’t actually beat faster.
But what does this have to do with size constraints?If the whale is bigger, it needs a bigger heart and more food.But, again, a larger heart beats more slowly and requires animals to spend more time on the water, reducing whales Time to look for krill. So basically, any larger animals, these animals may not be able to consume enough food to maintain their massive size.
That’s why it’s hard for Czapanskiy to imagine a hypothetical animal that would evolve to be bigger than a blue whale. These animals live in an environment with plenty of food, but their bodies limit the rate at which they can consume food. Unless a new, abundant, nutrient-rich food source emerges—or an animal evolves a highly novel and efficient physiology—blue whales may be not only the largest living animals, but also the largest living animals.
Anyway, this is a theory.
It’s worth noting that there may be other factors that limit body size, such as krill distribution and seasonal abundance, said Jeremy Goldbogen, associate professor at Stanford University and lead author of the 2019 EKG study. There are also unanswered questions about the ecology of blue whales, such as how much time they spend feeding. That’s where additional research — and Czapanskiy’s accelerometer data — might come in.
Solving the Mystery of Whale Stranding
Equipping whales with heart rate monitors can also benefit the animals. Just as the Apple Watch detects an elevated heart rate when we’re nervous or scared, sensors on whales can reveal when these animals are under duress.
The devices could even help solve the longstanding mystery of whale strandings, said Dave Haas, a marine scientist and co-founder of FaunaLabs, which develops Fitbit-like devices for whales, dolphins and other animals.
Thousands of whales strand each year, but scientists don’t really know why. In at least some cases, the strandings appear to be related to naval activity, leading some scientists to suspect that sonar may be interfering with the navigation of some whales and dolphins.
“If we can measure their physiology, we’ll be able to see in real time how those signals affect their heart rate,” Haas said.
With heart rate monitors, scientists might be able to determine the whale’s stress and even test potential solutions. In the best-case scenario, Haas said, subtle changes in the frequency or intensity of the sounds the boats make could prove to be less harmful to the whales. Haas said maritime organizations like the Navy — which funds research on how whales respond to sonar — may embrace these adjustments.
“This could lead to some major conservation outcomes,” Haas said.
In this way, eavesdropping on a whale’s heartbeat completes a cycle: it tells us how unique these animals are—and how unique their anatomy is in the animal kingdom. But it may also help us protect these ocean behemoths – the largest animals on Earth – for many years to come.