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Tuesday, 3 December 2019

For the first time, the heartbeat of the blue whale has been recorded


With a length of up to 30 meters and a mass of up to 170 tonnes, the blue whale is currently considered to be the largest living animal and possibly the oldest living on Earth. To assume the physiological needs of such a template, the heart of the blue whale must be strong enough. Although marine biologists already knew that the animal's heart rate changes relatively quickly as it dives for food, they were surprised to find out how much. Indeed, during a dive, the heart of a blue whale goes from about 30 beats per minute to only 2.

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This is what a team of marine biologists discovered after recording for the first time the heartbeat of a blue whale. After placing a pulse monitor on a blue whale off the California coast, the researchers watched the gigantic creature sink and return to the surface for nearly 9 hours, alternately filling her lungs with air and her belly with hundreds of Pisces.


A heart with rapid variations to ensure physiological needs

During these deep foraging dives, the whale's heart rate changes abruptly, going up to 34 beats per minute at the surface and only two beats per minute in the deepest waters - which is about 30 at 50% slower than the researchers expected.

According to the new study published in PNAS , the mere fact of catching prey could push the heart of a blue whale into its physical limits - which could explain why no larger creature than the blue whale has ever been spotted on Earth.

" Animals that work at physiological extremes can help us understand the biological limits of size, " says Jeremy Goldbogen, a marine biologist at Stanford University. In other words: If the heart of a blue whale could not pump faster to feed its daily foraging expeditions, how could the heart of a larger animal pump even faster for more even bigger energy?

A slow heart rate during the dive

Blue whales are the largest animals ever to have lived on Earth. As adults, they can be more than 30 meters long, about the size of two school buses parked end-to-end. It takes a big heart to propel a creature of this size. The heart of a blue whale can weigh up to 180 kg (2015 failed specimen), about the size of a golf cart.

Scientists already knew that the pulse of a blue whale had to slow down deeply. When air-breathing mammals dive underwater, their bodies automatically begin to redistribute oxygen; the heart and brain receive more O2, while muscles, skin and other organs absorb less O2. This allows the animals to stay underwater longer with a single breath, resulting in a significantly lower heart rate than normal.

Graphs showing heartbeat variations of the blue whale as a function of depth and position of the animal. Credits: JA Goldbogen et al. 2019

This is true for humans as well as for blue whales. However, given the gigantic size of the whale and its ability to dive more than 300 meters deep, their hearts are pushed to limits far beyond ours. To find out exactly how much a blue whale's heart rate changes during a dive, the authors followed a group of whales they had previously studied in Monterey Bay, California, and fixed a special mounted sensor at the end of a 6 m pole on one of them.

A cardiac transition from 30 to 2 beats per minute

The studied whale was a male first sighted 15 years ago. The sensor was equipped with a plastic shell the size of a lunch box, equipped with four suction cups, two of which contain electrodes to measure the heart rate of the whale.

The researchers set the monitor on their first attempt, and he stayed there for 8½ hours when the whale dipped and resurfaced during dozens of foraging "missions".

Most of this time was spent underwater: the whale's longest dive lasted 16.5 minutes and reached a maximum depth of 184 m, while it never spent more than 4 minutes on the surface to fill the lungs. The sensor showed that, deep within each dive, the heart of the whale beat on average four to eight times per minute, with a minimum of two beats per minute.

Graphs showing the heart rate of the blue whale according to its lung volume and depth. Credits: JA Goldbogen et al. 2019

Between these low-tempo beats, the stretched aortic artery of the whale slowly contracted so that the oxygenated blood slowly moved into the body of the animal. Back on the surface, the whale's heart rate accelerated to 25 to 37 beats per minute, which quickly loaded the animal's bloodstream with enough oxygen to support the next deep dive.

The biggest heart on Earth

During these quick stopovers, the heart of the whale skirted its physical limits - it is unlikely that the heart of a whale could beat faster than that. This natural heart limit may explain why blue whales reach a certain size and why no known animal on Earth has ever been so tall.



Since a larger creature would need more oxygen to support its long, deep dive for food, his heart would need to beat even faster to get oxygen back to the surface. According to the authors of the study, this does not seem possible on the basis of current data.

Video presenting the work done by the researchers:



Bibliography:

Extreme bradycardia and tachycardia in the world’s largest animal

ORCID ProfileJ. A. Goldbogen, ORCID ProfileD. E. Cade, J. Calambokidis, M. F. Czapanskiy, J. Fahlbusch, A. S. Friedlaender, W. T. Gough, S. R. Kahane-Rapport, M. S. Savoca, K. V. Ponganis, and P. J. Ponganis

PNAS first published November 25, 2019

https://doi.org/10.1073/pnas.1914273116

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