An explanation for menopause

Scott Compton galahad at leland.Stanford.EDU
Fri Jun 24 16:12:36 EST 1994

In article <ssolberg.772472243 at morgan>,
Robert Lewis <ssolberg at morgan.ucs.mun.ca> wrote:
>andre at bwh.harvard.edu (Andre Robatino) writes:
>>  Which animals have menopause....?  
>3.  "...approximately halfway through the maximum life span..."
>Menopause occurs early in the life course--well before other systems of
>the body senesce, strongly suggesting that menopause should be viewed as
>a maturational phase rather than a deteriorative aspect of aging.  In
>the non-human primates, the decline in the reproductive system is not
>distinct from the decline in other organs and systems in aging animals.
>Reproductive senescence in the non-human primates is characteristic of
>extreme old age.  Menopause in women is characteristic of middle age.

This statement sounds flawed to me if we are looking at evolution
of traits.  Life expectancies are most likely on the increase within
the past few hundred years.  The average lifespan of a women living
a million years ago might be *earlier* than menopause.  Furthermore,
how do we really know that the time in which menopause starts has 
stayed a constant (in the midlife range).  It would seem more likely 
that menopause has variability since some women start in their 30's 
and some women start in the 50's or even later.  Such a variance might
suggest that menopause could shift over time.  If the life expectancy
is about 50 and that is when menopause starts on average now, it might 
seem more reasonable to say that with the sudden change of lifespan
in our recent evolutionary past, the menopause rates are correlated
to mortality rates if we take the fitness of an organism into 
consideration.  I'd like to know if really young women have
undergone menopause: like a teenager or even earlier.  Another
factor we might consider that is somewhat related is the loss
of menstural cycles when women exhert themselves too hard (I'm
thinking of track atheletes).  If there is too much stress on
the woman, the body stops cycling possibly as a response of
survival to save blood.  Could this little fact be indirectly 
coorelated with menopause?

>Menopause does not seem to occur naturally in other species, though
>there is some evidence that it may in whales.  Very few mammels
>naturally live beyond 50 years.
Exactly, and that is why as humans we have artifically extended
our lifespans beyond the normal range.

Scott Compton
Biological Sciences
Stanford University
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