In some species, the older an animal gets, the larger it grows. Big sharks are old sharks, as are big crocodiles and snakes. In 1912, a reticulated python was found that measured thirty-two feet, ten inches in length, the longest ever, but no one knows how old it was. The age record for snakes goes to Popeye, a boa constrictor at the Philadelphia Zoo that died in 1977 at the great age of forty years and three months. Even for reptiles, however, Popeye's life span was modest. One Madagascar radiated turtle lived 188 years.
Most species, including humans, grow to a maximum size and then stop. The longest attested life span for a person is 122 years, achieved by France's Jeanne Louise Calment, who died in 1997, followed by Japan's Shigechiyo Izumi, who died just short of 121 years in 1986. Despite theories relating large body mass to long life span, some big and small animals in the wild have about the same life expectancy at birth: fifty years for both golden eagles and most whales, for instance, or twenty-four years for rhesus monkeys compared with twenty-five years for lions.
There is some disagreement over which animal species has the longest life span. The tubeworms growing on the ocean floor near hydrocarbon-seep sites in the Gulf of Mexico are thought to be as much as 250 years old, and so have been called the longest-lived noncolonial animals without backbones. However, some mollusks of the class Lamellibranchia, such as quahogs, a variety of clam, have also been called the longest-lived animal. They, too, can lie snug in their ocean bed for 250 years.
Whether classified as animals or not, bacteria hold the record for life span. If they are not eaten by predators or killed by environmental change, these single-cell creatures are theoretically immortal. Furthermore, they do not get larger with age: They divide.
tors 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 glyco-sylation 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 Athird 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).
See also: Aging; Birth; Death and dying; Demographics; Extinction; Extinction and evolutionary explosions; Genetics; Growth; Heterochrony; Mark, release, and recapture methods; Metabolic rates; Migration; Offspring care; Population analysis; Population fluctuations; Population genetics; Population growth; Predation; Reproduction; Reproductive strategies; Veterinary medicine; Wildlife management; Zoology; Zoos.
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