For the first time, researchers have monitored the heart rate of a blue whale in the wild.
The measurement suggests that blue whale hearts operate at extremes—which may limit the size of Earth’s largest species.
Encased in a neon orange plastic shell, a collection of electronic sensors bobbed along the surface of the Monterey Bay, waiting for researchers to retrieve them. A lunchbox-sized speck in the vast waters, it held cargo of outsized importance: the first-ever recording of a blue whale‘s heart rate.
The device was fresh off a daylong ride on Earth’s largest species—a blue whale. Four suction cups had secured the sensor-packed tag near the whale’s left flipper, where it recorded the animal’s heart rate through electrodes embedded in the center of two of the suction feet.
“We had no idea that this would work and we were skeptical even when we saw the initial data. With a very keen eye, Paul Ponganis—our collaborator from the Scripps Institution of Oceanography—found the first heart beats in the data,” says Jeremy Goldbogen, assistant professor of biology in the School of Humanities Sciences at Stanford University and lead author of the paper, published in PNAS.
“There were a lot of high fives and victory laps around the lab.”
Analysis of the data suggests that a blue whale’s heart is already working at its limit, which may explain why blue whales have never evolved to get bigger. The data also suggest that some unusual features of the whale’s heart might help it perform at these extremes. Studies like this add to our fundamental knowledge of biology and can also inform conservation efforts.
“Animals that are operating at physiological extremes can help us understand biological limits to size,” Goldbogen says. “They may also be particularly susceptible to changes in their environment that could affect their food supply. Therefore, these studies may have important implications for the conservation and management of endangered species like blue whales.”
Tagging the whale
A decade ago, Goldbogen and Ponganis measured the heart rates of diving emperor penguins in Antarctica’s McMurdo Sound. For years after, they wondered whether a similar task could be accomplished with whales.
“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,” Goldbogen says.
“We had to put these tags out without really knowing whether or not they were going to work.”
The tag performed well on smaller, captive whales, but getting it near a wild blue whale’s heart is a different task. For one thing, wild whales aren’t trained to flip belly-up. For another, blue whales have accordion-like skin on their underside that expands during feeding, and one such gulp could pop the tag right off.
“We had to put these tags out without really knowing whether or not they were going to work,” says coauthor David Cade, a recent graduate of the Goldbogen Lab who placed the tag on the whale. “The only way to do it was to try it. So we did our best.”
Cade stuck the tag on his first attempt and, over time, it slid into a position near the flipper where it could pick up the heart’s signals. The data it captured showed striking extremes.
When the whale dove, its heart rate slowed, reaching an average minimum of about four to eight beats per minute—with a low of two beats per minute. At the bottom of a foraging dive, where the whale lunged and consumed prey, the heart rate increased about 2.5 times the minimum, then slowly decreased again.
Once the whale got its fill and began to surface, the heart rate increased. The highest heart rate—25 to 37 beats per minutes—occurred at the surface, where the whale was breathing and restoring its oxygen levels.
The data was intriguing because the whale’s highest heart rate almost outpaced predictions while the lowest heart rate was about 30% to 50% lower than predicted. The researchers think that a stretchy aortic arch—part of the heart that moves blood out to the body—explains the surprisingly low heart rate.
In blue whales, the arch slowly contracts to maintain some additional blood flow in between beats. Meanwhile, the impressively high rates may depend on subtleties in the heart’s movement and shape that prevent the pressure waves of each beat from disrupting blood flow.
Looking at the big picture, the researchers think the whale’s heart is performing near its limits. This may help explain why no animal has ever been larger than a blue whale—because the energy needs of a larger body would outpace what the heart can sustain.
Now, the researchers are hard at work adding more capabilities to the tag, including an accelerometer, which could help them better understand how different activities affect heart rate. They also want to try their tag on other members of the rorqual whale group, such as fin whales, humpbacks, and minke whales.
“A lot of what we do involves new technology and a lot of it relies on new ideas, new methods, and new approaches,” Cade says. “We’re always looking to push the boundaries of how we can learn about these animals.”
Source: Stanford University