Between Zeus and the Salmon The Biodemography of Longevity (1997) / Chapter Skim
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9 Postreproductive Survival
9 Postreproductive Survival
Pages 161-174
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From page 161... ...
Some of the special pleading for a novel evolutionary explanation for complete reproductive cessation and a long postreproductive life in human females derives from the misimpression that the more rapid senescent decline in reproductive capacity in women relative to men is a life-history feature unique to humans (Hill and Hurtado, 1991; Lancaster and King, 1992~. On the contrary, such a pattern is common among well-maintained captive species such as laboratory rats (Austad, 1994)
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From page 162... ...
By contrast, male gamete production is continuous throughout life. Therefore under captive conditions in which the major sources of natural mortality are removed as life span is much extended, it is not surprising that males continue to be reproductively competent somewhat longer than females.
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From page 163... ...
Consequently, they live much longer than in nature and, in fact, can frequently outlive the period over which natural selection has molded their reproductive physiology to remain intact. A well-documented example of this situation is the house mouse (Mus musculus)
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From page 164... ...
Life expectancy and maximum reported age is dramatically shorter in males relative to females in both pilot and killer whales. In neither of these cetacean species is there any evidence that mortality rates have plunged recently, so this long period of postreproductive life does seem to be a product of natural selection rather than an artifact of a recent rapid increase in longevity.
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From page 165... ...
Specific quantitative models of adaptive menopause reveal that the conditions under which it is favored are stringent, however, requiring enormously effective assistance of young individuals by postreproductive females (Hill and Hurtado, 1 99 1; Rogers, 1 993 )
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From page 166... ...
In this case, there is also no reason to suppose that an uncontrolled degenerative loss of homeostasis will proceed similarly from species to species, and there is no reason to suppose that rodent or even primate models of the postreproductive state will provide revealing mechanistic details of the postreproductive state in humans. On the other hand, if menopause is an adaptive physiological state molded by evolution to maximize fitness in the face of generalized senescence, then natural selection would presumably have tailored postreproductive physiology to the hyposteroidal state, suggesting that medical interventions to alleviate menopausal conditions should be approached with considerably more caution.
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From page 167... ...
With such a fundamental contradiction in evidence, this aspect of the adaptive menopause hypothesis must await further evidence. The second aspect of the hypothesis whether postreproductive females, if alive, could provide sufficient help to kin to offset their own potential reproduction has been approached primarily from a theoretical perspective.
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From page 168... ...
A somewhat different approach was used by Rogers (1993) , who used the particularly detailed demographic information from Chinese farmers in turn-ofthe-century Taiwan as a surrogate demographic model of Paleolithic human life to ask mathematically exactly how much help postreproductive mothers would have had to provide to their children for adaptive menopause to have evolved.
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From page 169... ...
Thus in many natural populations, demographic studies allow the definition of ages beyond which animals will probably not survive, at which the likelihood of death is substantially increased, and those ages might sensibly be considered elderly, regardless of whether those animals are obviously debilitated at that time in life. Given this definition of elderly, animals that acquire or accumulate important resources such as food caches, complex and costly burrow systems or nests, or foraging and breeding territories during their lifetime will inevitably influence their own reproductive success and the ultimate genetic structure of populations by the manner in which those resources are disposed of as they approach the end of life specifically, whether those resources will be available to offspring.
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From page 170... ...
Because mound improvement and renewing of cached seeds occur throughout life, resources acquired during the lifetime of females are passed along to their offspring. No information exists on whether males bequeath mounds to their offspring (Jones, 1987~; however, it is improbable that males of this polygynous species can even recognize their own offspring.
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From page 171... ...
in the coastal waters of British Columbia Canada and Washington State USA. Report of the International Whaling Commission (Special Issue 12)
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From page 172... ...
Report of the International Whaling Commission (Special Issue 6)
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From page 173... ...
Kasuya 1984 Changes in the ovaries of the short-finned pilot whale, Globicephala macrorhynchus, with age and reproductive activity. Report of the International Whaling Commission (Special Issue 6)
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From page 174... ...
Waser, P.M. 1978 Postreproductive survival and behavior in a free-ranging female mangabey.
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