The heartbeat hypothesis postulates that every living creature has a limited number of heartbeats or breaths[dubious ]. 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.
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. Additionally, the life span of houseflies can be extended by preventing physical activity.
Support for this theory has been bolstered by several studies linking a lower basal metabolic rate (evident with a lowered heartbeat) to increased life expectancy.  This is thought to be the key to why animals like the Giant Tortoise can live over 150 years.  Studies in humans with 100+ year life spans have shown a link to decreased thyroid activity (lowered metabolic rate) to their longevity.
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.
- John R. Speakman (2005). "Body size, energy metabolism and lifespan". Journal of Experimental Biology 208 (Pt 9): 1717–1730. doi:10.1242/jeb.01556. PMID 15855403.
- Miquel J, Lundgren PR, Bensch KG, Atlan H (1976). "The effects of temperature on the aging process have been investigated in approximately 3500 imagoes of male Drosophila melanogaster". Mechanisms of Aging and Development 5 (5): 347–370. doi:10.1016/0047-6374(76)90034-8. PMID 823384.
- Ragland SS, Sohal RS (1975). "Ambient temperature, physical activity and aging in the housefly, Musca domestica". Experimental Gerontology 10 (5): 279–289. doi:10.1016/0531-5565(75)90005-4. PMID 1204688.
- Speakman JR, Selman C, McLaren JS, Harper EJ (2002). "Living fast, dying when? The link between aging and energetics". The Journal of Nutrition 132 (6, Supplement 2): 1583S–1597S. PMID 12042467.