The heart rate of the blue whale – the largest animal on Earth – has been recorded for the first time, scientists say.
Researchers waited for the whale to surface before reaching out with a long pole to attach a non-invasive electrocardiogram to its skin. Four suction cups secured the sensor-packed tag near the whale’s left flipper, where it recorded the heart rate through electrodes embedded in the centre of two of the suction feet.
Jeremy Goldbogen, assistant professor of biology in the School of Humanities Sciences at Stanford University in America, and lead author of the paper, said: “We had no idea that this would work and we were sceptical 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. There were a lot of high fives and victory laps around the lab.”
Scientists say their analysis of the data suggests the blue whale’s heart is working at its limit, which may explain why the creatures have never evolved to be bigger. They say this may also 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.
The research published in Proceedings of the National Academy of Sciences shed light on the extreme ranges of heart rates in blue whales during diving, feeding, and surfacing.
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Blue whales slow their heart rate for deep dives but expend energy to lunge forward and engulf water for filtering and feeding.
Researchers monitored the heart rate of the whale along the surface of Monterey Bay during 8.5 hours of diving. Foraging dives lasted as long as 16.5 minutes and reached a maximum depth of 184 metres, whereas surface intervals were typically less than four minutes.
Reader Q&A: How big could an animal get?
Asked by: Liam Farmer, Birmingham
Based purely on the physical strength of bone and muscle, it has been calculated that land animals of at least 100 tonnes and possibly as much as 1,000 tonnes ought to be able to support their own weight and move around.
That’s much bigger than even the largest dinosaur (Argentinosaurus probably weighed 80 tonnes at most), but that’s because other limits cut in first.
The largest animal to have ever lived is the blue whale. At 180 tonnes, it already has to eat 1.5 million calories a day. Blue whales eat krill, which is one of the most abundant food sources in the ocean.
Even so, about half the global population of krill is eaten every year by whales, seals and fish. A single freak blue whale that was double the normal size could still probably find enough food to sustain itself.
But if all blue whales grew this big, the population would need to be smaller and they would reproduce even more slowly than they do now, making them more vulnerable to extinction.
Heart rates during dives reached a minimum of two beats per minute, well below the predicted resting heart rate of 15 beats per minute, and surged to 2.5 times the minimum heart rate during lunge feeding.
During surface intervals, the heart rate reached 37 beats per minute after very deep dives, near the blue whale’s maximum heart rate, as the whale worked to re-oxygenate its tissues. Scientists say their results show how the circulatory system of whales adapts to accommodate diving and feeding.
Ten years ago Prof Goldbogen and Mr Ponganis measured the heart rates of diving Emperor Penguins and for years wondered if they could do the same for whales.
Prof Goldbogen said: “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.”