Big-Hearted: Arrhythmia in the World’s Largest Living Animal

Mariam Malik ‘22

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Figure 1. Among the many challenges of recording the blue-whale’s heartbeat, it was essential that the tag was secured onto the whale’s flappy, rigged skin. 

Bradycardia is a slower-than-normal heart rate, and can vary depending on age and physical condition. According to the American Heart Association, a heart rate lower than sixty beats per minute (BPM) qualifies as bradycardia. Tachycardia, on the other hand, is a heartbeat that is too fast, specifically one that beats over a hundred times per minute. Both conditions vary by age and physical fitness but researchers at Stanford University have tracked the heartbeat of a free-diving blue whale, the largest animal ever known to live on Earth, and found extreme cases of both bradycardia and tachycardia. 

Jeremy Goldbogen, assistant professor of biology at Stanford’s School of Humanities Science and lead author of the study, and Paul Ponganis, collaborator from Scripps Institution of Oceanography, used a custom-built ECG recorder that they incorporated into a unique suction-cup attached tag in which electrodes embedded. These electrodes in the suction feet recorded the mammal’s heart rate. Additionally, the development and decision of using the tag depended on finding a blue-whale, placing the tag on the right location on the whale and good physical contact with its skin, and ensuring the tag was functioning and recording properly. Due to the flappy and expansive nature of the skin, especially during feeding, the tag may easily come off. The team hypothesized that physical activity corresponds with the dive response and causes heart rate to accelerate during a whale’s dive. The researchers measured heart rates during dives as deep as 184m and lasting around sixteen minutes. Researchers found that when diving, the whale’s heartbeat decelerated to an average of about four to eight BPM, regardless of dive depth and duration, thereby categorizing it as extreme bradycardia Once the whale reached the bottom of a dive, where it would then lunge to consume prey, the heart rate would increase but then slowly decline. Then, after the whale began to reach the surface, heart rate would accelerate to its highest of 25 to 37 BPM. The lowest heart rate was approximately 30 to 50% lower than what the researchers expected it to be, and the highest heart rate surpassed predictions, assuming an average adult male whale of 23m and approximate body mass of 70,000kg. 

The extraordinary size and biology of blue whales has intrigued physiologists and the biological world for several decades. Goldbogen and Ponganis’s findings are the first ever recordings of a blue whale’s heartbeat. Furthermore, Goldbogen explains that animals that live with physiological extremes help scientists and researchers understand the biological limits of size. Such animals may also be vulnerable to change in the environment and/or food supply, thus such research could provide valuable insight into the handling of and conservation of endangered animals such as whales, dolphins, and sea turtles. 

 

References:

  1. J.A. Goldbogen, et al., Extreme bradycardia and tachycardia in the world’s largest animal. Proceedings of the National Academy of Sciences of the United States of America 116, 25329-25332 (2019). doi: 10.1073/pnas.1914273116
  2. Image retrieved from: https://en.wikipedia.org/wiki/Baleen_whale

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