Various species of plants and animals, including humans, have different lifespans. There is a well-developed evolutionary theory of aging, and general consensus in the academic community of evolutionary theorists; however the theory doesn't work well in practice, and there are many unexplained exceptions.
Evolutionary theory states that organisms that, by virtue of their defenses or lifestyle, live for long periods whilst avoiding accidents, disease, predation, etc., are likely to have genes that code for slow aging - which often translates to good cellular repair.
This is theorized to be true because if predation or accidental deaths prevent most individuals from living to an old age, then there will be less natural selection to increase intrinsic life span. The finding was supported in a classic study of opossums by Austad, however the opposite relationship was found in an equally-prominent study of guppies by Reznick.
One prominent and very popular theory attributes aging to a tight budget for food energy. The theory has difficulty with the caloric restriction effect observed in many animals, where tighter the calorific intake, lesser the rate of aging.
In theory, reproduction is costly and takes energy away from the repair processes that extend life spans. However, in actuality females of many species invest much more energy in reproduction than do their male counterparts, and live longer nevertheless.
In a broad survey of zoo animals, no relationship was found between the fertility of the animal and its life span.
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Last Updated: Feb 1, 2011