Life-Span

the duration of existence of an individual plant or animal and, sometimes, of a clone. A distinction is made between physiological, ecological, and average life-spans.

The physiological life-span is the maximum life expectancy for individuals of a particular species under optimum conditions of existence; thus, it is limited only genetically. The ecological life-span is the maximum age reached by individuals under natural conditions; it is dependent on many external factors. The average life-span is the age that individuals of a particular sample reach on the average, that is, the quotient obtained by dividing the total ages by the number of individuals. The average life-span is a variable statistical quantity that fluctuates during different periods of existence of a population. The physiological and ecological life-spans are constants for the corresponding species and population, and they are hard to differentiate. Reference is usually made to the maximum species life-span, regardless of whether it is observed under artificial or natural conditions.

In woody plants and some animals, including fossils, the lifespan can be precisely determined from annual rings. The lifespan of plants is also determined by comparing the thickness or size of the trunk with the annual increase, taking into account the number of whorls and the color and structure of the bark. The life-span of animals is determined by the habitus and by the extent of tooth abrasion or the development of bone sutures on the skull. Tagging, banding, and other types of direct registration are also used to determine the life-span of animals.

Differences in life-span have significant adaptive value, since individuals with a long reproductive period have the greatest opportunities to leave offspring. Some plants have exceedingly long life-spans. Sequoias, some cypresses, some yews, and some mosses encrusted with calcium carbonate have a life-span reaching 3,000 years or greater. Oaks, English walnuts, and chestnuts may live for more than 2,000 years, and the life-span of Siberian stone pines, lindens, and spruces is roughly 700 to 1,000 years. Plant clones have long life-spans; for example, vegetative reproduction has been traced for more than 300 years in the black poplar and some tulip species. However, the life-span of clones is usually much shorter than that of individuals of the same species grown from seeds. Most trees do not live more than 70 to 120 years. This is also the maximum life-span of algae (Laminaria), fungi (Phellinus), Pteridophyta and other spore plants, and many seed plants exposed to harsh ecological conditions—for example, in tundras (arctic willow) or deserts (Welwitschia, some wormwoods, and some tansies). Under severe ecological conditions there may also be a sharp decrease in the life-span, thus enabling a plant to pass through its life cycle in a short favorable season. Colonial plants, such as slime molds, some imperfect fungi, and certain algae live for approximately ten to 20 years. Anabiosis of spores and seeds represents a special case, since a marked lengthening of the life-span is achieved as a result of the almost total cessation of vital functions.

Among animals, maximum variation in the life-spans of both individuals and clones in relation to ecological conditions is characteristic of evolutionarily less advanced forms, unicellular organisms in particular. For example, suctorians of the genus Tokophrya live several days if food is abundant and several months if it is sparse. Some clones of the genus Paramaecium live no longer than two months, whereas others live as long as ten years. The maximum life-span of unicellular organisms is about a year. Judging from the results of experiments with labeled atoms, the physiological life-span of erythrocytes is about 120 days, although it is generally much longer for cells of multicellular organisms than for isolated cells. Neurons in the brain, like parenchymatous cells in the medullary rays of trees, may function 100 years or more.

Among multicellular animals, sponges live about ten to 15 years, some coelenterates (actinians) about 70 to 80 years, various worms from one to dozens of years, spiders four or five years (sometimes as long as 20 years, as in female tarantulas), and crustaceans from a few weeks (water fleas) to 50 years (lobsters). Insects in the imaginal stage usually do not live long, but some termites have a life-span of 40 to 60 years. The life-span of bivalve mollusks is about 100 years, but many mollusk species live only a few months or even weeks.

Among vertebrates, the maximum life-span of some fishes (sturgeons, pike) is more than 80 years; small fishes, such as anchovies and gobies, usually do not live more than 1½ to two years. Amphibians live about 60 to 70 years (giant salamander), turtles more than 150 years, birds about 70 years (great horned owl, condor), elephants 60 to 80 years (in captivity), gibbons about 32 years, and chimpanzees about 39 years.

In a number of cases the principle of proportionality between the physiological life-span and the period of growth—5:1—is applicable. However, there are other ratios, and many animal species, such as mollusks and fishes, grow all their lives. Large forms usually live longer than small ones. The cephalization index (ratio of the brain weight to the body weight) is most closely correlated with the life-span. It is expressed by the formula log ξ =0.6 log z; -0.23 log y +0.99, where ξ is the lifespan in years, y is the body weight in grams, and z is the brain weight in grams.

The mechanisms that determine the physiological life-span of each taxon may be passive or active. Passive mechanisms include the development of protective adaptations (for example, the shells of mollusks and carapaces of turtles) and low mobility. Active mechanisms include high mobility and intensification of the central integrating mechanisms and metabolic rates (in birds and mammals). Among animals with passive mechanisms of longevity, those that are the most mobile have the shortest life-span. For example, lizards and snakes have much shorter lives than turtles, and cephalopod mollusks live only three or four years, that is, one-tenth to one-twentieth as long as bivalve mollusks. On the other hand, among animals with active mechanisms for increasing the life-span, the hypodynamic forms do not live as long as animals with high mobility. For example, the life-span of rabbits is half as long as that of hares. Artificial hypodynamia markedly shortens the life-span of many mammals, whereas long periods of sleep or long periods without food sometimes lengthen it. The life-span of some organisms, including arthropods, decreases with rising ambient temperature.

Autoimmune diseases, accumulation of pigments and free radicals, and somatic mutations are among the more universal factors that shorten the life-span. Experiments have shown that it is possible to increase the life-span of mammals by acting on the endocrine system. Other factors that influence the life-span of animals and plants include reproduction, sex, heterosis, and the inheritance of biological traits that help to lengthen or shorten the life-span. Some fishes die after the first spawning, and monocarpic plants flower and bear fruit only once and then die. Female individuals of dioecious plants and female animals usually live longer than the males. Heterosis leads to an increase in the life-span of hybrid offspring.

The human life-span is determined not only by inherited biological traits but by social conditions (way of life, work, rest, eating habits). Humans live longer than most higher vertebrates, with some individuals reaching an age of 110 years or greater. “Record” life-spans, which are found in various countries and parts of the world, characterize the species, or biological, limit of human life expectancy. The maximum life-span, that is, the life-span of man as a biological species, does not seem to have changed significantly for many thousands of years. Individuals who attain the maximum life-span constitute only an insignificant percentage of the total population. For example, according to the 1970 census, only 19,300 of the 241.7 million persons living in the USSR were 100 years old or older (eight per 100,000 inhabitants).

The life expectancy can reflect the vitality of a population as a whole if the average number of years lived by a member, rather than record ages, is taken into account. In the Soviet Union, for example, more than half the inhabitants die between the ages of 70 and 75; the presence of several thousand long-lived individuals is not a significant factor. Special calculations based on probability (construction of mortality and average life-expectancy tables) are used to compute the life-span of a population as a whole and not that of individuals. It is virtually impossible to apply these calculations on the basis of observation to an actual generation of people. Such statistics would require many decades to accumulate, and the results would be meaningless, since they would represent the average life-span of a past generation and would reflect the influence of previous social and other factors. Therefore, the average life-span as a statistical index is computed as of a given time on the basis of actual ratios of living to dead at various ages. Precise figures on the number of living are obtained from censuses, and the same information on the deceased is found in earlier statistical materials. The computation of the average life-span is usually restricted to the two years closest to the date of the census.

Unlike the constant maximum life-span of man, the average life-span is variable. It reflects the efforts of society to prevent high mortality and to improve the health of the people. The average life-span in economically developed countries is 70 years. The highest life expectancy—74 years—is in Sweden, a country that has not taken part in war for more than 150 years. More than two-thirds of the world’s population lives in countries with high death rates and short life-spans. Life expectancy is lowest—about 40 years—in the countries of central and southern Africa.

The average life-span in Russia in 1896–97 was 32 years. Life expectancy in the USSR was 44 years in 1926–27, 69 years in 1958–59, and 70 years in 1970–71. This increase resulted from a rise in the standard of living, improvements in working and living conditions, recreational life, and eating habits; and advances in medicine and in health care.

The dynamics of the average life-span in most countries today is characterized by more favorable trends among women than among men. For example, the average life-span of women in Russia was two years longer than that of men. According to data from 1958–59, the life expectancy of women in the USSR was 7.4 years longer than that of men. It is now nine years longer: 74 years for women and 65 years for men. The lag among men is due to a number of social factors: specifically, “masculine” occupations (where the injury rate is quite high) and greater addiction to harmful habits. Advances in the prevention and treatment of diseases, lowering of the death rates (especially from injuries and accidents), and progress in gerontology and geriatrics should contribute to a further lengthening of the life-span.