Blue whales are the largest and heaviest animals on earth – and their hearts are correspondingly big. How does this gigantic organ beat? Researchers have overheard the heartbeat of a blue whale and found that when diving, the giant’s heart beats far slower than expected – only two to eight times per minute. After surfacing, however, the whale’s pulse rises extremely up to 35 beats per minute. According to the researchers, the big heart of the blue whale could move at the maximum of what is biologically possible.
The heartbeat is essential for us and all higher animals in order to transport blood and thus oxygen through the body. How fast the heart beats in mammals depends on the intensity of the metabolism and indirectly also on the body size. In small mammals like the mouse, the heart rate is 600 beats per minute, in a pipebat it can even reach more than 900 beats. Large animals like the elephant, on the other hand, live at a much slower rate: their heart beats an average of 25 to 30 times per minute. But what about the largest animal in the world? According to calculations, a blue whale around 23 meters in length and 70 tons in weight has a heart that weighs around 319 kilograms – and with each stroke it pumps around 80 liters of blood through the body. But how fast the heart of such a giant beats has long been unclear.
An EKG machine for a blue whale
Only Jeremy Goldbogen from Stanford University and his colleagues succeeded in measuring this. For their study, they combined a mobile EKG device with four powerful suction cups that were supposed to hold the sensor package to the whale skin. The researchers threaded electrodes into two of the suction cups, which were supposed to capture the whale’s heartbeats and transmit them to the EKG machine. “I was really not sure that this would work because so many things had to work: we had to find a blue whale, attach the sensor in exactly the right place and in good contact with the whale skin and then of course make sure that the sensor works and records data, ”says Goldbogen. To make matters worse, the ECG electrodes have to be attached to the left side of the chest, directly within the pectoral fin. Not only is this area difficult to reach, but there is also very wrinkled skin that makes sticking problematic.
But it succeeded: the researchers placed their EKG sensor on the chest of a 15-year-old male blue whale that had been in Monterey Bay in California for a long time. “This gave us an 8.5-hour EKG recording from which we were able to create the whale’s heart rate profile,” report Goldbogen and his colleagues. Because the whale undertook several dives to search for food during this period, the researchers were also able to see for the first time how the whale’s heartbeat changed. During such “forays” the blue whale typically first dives deep and then, with its mouth wide open, shoots from below through collections of its prey – crabs or very small fish. While the water then drains off, the small prey animals get caught in his whiskers. “It is estimated that during such feeding actions the metabolic rate increases to 50 times the base value”, explain Goldbogen and his colleagues. They therefore expected the heartbeat to accelerate accordingly.
From one extreme to the other
However, this was not the case, as the EKG recordings revealed. According to this, the heart rate of the blue whale initially drops sharply while it dives into the depths: its large heart then only beats four to eight times, in extreme cases even only two times per minute. Surprisingly, this accelerates only a little even during the strenuous soaring up: the heartbeat of the blue whale is hardly faster than the normal resting pulse rate of around 15 beats per minute. That is 30 to 50 percent slower than expected – and too little to avoid an oxygen deficit in the blood and muscles, say the researchers. In their opinion, this could explain why these whale food dives are usually very short – the bodies of the gigantic marine mammals cannot tolerate such a debt of oxygen for long. However, the blue whale’s aortic arch could at least provide some compensation for the slow heartbeat. Because, as Goldbogen and his colleagues explain, in whales this bow can contract and thus force additional blood into the body between heartbeats.
When the blue whale has completed its dive and swims back to the surface to breathe, its heart falls to the other extreme: it starts beating very quickly. According to the EKG values, the pulse of the blue whale then skyrockets up to 30 to 37 beats per minute. This means that the whale’s heart rate is close to its maximum, as the researchers explain. They suspect that this is necessary in order to pump oxygen-rich blood through the body again as quickly as possible and thus pay off the oxygen debt of diving. In the opinion of Goldbogen and his team, the big heart of the blue whale is therefore close to the limits of what is biologically possible. That could also explain why blue whales couldn’t get any bigger – their hearts just couldn’t have cope with more.
(Video: Stanford University)
Source: Jeremy Goldbogen (Stanford University) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.1914273116