Animal life spans

Life span has two common meanings, which are often confused. Popularly, the term can refer to the longevity of an individual, but in biology, it is more abstract, a characteristic of the entire species rather than individual members. In this sense, life span is the maximum time an individual can live, given its environment and heredity, and life expectancy is the amount of time remaining at any point during its life span.

The great variety within the animal kingdom complicates the definition of life span. For some species, the life span is essentially the same as its life cycle, the return to the same developmental stage from one generation to the next. Salmon, for instance, hatch from eggs in small streams, migrate to the sea where they reach maturity, then struggle up rivers to return to the site of their hatching to produce more eggs. After completing the reproduction cycle, both males and females die. For most animal species, however, an individual may produce several generations of offspring before dying. In the case of humans, individuals can live long after their fertility ends.

It is often difficult to measure the life span for specific species, and in fact, a time period is seldom definitive for all species members. Rather, scientists recognize that mortality—the percentage of individuals that die each year (or each day in some cases)—increases for a population of organisms until, at some age, it reaches 100 percent, and all individuals in a generation are dead. Finding the life span of laboratory animals is fairly simple: A population is given the best possible living conditions, and observers wait for the last individual to die. That, presumably, is the optimal life span for the species. Much the same procedure can determine the life spans of pets and animals in zoos, except that the population to be observed is much smaller, sometimes only a single individual. Likewise, the records of thoroughbred domestic animals, born and raised in captivity, provide evidence for the maximum species life span. Most animals live in the wild, however, where investigations face a great variety of conditions and anecdotal evidence can be misleading. For example, biologists long thought that bowhead whales lived only about fifty or sixty years, but in the late 1990s, various new kinds of historical and biochemical evidence identified bowheads that lived well into their second century. Moreover, species whose members can go into dormancy show a large variation in individuals’ apparent longevity, even when all members pass through a single life cycle.

In general, however, the life span variation among species falls into a fairly narrow range of time. The shortest life spans, which last a single life cycle, can be a matter of days, while the longest lasts more than two hundred years. Humans enjoy the greatest longevity among primates. Scientists estimate the theoretical maximum human life span to be from 130 to 150 years, more than double that of the species with the next greatest endurance, gorillas and chimpanzees. However, several kinds of invertebrates live for more than two centuries.

Life Span Limiters and Extenders

While each species has a theoretical maximum life span, few, if any, individual animals reach it. Three general influences limit longevity: environmental pressures, variations in physiological processes, and heredity.

Most domesticated species have longer life spans, frequently two times longer than their wild relatives, and wild animals in captivity often live longer than in their natural habitat. The gray squirrel, for example, lives for three to six years in the wild but from fifteen to twenty years in a zoo. The reason is the safer, healthier, less stressful environment. Predators are one of the biggest threats in the wild, as are fluctuations in climate that affect the availability of food and shelter. Natural calamities, such as hurricanes, wildfires, and earthquakes, also take their toll. Conversely, some animals live shorter lifespans in captivity. For example, elephants often live for over sixty to seventy years in the wild, but in captivity, most do not reach forty.

Disease and chance injury also kill off many organisms, but even if disease is absent, many physiological processes in the body appear to degenerate or stop with age. Biochemists find that individual cells age and die. Cross-connections among connective tissue, such as ligaments and cartilage, gradually reduce the body’s flexibility and inhibit motion. Chemical plaques build up in brain tissue, hindering the electrochemical connection among neurons. Highly reactive oxidants, the ionized molecular byproducts of cellular metabolism, also build up with age and degrade the operation of cells’ mitochondria, the generators of chemical energy, as well as other cellular organelles. Moreover, laboratory tests reveal that cells can divide only a certain number of times—about fifty for some human cells—and then they die, a limit called the Hayflick finite doubling potential phenomenon. In connection with this finding, geneticists discovered that the lengths of deoxyribonucleic acid (DNA) at the tips of chromosomes, called telomeres, shorten with age and appear to play a role in cell dysfunction and death.

Other genetic factors help determine the age limit of cells and the entire organism. Research in the late 1990s with mice and roundworms uncovered genes that regulate cell life, including one kind that causes cells to cell suicide if their DNA or internal structure is too damaged to function properly or if the cells turn cancerous. Loss of function by genes damaged during cell division (mitosis) or by environmental toxins can impair a cell’s ability to maintain metabolism or set loose functionless or even outrightly harmful proteins and enzymes into the bloodstream and lymph system—all of which can bring on illness. Organisms that escape such effects still may face genetic disease. Most human populations, for instance, now have a longer life expectancy than ever before; because of it, neurodegenerative diseases (such as Alzheimer’s disease), cardiovascular dysfunction, and immunological disorders, caused or made possible by genes, grow ever more common. Because many of these life-shortening maladies were rare or nonexistent before the twentieth century, natural selection has not had a chance to cull them from the human genome. Similar diseases crop up in domestic animals and wild animals in captivity.

Late-twentieth-century research also identified several ways to extend life. A sharply reduced diet extends the lives of mice and fruit flies beyond their normal life span (although for the brown trout, a richer diet is life-extending), and a lower than normal temperature has the same effect on fruit flies, fish, and lizards. Discovery of the relation of telomere length to cell death and isolation of genes that order cells to suicide allowed scientists to bioengineer animals with cells whose DNA self-repaired the telomeres and genes that failed to trigger cell suicide; the result was individuals that, in some cases, had life spans twice the normal length or more. However, the most pervasive life-extending method is the development of social life—colonies, herds, packs—in which individuals work together to protect, shelter and feed themselves, and rear their offspring. In the case of humans (and perhaps some whales), culture and intelligence permit the species to pass on recently acquired information from one generation to the next and even alter the environment to lengthen collective and individual life spans.

Size and Life Span

Biologists noted long ago that large animals live longer than small animals, but why this should be true and why there were so many exceptions to this rule was a mystery. In 1883, Max Rubner proposed that the relation had to do with metabolism. Large animals have a smaller skin surface-to-body mass ratio than smaller animals; accordingly, large animals lose heat more slowly, and so can maintain body functions with less energy burned per unit of mass; in other words, a slower metabolism. Scientists found that individuals of different animal species all use about the same amount of chemical energy during their lives, twenty-five to forty million calories per pound per lifetime. (There are significant exceptions: Humans consume about eighty million calories per pound.) Because small animals must burn energy at a higher rate, the argument holds that they physically wear out faster.

In 1932, Max Kleiber derived a mathematical relationship for Rubner’s proposal. According to Kleiber’s law, also known as the quarter-power scaling law, as mass rises, pulse rate decreases by the one-fourth power. So, elephants, which have 104 times the mass of chickens, have a pulse rate one-tenth as fast. Scientists suggest that the relation results from the geometry of circulatory systems and point out that the quarter-power scaling law is pervasive in nature, but the underlying reason for it remains unknown. In any case, plenty of exceptions to the mass-life span correlation exist. With a life span of about one hundred years, box turtles outlive fellow reptiles, for example, and humans outlive all mammals (with the possible exception of some whale species) regardless of size. Small birds often only live one to two years, while some parrots live for eighty years. Exceptions also occur among domestic species living sheltered lives: Cats have longer life spans than dogs.

Theories of Life Span

A 1995 review of data from earlier animal studies suggested that heredity accounted for about 35 percent of the variation in life spans among invertebrates and mammals; 65 percent comes from unshared environmental influences. Nonetheless, theorists in the life sciences continue to debate the relative influence of genetic and other biochemical factors on the one hand and environmental factors on the other hand. The debate derives from the premise that life spans are the product of the natural selection that ensured species’ reproductive success. The proposals fall into three categories: random damage (stochastic) theory, programmed self-destruction theory, and ecological theory.

Random damage theories emphasize the wear and tear on the body that accumulates with metabolic action. It is the source of damage that differs from one theory to another. One holds that the buildup of metabolically produced antioxidants is the key factor, a spinoff of the long-standing conjecture that the faster an animal’s metabolism is, the shorter its life span. A second theory focuses on proteins that change over time until their effect on the body alters for the worse, especially when the proteins are involved in cellular repair. There is, for example, the altered connective tissue that causes the cross-connections stiffening tendons and ligaments. Another such change is the glycosylation of proteins or nucleic acids, in which a carbohydrate is added. Glycosylation is involved in such age-related disorders as cataracts, vascular degeneration among diabetics, and possibly atherosclerosis. A third theory points to the buildup of toxins inside cells, and a fourth concerns the potential problems that come from errors in metabolism or viral infection which slowly impair or kill cells. Fifth, the somatic mutations theory proposes that chance mutations accumulate in a person’s nuclear or mitochondrial genome and induce cell death or produce proteins and enzymes that have aging effects.

Programmed death theories hold, as the name suggests, that a species’ genetic heritage includes a built-in timer or damage sensor. Telomeres shorten as DNA ages until the genes at the end of chromosomes are unprotected and subject to deterioration during the splitting and gene crossover of mitosis. The genes then lose their ability to produce essential biochemicals, whose absence harms the body or leaves it defenseless against damage from infection or injury. Damage sensors can include the genes that instruct cancerous or malfunctioning cells to die. Although such genes clearly are a means to check the spread of disease, their cumulative effect may be harmful. Furthermore, scientists discovered genes that produce much more of, or less of, their metabolic products as cells age, which also contributes to the overall aging of the body.

The ecological theory draws conclusions about life span from a species’ role in its environment. Small animals have faster metabolisms and live shorter lives, it is argued, because they are not likely to escape predators for very long. Therefore, they evolved to mature and reproduce rapidly. Large animals typically have more defenses against predators and can afford to take life slowly. Moreover, animals that evolve defensive armor, spines, or poison also avoid predation and live longer than related species that do not. Finally, species that evolve mechanisms to withstand environmental stress, as from extreme temperatures or food scarcity, also have long life spans.

The theories assume that the life span for individuals within a species serves the survival of the entire species. Yet, even a species’ days on Earth are numbered. Environmental change can slowly squeeze them from their habitats, a catastrophe may wipe them out indiscriminately, or they may evolve into a new species. Scientists estimate that the average life span for a multicellular species lasts from one to fifteen million years. That average is stretched by several notable exceptions in the animal kingdom—such living fossils as crocodiles (140 million years old), horseshoe crabs (200 million), cockroaches (250 million), coelacanths (a type of fish, 400 million), and certain mollusks of the genus Neopilina (500 million).

Some animals have a trait called biological immunity, which means they can regenerate and live a nearly infinite number of years if they do not catch a disease, get eaten by a predator, or experience an environmental change that makes their regeneration impossible. In Antartica's Ross Sea, a glass sponge is estimated to be as old as 15,000 years, and near Florida's Key Latgo, scientists discovered a giant barrel sponge estimated to be 2,300 years old. Other living creatures have been documented living exceptional life spans, including an Albatross that lived 72 years, a lobster that lived 72 years, and a Galápagos tortoise that was documented at 185 years old.

Principal Terms

Life Cycle: The sequence of development beginning with a certain event in an organism’s life (such as the fertilization of a gamete), and ending with the same event in the next generation

Life Expectancy: The probable length of life remaining to an organism based upon the average life span of the population to which it belongs

Life Span: The maximum time between birth and death for the members of a species

Metabolism: The biochemical action by which energy is stored and used in the body to maintain life

Mortality Rate: The percentage of a population dying in a year

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