Xref: utzoo sci.bio:1246 sci.med:5781
Path: utzoo!attcan!uunet!husc6!bloom-beacon!bu-cs!purdue!decwrl!nsc!voder!blia!heather
From: heather@blia.BLI.COM (Heather Mackinnon)
Newsgroups: sci.bio,sci.med
Subject: Re: Mortality
Keywords: aging, DNA damage, fertility, evolution of external testicles
Message-ID: <4810@blia.BLI.COM>
Date: 13 Jun 88 21:37:09 GMT
References: <36@feedme.UUCP> <1894@silver.bacs.indiana.edu> <1898@silver.bacs.indiana.edu>
Organization: Britton Lee, Los Gatos, CA
Lines: 36

In article <1898@silver.bacs.indiana.edu>, chiaravi@silver.bacs.indiana.edu (Lucius Chiaraviglio) writes:
> In article <31342@yale-celray.yale.UUCP> briscoe-duke@CS.YALE.EDU (Duke
> Briscoe) writes:
> |In article <1894@silver.bacs.indiana.edu> chiaravi@silver.UUCP (Lucius
> |Chiaraviglio) writes: 
> 
> Animals become less fertile or infertile with age as the gametes quit
> functioning due to somatic aging, and also in females because of running out
> of eggs (no more eggs are made after the early part of life (before birth in
> humans)).

Human females are some of the only animals who outlive their time of
sexual fertility.  I can't believe that running out of eggs is a cause
of menopause.  Human females are born with some 100,000 eggs.  If you
assume that 25 eggs are used per year (2 per menstrual cycle, a very
high estimate), a human female still has enough eggs for about 4,000
years of fertility.  Also, if running out of eggs was a major cause
of menopause, we would expect that women with a large number of children
(each pregnancy causes a cessation of ovulation for roughly 2 years) to
enter menopause significantly later than women who have fewer children.
In fact, women tend to enter menopause at roughly the same age that
their mothers did, regardless of the number of children borne.

Hormone levels stop dropping in human females around the age of 25.  They
continue to drop until the hormone levels are insufficient to cause follicle
ripening, ovulation and menstruation.  The reasons for this are not well
understood.

The above is fact, as far as I understand.  Conjecture follows.

It may be that menopause is programmed to occur at a certain point in a
woman's life, much as menarche occurs.  Late childbearing is a risk to
the mother and puts her other living children at risk also.  Since human
children are dependent upon their parents for such a long time, menopause
may be a way of helping to insure that the parents stick around long enough
for the children to achieve independence.