Different species of animals and plants have different lifespans.

According to evolutionary theory, organisms that live for long periods manage to avoid disease, predation and accidents due to their defence mechanism and lifestyle.

These organisms are likely to have genes that code for a slow aging process, which often means good levels of cellular repair are achieved. The theory is based on the logic that if predation or accidents could prevent long lifespans in most individuals, then there would be less natural selection to increase lifespan.

Another evolutionary theory states that a limited availability of energy in the form of food (caloric restriction) causes lifespan to lengthen. In many animal models of caloric restriction, particularly in mice and rats, lifespan is almost doubled when caloric intake is strictly limited.

Furthermore, several studies provide evidence to support this theory by demonstrating a link between a decreased basal metabolic rate and a longer life expectancy.

This is thought to explain why an animal such as the tortoise can live as long as it does. Research looking at humans who have lived beyond one hundred years have also shown a decreased level of thyroid activity and a decreased metabolic rate.