With a lot of ingenuity – and a little luck – researchers at Stanford University have monitored the heart rate of a blue whale in the wild for the first time.
Four suction cups secured a sensor-packed tag near the whale's left flipper, where it recorded the animal's heart rate through electrodes embedded in the centre of two of the suction feet.
"We had no idea it would work and were sceptical even when we saw the initial data,” says Jeremy Goldbogen, assistant professor of biology at Stanford, who led the project.
“But, with a very keen eye, Paul Ponganis – our collaborator from the Scripps Institution of Oceanography – identified the first heart beats in the data."
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 be bigger.
"Animals operating at physiological extremes can help us understand biological limits to size," says Goldbogen.
"They are particularly susceptible to changes in their environment, so these studies have important implications for the conservation and management of endangered species like blue whales."
Ten years ago, Goldbogen and Ponganis measured the heart rates of diving emperor penguins in Antarctica, and for years after wondered whether a similar task could be accomplished with whales.
"I 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 would work and record data," says Goldbogen.
The tag performed well on smaller, captive whales, but getting it near a wild blue whale's heart was muchharder, especially as blue whales have accordion-like skin on their underside that expands during feeding.
When the whale dove, its heart rate slowed, reaching an average minimum of 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 to around 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.
This data was intriguing because the whale's highest heart rate outperformed predictions, while the lowest heart rate was 50 per cent lower than predicted.
The team believe the surprisingly low heart rate may be explained by a stretchy aortic arch – part of the heart that moves blood out to the body – which, in the blue whale, 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.
The researchers are now hoping to add more capabilities to the tag – including an accelerometer that 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," says Goldbogen.