SREL Reprint #2641




Life history and demographic aspects of aging in the long-lived turtle (Emydoidea blandingii)

J. D. Congdon, R. D. Nagle, M. R Osentoski, O. M. Kinney, and R. C. van Loben Sels

More than 40 years ago Williams (1957) asked, "Why is it that after achieving the seemingly miraculous feat of morphogenesis, a complex metazoan is unable to perform the apparently much simpler task of merely maintaining that which is already formed?" Answering that question remains central to understanding the evolution of life histories in general, and aging and longevity specifically. The preponderance of relatively short-compared to long-lived organisms suggests that morphogenesis is easier to accomplish than is maintenance of soma, whereas the broad range of longevities of organisms (Finch 1999) demonstrates that maintaining soma for extended periods of time is possible. The underlying assumption of the "disposable soma" theory of aging (Kirkwood 1999) is that the expense of maintaining the immortal germ cells is always warranted, whereas investing in maintaining somatic cells depends on their contribution to the welfare of the germ cells. Because the death of many individuals results from extrinsic factors (predators, disease, accidents), large investiments in maintaining somatic cells are often not warranted. Kirkwood (1999) restated Williams' (1957) question to ask, ". . how long do germ cells need soma to last?" From a life history perspective, the question becomes, under what environmental conditions can selection favor prolonged investment in maintenance of soma?
To initiate the process of natural selection for longer life span, germ cells have to be housed within individuals that for some reason begin to survive longer and as a result produce more successful copies of their germ cell lines. Because most organisms are killed before they become old, escape from the extrinsic sources of mortality may be the most important mechanism initiating selection for traits that extend longevity. Escape from extrinsic mortality can happen by chance (e.g., invasion of islands without predators) and with little or no additional costs to individuals. For example, an insular population of the gecko Oplodactylus duvauceli delayed sexual maturity and lived much longer than did a closely related species on the mainland (Barwick 1982). Escape from extrinsic mortality can also occur by altering behavior, body design, or both (e.g., flight, development of poisons or irritants, or armor), mechanisms that are often associated with substantial costs to individuals. Ultimately, benefits to the germ cell line that are derived through increased investments in maintenance of soma are all that is important. . . .

SREL Reprint #2641

Congdon, J. D., R. D. Nagle, M. F. Osentoski, O. M. Kinney, and R. C. van Lobel Sels. 2003. Life history and demographic aspects of aging in the long-lived turtle (Emydoidea blandingii). pp. 15-31 In: Finch, et al. (Eds.). Brain and Longevity. Springer-Verlag Berlin Heidelberg.

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