als is subject to substantial error. Only in the last several decades have detailed longitudinal data been systematically gathered on individuals of known ages and greater emphasis been placed on understanding age structure of populations (Finch and Ricklefs, 1991). Researchers have found that the variability in the rate of aging in wild animals is as great as that in humans, and in most species, it is virtually impossible to determine the exact age of an individual by its appearance or a biomarker. This is particularly true at older ages. Indeed, "physiologic heterogeneity" is a consistent characteristic of the elderly population (Timiras, 1994).
The broad objective of this paper is to extract important facts about aging and social and community structure in nonhumans and to reassemble them on a foundation of ecology and population biology. Just as certain morphological traits in animals, such as antler type, leg length, wing shape, visual acuity, gut length, and ear design, have been successfully related to the ecology and life histories of different animal species, we expect longevity and mortality patterns to be interpretable in the same context. We believe that this approach will provide insight into questions such as: What aspects of natural history favor the evolution of greater longevity? To what extent is life span a function of ecology and social structure? Is slowing of mortality at older ages a consequence of selection for certain adaptive traits at younger ages?
We organized this paper into three broad sections and a discussion. In the first section we introduce foundational principles we believe are important for understanding the relationship between longevity and life history of animals. These include biodemographic concepts and behavioral characteristics, such as altruism, dominance, territoriality, learning, and culture. In the second section we present a synopsis of roles and life histories of the elderly in three selected animal groups, including elephants, cetaceans, and primates, to provide specific biological context. In the third section we explore the concept of extended longevity as a preadaptation for the evolution of eusociality in wasps. This concept is important because it broadens the evolutionary scope from microevolution, as is discussed by Rose (1991) and Kirkwood (1985), to macroevolution concerned with speciation and the evolution of broad taxonomic groups (Strickberger, 1996). In the discussion we attempt to synthesize the general findings of the natural history and roles of the elderly, discuss some common life-history patterns associated with extended life span, and suggest future directions.
Life span of animals is not an orderly unfolding of precisely timed events from fertilization to death, with the elderly as a distinct, definitive stage. There exists no objective physiological landmark for aging, such as occurs with sexual