For the first time, researchers have managed to record the heart rate of the largest animal that has ever lived on planet Earth — the blue whale.
The blue whale, also known as a sulfur-bottom whale, or by its Latin name, Balaenoptera musculus, is the largest animal known to have lived, with an average weight of 150 tons and a maximum length of 30 meters (m).
A host of mind-boggling figures characterize this colossal animal: Calves are about 8-m long and can weigh up to 90 kilograms (kg), an adult whale has about 100 long grooves on its throat and chest, and only its heart can weigh up to 700 kg — but how fast does this huge vital organ beat?
Understanding physiological parameters, such as this mammal's heartbeat, enable researchers to understand better its evolution, as well as better manage and preserve the species, which some list as endangered.
To find out how fast such a large heart can beat, researchers from Stanford University in California set out to place electrocardiogram sensors on a blue whale in Monterey Bay.
Jeremy Goldbogen, who is an assistant professor of biology in the School of Humanities Sciences at Stanford, is the lead author of the paper that details the exploits of the research team. The scientists collaborated with Paul Ponganis, from the Scripps Institution of Oceanography.
Goldbogen and team published their findings in Proceedings of the National Academy of Sciences.
Placing electronic sensors on a blue whale
The scientists had previously measured the heart rates of emperor penguins using a tag full of sensors, and they then decided to try out the system in whales.
The team trialed the sensor tag in small, captive whales, and it succeeded. However, applying the tag to a blue whale in the wild was a different feat altogether that entailed various other challenges.
Firstly, people have trained captive whales to flip their bellies up, which allows for easier access. Secondly, the grooves on the blue whale's underside enable the large mammal to expand a great deal when feeding, thus making it easy for the tag to detach.
"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 explains.
"We had to put these tags out without really knowing whether or not they were going to work," says study co-author David Cade, who also placed the tag on the whale. "The only way to do it was to try it. So we did our best."
Cade managed to stick the tag from the first try, and four suction cups secured the electronic tag near the mammal's left flipper, where it recorded its heart rate.
Blue whale's heart performs at extremes
Once the researchers had analyzed the data, it revealed intriguing insights. When diving, the whale's heart slowed to 4–8 beats per minute and a minimum of two beats per minute.
When the whale was at the bottom of the ocean feeding, that heartbeat raised 2.5 times more than the minimum, and then gradually slowed again.
When it rose back to the surface and breathed in oxygen, the whale increased its heart rate to 25–37 beats per minute — a rate that is "near the estimated maximum heart rate possible," as the authors write in their paper.
Overall, the whale's highest heart rate was near the extreme, and the low rate was 30–50 times lower than that which the researchers had predicted.
"Animals that are operating at physiological extremes can help us understand biological limits to size," says Goldbogen.
"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 think that the extreme limits near which a blue whale's heart operates may explain why there has never been another animal as large as this mammal — a heart would not be able to sustain the physiological needs of a larger body.