Blue whales are the largest animals that have ever lived on Earth. They can stretch up to 100 feet (30 meters) long and weigh 300,000 pounds (136 metric tons), roughly four times the length and 20 times the weight of an African elephant. They also have the biggest hearts in the animal kingdom — about the size of a bumper car, and weighing some 400 pounds (180 kilograms).
Until now, no one had managed to record the heart rate of a blue whale. That's understandable, given the logistical difficulties of measuring such a huge animal's pulse while it swims in the open ocean. Thanks to a team of U.S. researchers, though, we not only have the first recording of a blue whale's heart rate, but we also get to see how it changes as the whale dives to feed, going as deep as 600 feet (180 meters) for as long as 16 minutes at a time.
Led by Jeremy Goldbogen, assistant professor of biology at Stanford University, the team used a specialized tracking device equipped with electrodes and other sensors, which they attached via suction cups to a wild blue whale in Monterey Bay, California. Their findings were published Nov. 25 in the Proceedings of the National Academy of Sciences.
"The largest animals of all time, of course, can't be in the laboratory in a building," Goldbogen says in a video about the new study. "So we are bringing the biomechanics lab into the open ocean using these suction-cup attach tags."
The data show how a blue whale's heart helps it perform its deep feeding dives, the researchers report, and they also suggest this enormous organ is operating near its limits. This could help explain why no animal has evolved to grow larger than a blue whale, since the energy needs of a larger body might surpass what's biologically possible for a heart to accommodate.
When the whale dove to feed, its heart rate slowed to an average of about four to five beats per minute, the researchers found, with a low of two beats per minute. It rose as the whale lunged for prey at the deepest point of its dive, increasing by about 2.5 times the minimum rate, then slowly fell again. A final surge occurred as the whale returned to catch its breath at the surface, where the highest heart rates of 25 to 37 beats per minute were recorded.
As the planet's largest animal, blue whales have a lot to teach us about biomechanics in general. But they're also listed as Endangered by the International Union for Conservation of Nature, and since their gigantic bodies are so dependent on a large, consistent food supply, insights like this could be particularly valuable for protecting the species.
"Animals that are operating at physiological extremes can help us understand biological limits to size," Goldbogen says in a press release. "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."
The researchers plan to add more features to their suction-cup tag for future studies, including an accelerometer to shed more light on how heart rate changes during various activities. They also hope to use the tag with humpbacks and other whales.
"A lot of what we do involves new technology and a lot of it relies on new ideas, new methods and new approaches," says co-author and Stanford research assistant David Cade, who placed the tag on the whale. "We're always looking to push the boundaries of how we can learn about these animals."