This section contains brief descriptions of observations of living organisms and other empirical evidence regarding the nature of aging. Of special interest are observations that provide clues that distinguish between the main (programmed and non-programmed) theories of aging. Scientific reference literature relating to this section may be found in the Resources section.
Many plants and animals reproduce once and then die. This behavior is known as semelparity. Some other organisms such as some varieties of salmon are capable of multiple reproductive cycles but then die suddenly following one of them. In the case of the salmon, death follows dramatic rapid tissue deterioration similar to aging in other organisms but very rapid. A mammal, the male marsupial mouse, dies suddenly following mating.
The female octopus dies of starvation following brooding. This is evidence of a complex suicide mechanism involving the nervous system (behaviors). Some sort of suicide mechanism triggered by reproduction is signaling the nervous system such that the octopus does not feel hunger and therefore does not eat. Experiments have shown that removing optical organs interferes with this behavior and results in an octopus that can survive reproduction. This in turn suggests that sensing of some external condition may play a part in the behavior of the octopus and also demonstrates that the octopus does not die of "exhaustion" resulting from reproductive activity but rather as a result of a suicide mechanism.
Biological suicide is compatible with and expected by aging theories based on alternative evolutionary mechanics theories that hold that a limited life span generally provides evolutionary benefit. According to this view, gradual aging and semelparity are just different forms of a suicide mechanism or "life span management system." Some theorists consider gradual aging to be a more advanced form of such a system.
Biological suicide is incompatible with traditional evolutionary mechanics theory unless the behavior specifically and specially benefits mates of or direct descendents of the suicidal organism (or the organism itself). A female spider eating its mate and thus having more energy to reproduce could possibly be an example. The male spider's personal descendents benefit from its demise. In the case of the salmon, some traditional theorists suggest that the dead bodies of adult salmon provide food for their descendents. This is an example of the "cheater problem." Wouldn't descendents of non-suicidal salmon also benefit from the dead bodies of the suicidal salmon? Wouldn't the non-suicidal salmon then have a traditional evolutionary advantage over the suicidal salmon?
Some traditionalists contend that instances of sudden death following reproduction are not biological suicide but sexual or reproductive "exhaustion." The male marsupial mouse dies from exhaustion related to mating because using all its life energy in mating conveys a reproductive advantage over other less vigorous but non-suicidal mice that outweighs the disadvantage of not surviving for a subsequent mating season. The female mouse, even though it not only must mate but also support gestation and then nurturing of progeny can somehow survive, sometimes for several years. Other similar species have apparently worked out ways of reproducing without suicidal "exhaustion."
Life Span Regulation by Sensing of External Conditions
Some investigators report instances in which life span of simple organisms is mediated or regulated by sensing of external signals. This is typical of evolved mechanisms. In mammals, major internal biological processes are often regulated by sensing of external conditions. Examples: Circadian rhythms and annual reproductive cycles synchronize bodily processes to planetary cues.
Caloric Restriction and Life Span
Extensive experimental evidence confirms that mammal life spans are typically increased when food intake is restricted and that life span continues to increase all the way to semi-starvation levels. Programmed aging theorists suggest that this behavior has evolutionary benefit and was therefore selected. Caloric restriction has a group benefit in enhancing the survival potential of a group under famine conditions because a population that increased its life span while reducing its reproductive activity could survive as long with less food than another population of otherwise identical animals that did not extend their life spans and therefore had to reproduce more to maintain the same population. Merely surviving does not take as much energy as reproducing. This is a proposed example of an organism modifying an evolved genetically controlled behavior in real time to fit temporary external conditions. Non-programmed theories have difficulty explaining caloric restriction.
Stress and Life Span
Experimenters have found that several forms of stress in addition to caloric restriction counter-intuitively increase life spans in various organisms. For example, exercise increases life span and inactivity decreases life span. Followers of programmed aging theories suggest that this is also a selectable behavior with group benefit in a manner similar to caloric restriction. If a population of animals was under heavy predation, its members would no doubt feel more stress than another population that had few predators. If such a population increased its life span, that would tend to compensate for the higher death rate caused by predation. The adapting population would therefore have a competitive advantage over a non-adapting population. Non-programmed theories have difficulty with the stress response.
Several experimenters have reported discovering genes that limit life span in various organisms. Deleting the genes through genetic engineering has resulted in life span increases of as much as a factor of six. Genes and their associated processes are generally accepted to be evolved features of an organism. Followers of traditional evolutionary mechanics and non-programmed aging theories contend that the deleted genes must all have some individually beneficial function that compensates for their adverse nature. However, no such function has yet been found. See Aging Genes.
Cynthia Kenyon is a leading experimentalist in this area and has found aging genes, internal hormone signaling (e.g. between digestive system and aging function), and instances where a life span regulation system is mediated by detection of external signals. Valter Longo has also found experimental evidence for programmed aging.
Hutchinson-Guilford Progeria and Werner Syndrome
Hutchinson-Guilford progeria and Werner syndrome are single-gene human genetic diseases that dramatically accelerate multiple symptoms of aging. This suggests that there are mechanisms that are common to multiple manifestations such that a single-gene malfunction could affect multiple symptoms. This fits programmed aging theories (common life span management system) better than non-programmed theories.
Organisms that do not possess deterioration with age are important to aging theories and aging research because they suggest that aging is not the result of some fundamental and unalterable limitation and also provide clues distinguishing various theories. See Negligible Senescence for more.
Human Mortality Data and Animal Life Span Data
These pages provide information on human and animal life span characteristics that provide interesting insights into the aging process. One interesting factoid: Human death rates appear to decline after age 100.
See: Human Mortality Data by Age and Disease and Life Span Data for Various Animals.
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