Scientists have discovered something big in the heart of a blue whale

Blue whales are among the most fascinating creatures on the planet, existing at the limits of what we know to be possible. They are the largest animals to have ever lived on Earth, clocking in at up to 105-feet long and some 200 metric tons (that’s three double-decker buses and 30 T.rex worth of whale). They are also among the loudest-known animals on the planet — scientists think that, if ocean conditions are right, blue whales can communicate with another whale 1,000 miles away. Imagine being able to yell from New York and have someone hear you in Tampa. And though they dwarf us humans in so many respects, they can live just as long as we can, averaging 80-90 years.

Their solitary habits, relative scarcity (they are endangered), and sheer size mean that studying blue whales in the wild is difficult.

There is much we don’t know about these blue whales, including how their heart works. New research provides some clues about the heart of a blue whale, and like its other features, it seems to be operating at its limits, too.

Scientists have recorded the heartbeat of a blue whale

In a first, researchers have recorded the heartbeat of a blue whale over the course of eight and a half hours in the wild. What they found suggests blue whale hearts are working at their limits just to enable the whale to feed. As a result, these heart limitations may be what is stopping the whale from evolving to be even larger.

The findings were published this week in the journal Proceedings of the National Academy of Sciences.

What they found was something wonderfully big.

To measure the heartbeat of a blue whale, researchers attached an echocardiogram-depth monitor onto the underside of a 15-year-old male blue whale’s left fin as it swam in the waters of Monterey Bay, California.

The teenage whale is a regular to the bay, appearing there and elsewhere off the coast of California throughout its life. The lunchbox-sized sensor gathered 8.5 hours of data from the whale, capturing its heart rate as it undertook four different feeding dives of up to 16.5 minutes long and down to 184 meters in depth. Just getting the sensor to stay put was a feat in and of itself, the researchers said.

“I honestly thought it was a long shot because we had to get so many things right: finding a blue whale, getting the tag in just the right location on the whale, good contact with the whale’s skin and, of course, making sure the tag is working and recording data,” Jeremy Goldbogen, an author on the paper and associate professor at Stanford University, said in a statement.

What’s the heart rate of a blue whale? We now know.

Thought to be 70 metric tons in weight, the researchers estimate that the teen whale’s heart weighs 319 kilograms and pumps out 80 liters of blood with each beat. From that, they predicted the whale’s resting heart rate would be 15 beats per minute (bpm). But the patterns in heart rate they found defied their estimates.

When the whale descended into a dive, its heart rate plummeted to between 4-8 beats bpm, reaching as low as just 2 bpm on one dive — a third to a half as much as the resting heart rate predicted. On ascent, the whale’s heart rate accelerated quickly, maxing out at up to 37 bpm once back at surface, or more than 2.5 fold the predicted rate.

The results suggest something very big indeed: The heart of a blue whale is working at its physiological limits in the course of normal feeding behavior.

“This blue whale had heart rates ranging from 2 bpm to 37 bpm, which is more than an order of magnitude difference — 10-fold,” Goldbogen tells Inverse. “In comparison, human heart rates might typically range from 60 bpm to 200 bpm, which is a much lower range, just over a 3-fold difference.”

How do blue whales’ hearts do it?

Part of the reason why blue whales’ heart may have to work so hard is to do with how the creatures feed: To sustain themselves, the whales dive down into the water, briefly lunge up and take in huge volumes of water laden with their favorite prey, krill.

They then dive again, filter the water out through their comb-like teeth and swallow the prey before coming back to surface. This feeding strategy comes at a huge energy cost to the whales — it is just physically hard to dive, lunge up quickly, take in a volume of water larger than its own body (especially when the body is so big to start with), and keep moving with the high drag that weight creates, Goldbogen says.

The whale’s heart sustains this behavior due to certain unusual qualities, the study suggests. The heart’s aortic arch, which distributes blood from the left chamber of the heart to the body, may be highly elastic, the researchers posit. That would help ensure the necessary supply of oxygen got to the whale’s muscles as it moves through a feeding dive even though its heart rate drops and rises to such extremes, according to the study.

These unique qualities may also help explain why blue whales are the largest animal on Earth — and why they haven’t evolved to be even larger, the researchers say.

“New measures of vital rates and physiological rates help us understand how animals work at the upper extreme of body mass. What is life like and what is the pace of life at such a large scale?” Goldbogen says.

Existing at the limits of the possible does not come without potential costs, however.

“Given that the heart rates appear to be maximal during routine diving behavior, even larger whales might have trouble meeting physiological demands,” says Goldbogen. “Much more research needs to be done to explore the many possible explanations for why we currently do not see any animal that is larger than a blue whale.”