Heart rate, measured in beats per minute (bpm), is a physiological indicator of an animal’s metabolic pace. This rate is heavily influenced by body size; smaller creatures generally possess faster metabolisms and require more heartbeats to circulate oxygen and nutrients. Environmental factors, such as ambient temperature and physical activity level, also modulate how quickly a heart must pump to meet energy demands. The relationship between size and metabolism means that the largest animals tend to have the lowest baseline heart rates, setting the stage for one species to hold the record for the slowest measured heart rhythm.
The Animal with the Slowest Heartbeat
The animal with the slowest heart rate is the blue whale, the largest animal known to have ever existed. When feeding at great depths, its heart can slow to an astonishingly low rate of just two beats per minute (bpm). This extreme slowing, known as profound bradycardia, is a temporary state critical for deep diving. The typical heart rate during a deep foraging dive remains in the range of four to eight bpm. For context, a typical human resting heart rate falls between 60 and 100 bpm. When the blue whale surfaces to breathe, its heart rate accelerates dramatically, typically reaching between 25 and 37 bpm to rapidly re-oxygenate its massive body.
Physiological Reasons for Extreme Bradycardia
The blue whale’s ability to tolerate a slow heart rate is rooted in its immense size, which dictates a low mass-specific metabolic rate. The whale possesses a low surface-area-to-volume ratio, meaning it loses heat and energy slowly compared to a smaller mammal. This low basal metabolic demand is a prerequisite for sustaining a slow heart rhythm even at rest, where its predicted baseline heart rate is around 15 bpm.
The dramatic drop during a dive is a result of the mammalian dive reflex, an automatic physiological response activated upon submergence. This reflex triggers an immediate reduction in heart rate to conserve the body’s limited oxygen supply. The body redirects blood flow away from non-essential organs and tissues—a process called peripheral vasoconstriction—prioritizing the heart and brain. The whale’s circulatory system also features a specialized, large-diameter, elastic aortic arch. This highly compliant structure absorbs the large volume of blood ejected during a beat and then slowly releases it, maintaining continuous blood flow during the long pauses between heartbeats. This unique hemodynamic design allows the heart to function efficiently at rates that would cause circulatory failure in other mammals.
Dramatic Heart Rate Slowing in Other Species
While the blue whale holds the record for the lowest instantaneous heart rate, other species also exhibit controlled slowing as a survival strategy, often linked to induced states like torpor or prolonged diving. Hibernating mammals suppress their metabolic functions for months, leading to profound bradycardia.
Mammalian Examples
The thirteen-lined ground squirrel, a small rodent, can drop its active heart rate of 200 to 400 bpm down to 3 to 10 bpm when in deep winter torpor. This conserves energy when food is scarce and temperatures are low. Larger hibernators, such as bears, reduce their heart rate from a normal resting rate of 80 to 90 bpm down to 8 to 19 bpm during their winter sleep. Unlike smaller true hibernators, bears maintain a relatively high body temperature, suggesting their heart rate suppression is a method of energy conservation independent of the deep hypothermia seen in squirrels.
Marine Examples
The Northern elephant seal exhibits a remarkable response, with its surface heart rate of over 100 bpm occasionally plummeting to as low as three bpm during deep foraging dives. Aquatic reptiles also utilize this method of oxygen rationing. The leatherback sea turtle, a deep-diving species, has been recorded with instantaneous heart rates as low as 1.05 bpm during extended dives. Similarly, the loggerhead sea turtle can temporarily drop its heart rate to around two bpm when making deeper dives. These examples across different classes of vertebrates demonstrate that controlled, extreme heart rate reduction is a widespread and highly effective mechanism for extending survival time in oxygen-deprived environments.