Discussion

General Themes

The overall theme of this paper is that, as a subgroup, the elderly play important roles in the cohesion and dynamics of groups, populations, and communities. They serve as care-givers, as guardians, as leaders, as stabilizing centers, as teachers, as sexual consorts, and as midwives. The elderly are at an evolutionary forefront when their brood-care behavior contributes to population fitness, which, in turn, may lead to evolution of sociality in insects and perhaps to other evolutionary pathways in other species. Virtually all behaviors exhibited by the elderly have an evolutionary basis, yet evolutionary aspects of behavior are seldom considered in the context of aging biology. Perhaps the time has come to expand the scope of gerontology and demography of aging to include behavioral to better incorporate the contributions of the elderly to the fitness of populations.

Another theme of this paper, particularly concerned with longevity as a preadaptation for the evolution of eusociality, is that it is important to distinguish between micro- and macroevolutionary aspects of aging and longevity change. Clearly, the evolutionary scale of change for fruit fly selection experiments (Rose, 1991) involving the cost of reproduction is different from changes involving the emergence of major taxonomic groups, new structures, physiological processes, or major adaptive shifts (i.e., eusociality). It is important to recognize differences in evolutionary scales because the macroevolutionary scale provides context for broad differences in life spans, which may shed light on more subtle, microevolutionary differences. Cost of reproduction tradeoffs or metabolic-rate differences may help explain longevity constraints in microevolutionary settings but provide little insight into why, for example, bats and birds live longer than similarly sized mammals. Explanations for these life span differences may have more to do with macroevolutionary causes, such as the evolution of sociality or changes in ecological factors, than with proximate factors, such as reproductive costs.

Biodemographic Perspectives on Life Span

In many species a substantial amount of energy is devoted to offspring care. Demographically, this results in reduction of birth rate. For example, Van Lawick-Goodall's (1976:86) description of the chimpanzee's early life is informative:

The infant does not start to walk until he is six months old, and he seldom ventures more than a few yards from his mother until he is over nine months old. He may ingest a few scraps of solid food when he is six months, but solids do not become a significant part of his diet until he is about two years of age and he



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