Blue whales are the largest and heaviest animals on earth - and their hearts are correspondingly large.
|How often does a blue whale's heart beat.|
However, biologists could only guess how fast the pumping organ of these marine mammals beats.
Now, however, a research team has succeeded for the first time in monitoring the heartbeat of a blue whale. The sensor data reveal: During the dive, the giant's heart beats far more slowly than expected - only two to eight times per minute.
After the dive, however, the whale's pulse rises extremely to 35 beats per minute. According to the researchers, the large heart of the blue whale might already be operating at the maximum of what is biologically possible.
The heartbeat is essential for us and all animals to transport blood and oxygen through the body. In mammals, how fast the heart beats depends on the intensity of the metabolism and, indirectly, on body size.
In small mammals such as mice, the heart rate is 600 beats per minute; in dwarf bats it can even reach more than 900 beats per minute. Large animals such as elephants, on the other hand, live at a much slower rate: their heartbeats on average around 25 to 30 times per minute.
How much blood does a blue whale heart pump?According to calculations, a blue whale about 23 meters long and weighing 70 tons has a heart weighing about 319 kilograms - and with every beat it pumps about 80 liters of blood through its body. But nobody has yet been able to measure how fast the heart of such a giant beats.
An ECG device for a blue whaleJeremy Goldbogen from Stanford University and his colleagues have now succeeded in making this measurement for the first time. For their study, they combined a mobile ECG device with four powerful vacuum cups that were supposed to hold the sensor package on the whale skin.
|Stanford researchers recording of blue whale heart rate.|
Researchers monitored the heart rate of a blue whale in the wild.The researchers threaded electrodes into two of the vacuum cups to capture the whale's heartbeats and transmit them to the ECG device. "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 more difficult, the ECG electrodes on the left side of the chest must be placed directly inside the pectoral fin. This area is not only difficult to reach, it also has very wrinkled skin, which makes attachment problematic.
The researchers placed their ECG 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 ECG record from which we could create the whale's heart rate profile," Goldbogen and his colleagues report.
|Monitored the heart rate of a blue whale in the wild.|
Because the whale undertook several dives in search of food during this time, the researchers were also able to follow for the first time how the heart rate of the whale changed. In such "prey raids", the blue whale typically dives deep and then rushes from below with its mouth wide open to pass through the masses of its prey - crabs or very small fish. While the water then flows off, the small loot-animals get caught.
"During such feeding actions, the metabolic rate is estimated to increase to 50 times that of the base level," explains Goldbogen and his colleagues. They therefore expected the heartbeat to accelerate accordingly.
From one extreme to the otherHowever, this was not the case as the ECG records revealed. Accordingly, the heart rate of the blue whale initially drops sharply while it dives into the depths: its large heart then beats only four to eight times, in extreme cases even only twice per minute. Surprisingly, this accelerates only slightly even during the demanding high speed movement: 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 per cent slower than expected - and too little, in order to avoid an oxygen deficit in the blood and the muscles, say the researchers. In their opinion, this could explain why these whale feed dives are usually very short - the bodies of the gigantic marine mammals cannot tolerate such an oxygen deficit for long. However, the Blue Whales' aortic arch could at least compensate for the slow heartbeat. As Goldbogen and his colleagues explain, this arch can contract with the body of the whale and press additional blood inside the body in between the heartbeats.
Once the blue whale has completed its dive and swims back to the surface to breathe, its heart drops to the other extreme: it begins to beat very fast. According to the ECG values, the pulse of the blue whale then rushes 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.
|How often does a blue whale's heart beat.|
They suspect that this is necessary in order to pump oxygen-rich blood through the body as quickly as possible and thus settle the oxygen deficit from diving. According to Goldbogen and his team, the large heart of the blue whale as a whole is thus close to the limits of what is biologically possible. This could also explain why blue whales couldn't get any bigger - their heart simply couldn't take any more.