Blue Whale and Humpback Whale Heart Rates Rise During Feeding Lunges

When a blue whale descends beneath the surface, its heart rate drops dramatically to save oxygen.

While diving hundreds of metres to feed, its enormous heart can slow to only two beats a minute.

If your own heart rate fell that far, you would probably lose consciousness.

Yet filter-feeding cetacean divers such as the blue whale (Balaenoptera musculus) and humpback whale (Megaptera novaeangliae) can still surge towards food despite the profound bradycardia affecting their bodies.

This behaviour has been called the "largest biomechanical event on Earth": a remarkable sequence of steep dives, turns and barrel rolls at depth as a whale gathers prey, all while its heart is beating at a sluggish pace.

Marine scientist Ashley Blawas, a researcher at Stanford University's Hopkins Marine Station, led a study recently published in PNAS examining how adaptable whales' heart rates can be when they are also expending substantial energy underwater.

"The challenge whales and other divers face is that they have to balance this oxygen-conserving dive response with the need to perform potentially oxygen-demanding exercise while underwater to capture prey," Blawas told ScienceAlert.

Earlier research by Blawas and her colleagues measured the energy whales use while feeding. For this study, the team instead concentrated on the animals' heart rates.

"What we found in this study," Blawas said, "is that blue whales and humpback whales have flexible heart rates underwater during foraging that increase to support prey capture via sprint-like lunges, and decrease slowly between lunges."

Blue whale and humpback whale heart-rate tracking

The team followed and tagged five blue whales and four humpback whales in Monterey Bay, the Channel Islands in California, and the Western Antarctic Peninsula.

To capture biometric readings, Blawas and her team fixed a suction-cup tag to every whale as it surfaced for air. The tag contains an accelerometer, gyroscope and magnetometer to monitor whale movement, alongside an ECG for measuring heart rate.

"Successfully recording an ECG from a whale requires stable attachment of electrical points of contact to the animal, elimination of saltwater intrusion that could electrically short your measurement, and placement of the device close enough to the heart to pick up these small signals," Blawas explained.

The whales' blubber prevents researchers from positioning the tag directly above the heart, however, which can leave the signal rather weak.

Even so, after multiple efforts to engineer the tags, the researchers produced a design suited to the task.

The devices remain in place for roughly 12-36 hours. Once detached, they float and transmit a satellite signal, enabling the scientists to retrieve them and download the gathered data.

Heart rates rise after feeding lunges

The recordings showed that directly after lunging for prey, blue whales reached heart rates of about 21.9 beats per minute (bpm), while humpbacks reached 28.2 bpm.

Their heart rates stay relatively high after a lunge before slowly falling as the whales filter seawater from their mouthfuls.

"This is similar to what we see in the heart rate patterns of humans recovering between interval sprints, where heart rate stays high even after the sprint is over and then slowly recovers," Blawas explained.

"These results suggest that, just like humans, rorquals whales likely rely on some amount of anaerobic energy stores for lunge sprints that oxygenated blood helps to renew between lunges."

A resting humpback whale's heart rate

Blawas said that one of the most exciting observations came from a humpback whale that was not diving at all, but resting instead.

"We happened to record the heart rate of a humpback that was doing what we call 'logging'… a restful behavior characterized by low to no movement when the animal stays at the surface for many minutes at a time," she explained.

"The heart rate data during this time is likely the closest anyone has come to a measurement of resting heart rate in a free-ranging whale!"

The whale's resting heart rate was 18.5 bmp, substantially lower than the typical human average of around 60 to 100 bpm. However, it matched what scientists would anticipate for an animal of this scale if it lived on land. Elephants, for example, have a resting heart rate of 30 bpm.

"It suggests that the same drivers of metabolism and oxygen demand during rest appear to be shared between terrestrial mammals and these big whales," Blawas added.

"Changes in resting heart rate can be used as an indicator of health, stress, and more, so starting to collect this data in whales will be useful for future studies assessing the impacts of things like changing prey populations or noise pollution."

The research appeared in PNAS.

This article was fact-checked by Clare Watson and edited by Clare Watson. Although we take pride in our process, we are only human. If you notice an error, please tell us.