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WELCOME
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| I recommend this comprehensive
overview of the possible reasons for human menopause for a first look
at the question. The extract below is only the conclusion of a long article.
http://www.mc.maricopa.edu/anthro/origins/women_change.html
The winners of evolution's race are those who can leave behind the most offspring to carry on their progenitors' genes. So doesn't it seem odd that human females should be hobbled in their prime by menopause? If one were playing God and deciding whether to make older women undergo menopause, one would do a balance sheet, adding up the benefits of menopause in one column for comparison with its costs in another column. The costs of menopause are the potential children of a woman's old age that she forgoes. The potential benefits include avoiding the increased risk of death due to childbirth and parenting at an advanced age, and thereby gaining the benefit of improved survival for one's grandchildren, prior children, and more distant relatives. The sizes of those benefits depend on many details: for example, how large the risk of death is in and after childbirth, how much that risk increases with age, how rapidly fertility decreases with age before menopause, and how rapidly it would continue to decrease in an aging woman who did not undergo menopause. All those factors are bound to differ between societies and are not easy for anthropologists to estimate. But natural selection is a more skilled mathematician because it has had millions of years in which to do the calculation. It concluded that menopause's benefits outweigh its costs, and that women can make more by making less. |
One
of the possible reasons for menopause is often called the Good Mother
theory. This is variously described, but this extract
from an 1834 medical text spells out the essentials with archaic
charm:
The reason of the discharge leaving the woman at this time of life, appears to be founded in Divine wisdom and beneficence; childbearing being thereby prevented beyond that period at which the mother would be capable of extending her care to her offspring, in the ordinary probabilities of human life; and thus consequently submit her child to the doubtful management of strangers, or subject it to the waywardness and caprice of those who could not feel a parent's affection, or would not yield a mother's devotion to its many necessities and wants, at a period at which its helplessness would most require the kindest offices. Here is a small part of a transcript of The Health Report with Norman Swan (an Australian radio show) at http://www.abc.net.au/rn/talks/8.30/helthrpt/stories/s11300.htm Other theories are also discussed Menopause
Natasha Mitchell: The other way that evolutionary biologists go about testing the adaptive advantage of menopause is by building mathematical models. These yield different results but some reveal that Good Mothering is an altogether unrealistic explanation. |
| The
Grandmother Hypothesis has been proposed as an alternative to the
Good Mother Theory. Here is part of an article about it at http://cas.bellarmine.edu/tietjen/images/why_life_after_menopause.htm
Why Life After Menopause?
Why do women live so long after they stop reproducing? <snip> And here you have Real Audio files from National Public Radio http://www.npr.org/ramfiles/980203.me.05.ram
More
theorizing, again focussing on grandmothers can be found at
... it is possible that the
menopause is an unintended outcome of some other biological shift.
The second model derives
from sociobiology and behavioral ecology. In this view, the reproductive
success of a female is measured not only by the number of her children
that survive, but the number of copies of her genes that survive into future
generations in the form of her grandchildren and other relatives. <snip>
Women who devote their time to helping raise grandchildren, instead of
having more children of their own, may therefore improve their representation
in successive generations more than those who try to reproduce healthy
children when they are over 40. |
| And
then there's the non-adaptive pure chance theory - if you are a baboon
- which is outlined at given at
http://www.augustachronicle.com/stories/042398/fea_124-4479.shtml Study challenges idea that menopause gives evolutionary advantage The researchers concluded that menopause -- instead of conferring a longterm evolutionary advantage -- is just a result of aging, like failing eeyesight or thinning hair Web posted Apr. 23 [98] at 01:15 AM By Malcolm Ritter Associated Press NEW YORK -- Scientists may have found the answers to two questions that might nag a woman entering menopause: Why is this happening? And why now? A different more serious report of the same study at http://www.brown.edu/Administration/George_Street_Journal/v22/v22n28/menopause.html Research team offers fresh findings on reason for menopause concludes: An author of the grandmother hypothesis says the latest findings still do not explain why, unlike baboons and lions, up to 40 percent of women live well beyond the decade needed to ensure that their offspring survive. |