Aging (zoology)

Progressive and irreversible change has been called the single common property of all aging systems. When change is reversible or self-maintaining, such as one would see in a forest, for example, the effects of aging are often not observable. Growth of the forest is evident, but with the right conditions, trees within the forest may grow for hundreds of years in the absence of disease. Certain conditions of the forest system help to regenerate, renew, and reverse changes that happen within that system.

However, in animals, some changes are not reversible. The changes in the cells of the body accumulate over time and result in a steady downward trend. The endpoint of this trend is the death of the organism. Aging is a normal part of the life cycle. This is known to be true because aging changes within populations are rather predictable. The changes associated with aging that are seen in all animal species may occur for similar reasons. These may include chemical aging, extracellular aging, intracellular aging, and aging of cells.

Aging occurs within body systems as a result of unseen changes at the molecular and cellular levels. Although the mechanisms through which aging occurs may be understood, the causes are less clear. The fact remains that due to changes in chemical balances, such as those of hormones, and to the dying of cells within the body, each of the bodily systems shows deterioration over time.

Changes that occur in domestic animals over the life span can be similar to those that occur in humans. Dogs experience the graying of their hair, a decrease in vision, and a slowing of movement with age. They also experience cataract formation, arthritis, skin problems, cancer, and diabetes. Certain breeds of animals may demonstrate a tendency toward specific illnesses or diseases. For example, German shepherds often develop hip problems, and collies commonly develop progressive arthritis that may seriously inhibit mobility by around ten years of age.

Common Effects of Aging

There are many variations in the effects of aging among the species of animals. The life span of animals may range from a few days (among insects) to thirty years or more, with great variation depending upon many factors. Animals that live in captivity as pets or in zoos where they are sheltered from the effects of predation, disease, and adverse climate also tend to live significantly longer than animals in the wild.

Little research has been done on the aging of most animal species because of the difficulty of observing animals over a long period of time in their natural habitat. Aging in monkeys has been studied more than in other animals because of the notion that aging patterns may closely reflect those of humans.

Aging monkeys show changes in their circulatory systems similar to those found in humans: There is notable atherosclerosis and arteriosclerosis or hardening of the arteries. The heart pumps less effectively, and vessels show a buildup of plaque. These changes often result in cardiac problems, including heart attacks. The respiratory system also shows a decrease in elasticity. Senile emphysema has been noted. The kidneys show signs of atrophy and sclerosis in aged monkeys. The kidneys of humans may lose up to half of the functioning nephrons with advanced age and thus become less effective in filtering waste products from the body.

Physical function or capacity tends to decline with age. This is largely due to the atrophy of muscles, which is more common as the body gets older. The joints tend to become stiffer and less mobile. Range of motion may be restricted. Changes in bone density may lead to loss of teeth, osteoporosis, and subsequent fractures. Tooth loss and osteoporosis have been documented in monkeys over the age of twenty years. Pictures of such older monkeys reveal a stooped posture, with shoulders hunched forward, similar to the kyphosis observed in many older human women.

Physical function among animals has been less studied than that in humans, but certain physiological characteristics are similar. For example, survival times after severe physical injury with blood loss and trauma decrease in both humans and animals as age increases. Male monkeys do not lose reproductive capabilities until toward the end of the life span, while females have a more restricted period of time to bear offspring. Fertility among all females tends to decline with age after its peak.

The immune system functions less effectively as age increases. This leaves the body more susceptible to a range of illnesses and diseases. Neoplasms, or tumors, are most common among mammals as they age. An impaired immune system allows various types of tumors or cancers to spread more rapidly in the older body. Response to stress and ability to adapt to stressors also decline with age. For example, older mice become less able to adapt to cold temperatures.

Social roles and behaviors among animals may also change with age. Longitudinal studies on animals in the wild are scarce, so only generalities may be speculated upon. Even studies done within controlled laboratory settings yield only broad suggestions since the number of animals available for study is limited. Males generally tend to dominate females in both physical strength and social ranks. Some nonhuman primates show different characteristics with advanced age. That is, some monkeys and baboons allow older males to remain part of the social group, while other species support the male leader in the group only as long as the female harem supports him, whether younger or older. Individual monkeys in stable groups have been observed to resort less frequently to aggressive behavior to maintain their status within the group.

Aging in animals provides a research foundation for improving human health and understanding aging. Fruit flies, roundworms, nonhuman primates, rodents, and birds are among the many animals that researchers use to investigate aging. Various genes have been identified as indicators of animal lifespan and certain diseases. For example, in roundworms, which only live around twenty days, over four hundred genes that extend their lifespan have been identified. Scientists also identified a gene they named Methuselah in fruit flies that can increase the fly's lifespan by 35 percent and developed a "supermodel granny" drug that extended the life of mice and other laboratory animals by up to 25 percent. Further research in zoological aging revealed that turtles and tortoises that experience an improvement in their environment may begin to age more slowly, and mammals under a calorie restriction may produce fewer free radicals, increasing their lifespan.

Causes of Death

Among nonhuman primates, the leading cause of spontaneous death is digestive problems. Older animals that die do not always show advanced signs of tissue aging. Since much less research has been done on aging among animals than among humans, data about causes of death are rare. However, as animals age, there is an increased probability of dying from trivial illnesses, perhaps due to decreased resistance factors. The lifespan of most animals can be estimated using correlations between brain weight, body weight, and resting metabolic rate. However, social and environmental factors impact lifespans significantly.

Predator-prey relationships among animals are particularly significant as causes of death. Thus, the effect of the environment on animal aging and death requires more investigation. Do animals age more quickly if they are objects of prey? Do animals relate to stress in ways similar to those of people, thus showing signs of wear and tear that are seen with premature aging under stress? Are there risk factors among animals that affect their life span? These are some of the questions that remain to be answered on the topic of aging among animals.

Principal Terms

Aging: A process common to all living organisms, eventually resulting in death or the conclusion of the life cycle

Cognition: Ability to perceive or understand

Death: The cessation of all body and brain functions

Function: Ability, capacity, performance

Life Span: Length of life from birth to death

Longevity: Length of life

Bibliography

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Gallagher, James. "'Supermodel Granny' Drug Extends Life in Animals." BBC, 17 July 2024, www.bbc.com/news/articles/cv2gr3x3xkno. Accessed 20 Sept. 2024.

Kohn, Robert R. Principles of Mammalian Aging. Prentice-Hall, 1971.

Newcomb, Beth. "University Professor Caleb Finch Discusses the Wide Spectrum of Aging Rates in the Animal Kingdom and What it Could Mean for Human Aging Study." USD Leonard Davis, 23 June 2022, gero.usc.edu/2022/06/23/aging-rate-varies-species. Accessed 25 June 2023.

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