An
evolutionary explanation for menopause suggests a “grandmother” hypothesis
which proposes that post-reproductive survival is favoured by kin selection as
relatives are helped by older females with reproduction (Johnstone & Cant.
2010). Another evolutionary explanation suggests that menopause is a hominine
adaptation, and the climacterium (a developmental phase accompanying menopause (McKinlay.
1996)) evolved due to intense reproductive investment early in life, or to
‘prevent’ reproduction late in life as there are such large costs following
with only small benefits (Peccei. 2001).
Jacobson, M. The
Sandwich Generation: Raising Kids, Caring for Parents (2015).
Reproductive
investment is costly in humans as offspring require a long period of dependence
as they are born helpless. This high cost becomes disadvantageous as there is a
large number of dependent offspring due to such short inter-birth intervals,
and all of which are nurtured simultaneously in humans. However, there are only
small benefits in late reproduction as there is an elevated risk of the fetus
having a defect, being born small or being miscarried (Peccei. 2001).
Late
reproduction may be costly, but, for a young mother to die during or shortly
after childbirth jeopardizes the life of not only the current child but also
for the children which are still heavily dependent on their mother for not only
protection but for nourishment also (Johnstons & Cant. 2010).
Thus
concluding that menopause may have evolved due to the costs of late
reproduction out-weighing the benefits that accompany it, as well evolving
genes selected to cease reproduction early compared to the genes that would
allow for risky reproduction late in life (Lahdenpera et al. 2004 & Peccei.
2001).
So
now it is known how this maladaptive trait has evolved, my next question is;
what animals, along with humans, experience menopause and menstruation?
References:
Jacobson, M. (2015). The Sandwich Generation: Raising Kids,
Caring for Parents. https://www.parentmap.com/article/the-sandwich-generation-raising-kids-caring-for-parents;
retrieved 20/03/2015.
Johnstone, R.A., & Cant, M.A. (2010). The evolution of
menopause in cetaceans and humans: the role of demography, 277(1701):3765-3771.
Doi: 10.1098/rspb.2010.0988.
Lahdenpera, M., Lummaa, V. & Russel, A.F. (2004). Menopause:
why does fertility end before life? 7:327-332.
McKinlay, S.M. (1996). The normal menopause transition: an
overview, 23(2): 137-145. Doi: 10.1013/0378-5122(95)00985-X.
Peccei, J.S. (2001). Menopause: Adaptation or Epiphenomenon.
Evolutionary Anthropology, 10:43-57.
Very interesting. You mention that, for humans, reproductive investment is costly due to long periods of dependence. But, elephant also have long periods of dependence. Similarly, you mention that costs to humans increase because of short inter-birth intervals and simultaneously nurturing. However, mice have short inter-litter intervals and can nurture up to 12 pups simultaneously. Why are these costs so important to humans, if they are shared by other species? I’m interested to learn more about menopause in other animals.
ReplyDelete