Post Reproductive Lifespan and the “Evolutionary Pass”
Why do Femina Sapiens can live even after reproductive senescence?
Context: There’s a correlation observed in nature; from plants to animals to fungi. An organism’s reproductive senescence coincides with its somatic senescence, i.e., they die after they lose their ability to reproduce. Technically, reproductive senescence is the process of ceasing reproductive ability or decrease in fertility, and somatic senescence is the process of ceasing cellular functioning, leading to the organism’s death.
From the evolution’s perspective, reproductive and somatic senescence is positively correlated, i.e., as and when the organism loses its ability to reproduce, its cellular functioning starts deteriorating exponentially. In other words, there is no point in keeping an organism alive if it cannot reproduce. A pattern most common in nature. Also, there is no incentive for evolution, as a lot of energy goes into repairing that organism to be able to live.
However, there is an exception. We, humans. Specifically, Femina Sapiens. The technical name for reproductive senescence in females is menopause, i.e., ceasing of the process of ovulation, starting sometime between the age of 45 to 50.
This begs the question: Why do females get this “evolutionary pass” to live even after reproductive senescence?
This is primarily because of two reasons:
- Firstly, human babies are dumb and helpless. They entirely depend on their parents, or more precisely on their mothers for their needs. In contrast, a baby chimp starts gathering food as they are weaned. A baby giraffe starts walking independently after an hour or so after birth, which is true for other animals like zebra, camel, pigs, etc. The development is so slow that the “human baby” is still dependent on their mother till they reach adulthood (or in some cases, maybe even later).
- Our inability to accomplish such feats just after birth was necessary, as evolution made a trade-off of an under-develop brain for a smaller body, as the female would die otherwise. Childbirth was (and even now, is) fatal; up until the developments in healthcare over the last century. A hunter-gatherer mother was gambling the lives of her offspring every subsequent childbirth. Hence, natural selection opted for a menopausal shutdown to protect the mother’s prior investment in children.
- In a study published in 2018, title: ‘What Did You Do Yesterday? A Meta-Analysis of Sex Differences in Episodic Memory’, the researchers analyzed which sex can remember episodic memories more. The answer went in the favor of females.
Why is it important? Well, memory played an integral role in the survival of the human species and its eventual dominance over other species. Our “presumably” dead cousins (Neanderthals, or Cro Magnons) didn’t have a lifespan, long enough to share their hunting experiences and mistakes to their offspring for better survival. Hence, if one of the members of the species (who lives longer and is infertile) can remember the events and be able to tell their offspring. And it even makes sense, because no caveman would want to mate with a female as the result was unfruitful.
In conclusion, although females go through monthly “hassle” for roughly 40 years, they get an “evolutionary pass” to live beyond their reproductive senescence age. (Although not a valid justification for the ‘“hassle”.) Let’s say an evolutionary trade-off!
SOURCES:
- The rise and fall of the third chimpanzee — Jared Diamond.
- Herlitz, A. et al. (2019) What did you do yesterday? A meta-analysis of sex differences in episodic memory. Psychological Bulletin. doi.org/10.1037/bul0000197
- Darren P. Croft, Lauren J.N. Brent, Daniel W. Franks, Michael A. Cant, The evolution of prolonged life after reproduction, Trends in Ecology & Evolution, Volume 30, Issue 7, 2015, Pages 407–416, ISSN 0169–5347, https://doi.org/10.1016/j.tree.2015.04.011
