Scientists have recorded the heart rate of a blue whale (Balaenoptera musculus) in the wild and found considerable extremes in how fast it beats.
When the whale dove for food, its heart rate dropped as low as two beats per minute, but back at the water’s surface it sped up to nearly 40.
This suggests it is working at its limit, write Jeremy Goldbogen, from Stanford University, US, and co-authors in the journal Proceedings of the National Academy of Sciences.
And it could explain why the world’s largest animal – with a heart weighing as much as a car – doesn’t get any bigger. Its heart wouldn’t be able to sustain higher energy needs.
Body size is intimately related to physiological functions, an observation known as allometry.
The quest to understand this has led researchers like Goldbogen and colleagues to explore the limits of body mass.
“From the smallest shrews to the largest whales, physiological performance at the extremes may shed light on constraints to body size,” they write.
But measuring the heartbeat of giant, wild roaming whales has proven somewhat difficult until now. The inspiration came from work with penguins.
Ten years ago, Goldbogen and colleague Paul Ponganis, from the Scripps Institution of Oceanography, measured the heart rates of diving emperor penguins (Aptenodytes forsteri) in Antarctica and wondered if it would be possible to do the same 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,” says Goldbogen.
They successfully tried the tag on smaller whales in captivity but getting it to the accordion-like belly of a blue whale and keeping it there was another challenge altogether.
David Cade was the lucky team member who successfully placed a non-invasive sensor tag on the whale, secured with four suction cups, which slid to a position near its left flipper where it could detect the heartbeat.
Once they managed to decipher the data, the researchers found the lowest rate averaged four to eight beats per minute, up to 50% lower than predicted. The upper rate averaged 25 to 37 beats, nearly outpacing predictions.
When blue whales dive for krill, their main food source, their heart slows down to lower the rate of oxygen store depletion as well as its own oxygen needs.
Goldbogen and colleagues found that the extremely low rate at the bottom of the dive increased by 2.5-fold as the whale powerfully ascended in a feeding lunge, then gradually decreased while it glided to filter the water out from the catch.
It rose to near maximum capacity when the whale resurfaced to replenish its oxygen stores.
The researchers think this explains why the whale’s heart has a high yielding, stretchy aortic arch, to accommodate blood pumped out by the heart and keep it flowing between beats.
Pete Gill, lead researcher from the Blue Whale Study, Australia’s longest-running research program on the species, is very impressed with the study, which he notes was carried out by an “exceptionally accomplished group of scientists”.
“This study appears to offer a new window into understanding the foraging ecology and energetics of this giant mammal by recording their heartbeat patterns during different behaviours,” he says.
“This may help to understand how blue whales balance energy expended in searching for food, against the nutritional value of the food they obtain, the razor’s edge that all animals walk. Or swim.
“It may also have useful applications in assessing stress of blue whales when faced with various anthropogenic stressors.”