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Aging in Animals: A Natural and Irreversible Process

Progressive and Irreversible Change

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 change is not reversible. The changes in the cells of the body accumulate over time and result in a steady downward trend. The end point 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
• Aging of cells

The Molecular and Cellular Basis of Aging

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.

Common Signs of Aging in Domestic Animals

Changes that occur in domestic animals over the life span can be similar to those that occur in humans. Dogs experience:

• Graying of hair
• Decrease in vision
• Slowing of movement with age
• Cataract formation
• Arthritis
• Skin problems
• Cancer
• Diabetes

Certain breeds of animals may demonstrate a tendency toward specific illnesses or diseases.

• German shepherds often develop hip problems.
• 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

Animals in captivity (pets or in zoos) where they are sheltered from predation, disease, and adverse climate tend to live significantly longer than those in the wild.

Very little research has been done on the aging of most animal species. The reasons include the difficulty of observing animals over a long period of time in their natural habitat.

Aging in Monkeys

Aging in monkeys has been studied more than that 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:

• Atherosclerosis and arteriosclerosis
• Decreased heart pump efficiency
• Buildup of plaque in vessels
• Cardiac problems, including heart attacks
• Decreased respiratory elasticity (senile emphysema)
• Kidney atrophy and sclerosis

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.

Decline in Physical Capacity

Physical function or capacity tends to decline with age. This is largely due to:

• Atrophy of muscles
• Stiff and less mobile joints
• Restricted range of motion
• Changes in bone density
• Tooth loss
• Osteoporosis
• 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.

Survival After Injury

Survival times after severe physical injury with blood loss and trauma decrease in both humans and animals as age increases.

Reproductive Aging

• Male monkeys do not lose reproductive capabilities until toward the end of the life span.
• Female fertility tends to decline with age after its peak.

Immunity, Disease, and Aging

The immune system functions less effectively as age increases, leaving the body more susceptible to:

• Illnesses
• Diseases
• Tumors or cancers

An impaired immune system allows various types of tumors to spread more rapidly in the older body.

Stress Response

Response to stress and the ability to adapt to stressors decline with age. For example:

• Older mice become less able to adapt to cold temperatures.

Social Roles and Aging Behavior

Social roles and behaviors among animals may also change with age. Longitudinal studies in the wild are scarce. Even controlled laboratory studies yield only broad suggestions. Observations include:

• Males generally dominate females in physical strength and social ranks.
• Some nonhuman primates allow older males to remain in the group.
• Other species support a male leader only with continued support from a female harem.
• Stable group monkeys resort less frequently to aggression with age.

Causes of Death in Aging Animals

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. There is an increased probability of dying from trivial illnesses, possibly due to decreased resistance factors.

Environmental and Predatory Factors

• Predator-prey relationships are significant causes of death.
• The environment’s role in aging and death requires further research.

Unanswered Questions

• Do animals age more quickly if they are preyed upon?
• Do stress and environmental challenges contribute to premature aging?
• What risk factors affect an animal’s life span?

These questions remain critical to advancing our understanding of aging among animals.

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References

• Bowden, Douglas M. Aging in Nonhuman Primates. New York: Van Nostrand Reinhold, 1979.
• Kohn, Robert R. Principles of Mammalian Aging. Englewood Cliffs, N.J.: Prentice-Hall, 1971.
• Schmidt-Nielsen, Knut. Animal Physiology: Adaptation and Environment. New York: Cambridge University Press, 1997.
• Slater, P. J. B. Essentials of Animal Behavior. New York: Cambridge University Press, 1999.
• Slobodkin, Lawrence B. Growth and Regulation of Animal Populations. New York: Holt, Rinehart and Winston, 1961.

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