How to tell an animal’s age in a heartbeat
For Newton, time was absolute, ‘outside’ all natural events. Relativity theory says, however, that time flows differently depending on how fast one is moving and any gravitational force acting. Then there is ‘physiological time’; species age at differing rates. According to an old rule of thumb, a heart is good for 800 million beats. Small creatures lose body-heat rapidly, so their hearts and lives move faster. Your collie dog, with up to 140 beats a minute, is lucky to celebrate their 12th birthday.
Human hearts, averaging 60 to 100 per minute, should fail by age 25 but, an exception to the rule, they last three times longer. Ireland’s smallest mammal, the pygmy shrew, clocks up 1,000 heartbeats every minute and lives for just a year. So for how does long can the world’s largest animal, the blue whale, survive? It is 12 million times heavier than a shrew. A child could swim along its aorta and the giant heart is the size of a small car. How slowly, therefore, does it beat?
Scientists from Stanford University have come up with the answer. In a paper just published, they describe placing an ‘ECG-depth recorder’, with electronic sensors, inside a small plastic box, equipped with four suction cups to hold it in place on a whale’s body. The suckers had electrodes buried within them, to record the creature’s heartbeats.
Finding a suitable whale, and approaching it in a small boat, was a challenge. When a 22m-long giant was spotted in Monterey Bay, a co-author of the paper, David Cade, used a 6m-long pole to place the recording device just behind the animal’s left flipper, where it could sense the creature’s heartbeat. The little brightly-coloured unit was recovered from the sea a day later. It had recorded nine hours of data.
During the monitoring period, the whale dived to 184m and remained submerged for up to 16.5 minutes. Its heart-rate during dives was four to eight beats per minute.
The lowest rate recorded was two beats per minute, 30% to 50% lower than theoretical calculations had suggested. As the animal ascended from a dive, the heart speeded up. The highest pulse rates were logged when it broke the water surface, ‘blowing’ and breathing rapidly to restore blood-oxygen levels. The 25 to 37 beats per minute recorded then, were close to “the estimated maximum heart-rate possible”, the authors say.
Blue whales indulge in ‘plunge feeding’, which the researchers describe as ‘a high-cost feeding mechanism, whereby large volumes of water are intermittently engulfed and filtered during dives’. Lunging has ‘an estimated metabolic rate 50 times the basal value’. That the heart-rate is slowest during this activity seems counter-intuitive.
The life-expectancy of another baleen whale species is known. A type of harpoon tip, not used by the Inuit for 200 years, has been found embedded in a bowhead whale’s skin; some individuals alive today, therefore, may have been born during the Napoleonic Wars. Blue whales, being larger, must have slower heart-rates. So how old can some of them be?
JA Goldbogen et al. Extreme bradycardia and tachycardia in the world’s largest animal. Proceedings of the National Academy of Sciences. 2019.
