To date, no heart rate data has been gathered for Earth’s largest creature at sea – the blue whale. To investigate the blood-pumping abilities of their 400-pound heart, a team of researchers tagged a whale off the coast of Monterey Bay, California, with sensor-packed suction cups. 

To do so is no easy task. With a 6-meter carbon fiber pole in hand, David Cade stood on the boat's custom-built platform as his colleague maneuvered the team near the creature. On the end of Cade's pole was a neon tag with four suction cups and two sensors. Despite the logistical difficulties of tagging a whale at sea and making sure its accordion-like underbelly didn’t pop the tag off in one gulp, the Stanford University team did it.

"It's really a team effort. You can't tag a whale by yourself," Cade, who is co-author of the study published in Proceedings of the National Academy of Sciences, told IFLScience. "My colleague James Fahlbusch is one of the best drivers out there and we've tagged a lot of whales together."

The tricky bit was getting the tag exactly where they wanted it. "We had to get it on the exact right spot (far forward and low on the left side of the whale) since we needed the tag to be close to the whale's heart in order to get a signal," he explained.

"Meanwhile, my colleague Shirel takes a biopsy sample and others are taking pictures (so a lot happens in those 4 seconds that the whale is at the surface)."

The effort paid off. The sensors recorded 8.5 hours of data from the whale as it surfaced, dived, and swam the open seas. Photo identification and biopsy analysis pegged the whale as a male around 15 years old, first sighted in 2003. 

At the deepest points of the whale’s foraging dives, its heart beat at 4 to 8 times per minute, reaching as low as 2 beats per minute – about 33 to 50 percent lower than predicted.

"We were interested in whether the heart rate would remain low or if the energy demands of feeding would override the typical physiological response of diving bradycardia," said lead author Jeremy Goldbogen, speaking to IFLScience. "What we found is that whales generally keep their heart rates low during dives to conserve oxygen during a breathe hold despite the high cost of feeding."

Blue whales are filter feeders with expandable throats to gulp down swarms of shrimp-like crustaceans. As it feeds, water is pushed out of its bristly baleen plates by its tongue, leaving just the prey trapped inside. The team suspect the low heart rate during a lunge dive is due in part to the species’ stretchy aortic arch, which it contracts to maintain blood flow in between heartbeats. 

After a deep dive, the whale’s surface heart rate pounded at between 25 to 37 times per minute as it recovered from its oxygen debt. The upper range is near the estimated maximum heart rate for such a creature, which may be why blue whales haven't evolved to be even bigger than they are. Shallower dives didn’t increase the whale’s heart rate as much, landing the male around 20 to 30 bpm at the surface.

"Whales appear to pay back the cost of feeding while at the sea surface by reaching maximum heart rate during a series of many breaths," said Goldbogen. However, more research is needed to explore why we currently do not see any animal that is larger than a blue whale.

"As an interesting comparison, the smallest mammals, shrews, have heart rates upwards of a thousand beats per minute," added Goldbogen. "Together, comparative data across the great diversity of mammalian body size tells us what the pace of life is like at different scales."