Since there is no verified instance of a person having lived 150 years, this number may, for purposes of illustration, be arbitrarily accepted as the maximum limit of the span of human life.
Species differences in longevity and aging
That there are large differences in life span between some species of animals has long been known, but only recently have the data become adequate for statistical analysis. Maximum life span provides an estimate of the potential longevity of mammalian and avian species because of the sharp upper limit of the survival curves in life tables. Also, it is superior to the average life span because the latter is influenced by environmental factors unrelated to aging (e.g., human protection).
The taxonomic stratification of longevity can be seen among the mammals. Primates, generally, are the longest lived group, although some small prosimians and New World monkeys have relatively short life spans. The murid (mouselike) rodents are short-lived; the sciurid (squirrel-like) rodents, however, can reach ages two to three times longer than the murids. Three traits have independent correlations with life span: brain weight, body weight, and resting metabolic rate. The dependence of life span on these traits can be expressed in the form of an equation: L = 5.5E 0.54S −0.34M −0.42. Mammalian life span (L) in months relates to brain weight (E ) and body weight (S ) in grams and to metabolic rate (M ) in calories per gram per hour. The positive exponent for E (0.54) indicates that longevity of mammals has a strong positive association with brain size, independent of body size or metabolic rate. The negative coefficient for metabolic rate implies that life span decreases as the rate of living increases, if brain and body weight are held constant. The negative partial coefficient for body weight indicates that the tendency for large animals to be longer lived results not from body size but rather from the high positive correlation of body weight with brain weight and its negative correlation with metabolic rate. The same kind of relation of L to E, S, and M holds for birds, but there is a tendency for birds to be longer lived than mammals of comparable brain and body size despite their higher body temperatures and metabolic rates. The larger reptiles have life spans exceeding those of mammals of comparable size, but their rates of metabolism are about ten times lower, so that their total lifetime energy expenditures are lower than those for mammals. The more highly cephalized animals (i.e., those with higher brain weight), especially the primates, have greater lifetime energy outputs; the total lifetime energy output per gram of tissue is about 1,200,000 calories for man and 400,000 calories for domestic animals such as cats and dogs.
The above relations hold for the homeothermic mammals, those with nearly constant body temperature. The heterothermic mammals, which are able to enter daily torpor, or seasonal hibernation, thereby reduce their metabolic rates more than tenfold. The insectivorous bats of temperate latitudes are the most dramatic example; although they have life spans in excess of 20 years, almost 80 percent of that time is spent in deep torpor. As a result, their lifetime energy expenditures are no greater than are those of other small mammals.
The longevities of arthropod species extend from a few days to several decades. The extremely short-lived insects have a brief single reproductive phase; the longer lived spiders and crustaceans are iteroparous, with annual reproductive cycles.
The inheritance of longevity
The inheritance of longevity in animal populations such as fruit flies and mice is determined by comparing the life tables of numerous inbred populations and some of their hybrids. The longevity of sample populations has been measured for more than 40 inbred strains of mice. Two experiments concur in finding that about 30 percent of longevity variation in female mice is genetically determined, whereas the heritability in male mice is about 20 percent. These values are comparable to the heritabilities of some physiological performances, such as lifetime egg or milk production, in domestic animals.
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