experiential processes, all of which are systematically interactive over the life course. (3) The life-course pattern of any particular individual is affected by changes in the group and in the environment. (4) Not all individuals survive to old age. However, the importance of the elderly to the overall dynamics and welfare of the population may be disproportionately greater than their numbers.

Long life has several consequences for individuals in animal populations (Riley and Riley, 1986). First, long life prolongs opportunity to accumulate social and biological experiences. For example, long-lived individuals in social species may possess unique knowledge, such as the location of water during a drought or how to manufacture and use tools. Second, long life maximizes an individual's opportunities to complete or to change role "assignments." For example, a long-lived animal can ascend, lose, and re-attain alpha status within a troop, serve as a grandparent "caregiver"; become a forager after having been a colony "nurse"; become a breeder, by inheriting breeding territory, rather than remain a nest helper. Third, greater longevity prolongs an individual's relationships to others, including mates, parents, offspring, and associates. This feature increases the structural complexity of an individual's social networks, such as kinships, alliances, and communities. For example, when a monogamous pair of birds survives over multiple years, the relationship can become highly complex, leading to large clutch sizes and increased breeding success not seen in short-lived species (Coulson, 1966). In general, these consequences demonstrate that longer-lived individuals can significantly influence change within their group.

The elderly often exhibit behaviors fundamental to behavioral ecology. This section provides background on several of these concepts that frequently appear in literature on the elderly and thus are basic to understanding the broader nature of the roles of elderly individuals.

Altruism, defined by Wilson (1975) as "self-destructive behavior performed for the benefit of others," is relevant to understanding the role of the elderly in nature in several social contexts, including care-giving, allomothering, group defense, and nest care. Altruism is also apparent among the elderly in some nonsocial species. For example, altruism is demonstrated by older, foul-tasting Saturnid moths that sacrifice themselves to "teach" predators that younger counterparts are repugnant (Blest, 1963).

Hamilton (1964) proposed the primary theoretical background for explaining the evolution of altruistic behavior. Hamilton's theory based altruism on the