DEGENERATIVE CHANGES

The decline in organ function is very important in relation to aging, but the end result of aging, death, is related to this decline in only a very peripheral way if at all. In general, the cause of death of older domestic animals is degenera­tive diseases.

25.4.1 Maximum Life Span and Life Expectancy

The maximum life span is a characteristic of the species. The maximum lifespan of a species is how long its longest observed member has lived. Life expectancy is the time at which approximately 50% of the members of a given popu­lation of a species still survive. It is the maximum num­ber of years a member of that species has been known to survive. The maximum human lifespan is estimated to be 121 years (Arking, 1998). The life spans of tortoises and

FIGURE 25.2 Model organisms of aging

lake trout are both unknown but are estimated to be more than 150 years. The maximum lifespan of a domestic dog is about 20 years, and that of a laboratory mouse is 4.5 years. Life expectancy, the amount of time a member of a spe­cies can expect to live, is not characteristic of species but of populations. It is usually defined as the age at which half the population still survives.

The phenomena of senescence and the diseases of aging are much more common today than they were a century ago. In 1900, people did not have the “luxury” of dying from heart attacks or cancers. These diseases generally occur in people over the age of 50. Rather, people died (as they are still dying in many parts of the world) from infectious dis­eases and parasites (Arking, 1998). Similarly, until recently, relatively few people exhibited the more general human senescent phenotype: graying hair, sagging and wrinkling skin, joint stiffness, osteoporosis (loss of bone calcium), loss of muscle fibers and muscular strength, memory loss, eyesight deterioration, and the slowing of sexual respon­siveness. Life expectancy is the key metric for assessing population health. The ages of some wild animals can be determined by annual marks in scales or bones similar to treerings.

25.5