The Heartbeat hypothesis postulates that every living creature has a limited number of heartbeats or breaths. The hypothesis is based on two observations. First, that small mammals (such as a mouse) have rapid resting heart rate compared to a larger mammal (such as an elephant), and that their respective lifespans are inversely proportional to those rates. Second, is that athletically fit people tend to have a lower resting heart rate and tend to live longer than unhealthy people.^[1]

In 1926 Raymond Pearl proposed that longevity varies inversely with basal metabolic rate (the "rate of living hypothesis"). Support for this hypothesis comes from the fact that mammals with larger body size have longer maximum life spans and the fact that the longevity of fruit flies varies inversely with ambient temperature.^[2] Additionally, the life span of houseflies can be extended by preventing physical activity.^[3]

However, the ratio of resting metabolic rate to total daily energy expenditure can vary between 1.6 to 8.0 between species of mammals. Animals also vary in the degree of coupling between oxidative phosphorylation and ATP production, the amount of saturated fat in mitochondrial membranes, the amount of DNA repair, and many other factors that affect maximum life span.^[4]

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