Sunday, January 6, 2013

5 Evolutionary Theories of Why Do Women Experience Menopause?

While it is absolutely clear why women experience menopause from biological perspectives, when you start looking from the global viewpoint, the answer to this question becomes blurred and distorted. After all, chimpanzees and baboons, our closest evolutionary relatives who we share 99% of our DNA with, are reproductive throughout their lifespan but human women can spend the last third of their lives infertile. Actually, there are just three species on the planet — humans, killer whales and pilot whales — where females routinely stop breeding years before the end of their lives.

According to evolutionary theory, the goal of any organism is to procreate—to pass on its genes. Thus, it has always been an enigma to evolutionary theorists why human females live so long after they have lost the ability to reproduce. They are the only ones in the primate family that have a long post-reproductive life. It is confusing in and of itself that they lose the ability to reproduce during their lifetime, as it is quite rare in the rest of the animal kingdom.

Female Lifespan Increase Hypothesis

One hypothesis to explain menopause is that humans didn’t live as long as we do now with modern medicine and that, in our natural environment (i.e. the African savanna 100,000 years ago), women never would have lived long enough to reach menopause. It is a known fact that modern indigenous people (many of whom live in environments similar to that of our distant African ancestors) have an average lifespan between 35-40. But the high infant mortality in these populations is skewing that statistic to make it seem that adults are dying younger than they are. Most modern hunter-gatherers, if they make it through childhood, will reach a ripe old age. So that can’t be the reason for menopause.

Grandmother Hypothesis

Dr. George C. Williams offered the first reasonable evolutionary explanation in 1957. He hypothesized that menopause is adaptive because it keeps older women from being exposed to the risks associated with childbirth (which were much higher for our ancestors). This allows them to remain alive long enough to ensure their children are raised to maturity to have their grandchildren (thus continuing the original mother’s gene line). This became known as the grandmother hypothesis.

Biologist Virpi Lummaa, whose recent work on evolutionary theory and birth control has conducted numerous highly regarded studies verifying the role of natural selection in human populations. Recent Lummaa’s work has also looked at a key prediction of evolutionary theory, namely, that an individual’s fitness will always be a trade-off between reproductive and somatic investment. In other words, the fitness differences (and by fitness we’re talking reproductive success) of an individual that has numerous offspring at an early age versus focusing instead on their somatic interests (physical health and growth) in order to reproduce at a later time.

This is what is known as Life History Theory. Many species go through what is called an r-selection strategy and put all of their investment in as many offspring as possible, many of whom will never reach sexual maturity. Others take a K-selection approach and produce fewer offspring but help ensure that each one grows to reproduce. What’s more, these strategies are not fixed but will frequently shift depending on available resources.

Utilizing life history theory, Lummaa suggests that reproductive senescence is actually an adaptation, and a fitness enhancing adaptation at that. So how exactly would cutting off 1/3 of an individual’s reproductive potential result in higher reproductive success? Therein lies the grandmother hypothesis and a key contribution of Dr. Lummaa’s work.

The grandmother hypothesis suggests that humans have “given up” their reproductive potential in later years in order to invest in the children they already have as well as their grandchildren. Naturally, this is an unconscious, biological adaptation that emerges over many generations and is not the result of individual decision-making. For such a hypothesis to be confirmed it would have to be demonstrated that children are significantly more likely to survive when a grandmother is present than when she isn’t.

Dr. Lummaa has done just that in her study published in the journal Nature, demonstrating that children are 12% more likely to survive to adulthood when they have a grandmother’s support than when they don’t. That may not seem like a lot, but consider all of the descendants from that surviving 12%, each carrying the trait for reproductive senescence, and you can see how it wouldn’t take long for the trait to become fixed in a population. Furthermore, one of the key innovations of her study was her choice of sample set. By using Finnish records dating from the 18th and 19th centuries she could ensure that any modern health benefits wouldn’t influence the results and would therefore accurately pinpoint the grandmother’s role.

Absent Father Hypothesis

While we are talking mainly to the process, where female is involved, the male influence should also be considered since you need “two for tango”. The absent father hypothesis proposes that reduced paternal investment linked with increasing maternal age was an additional impetus for the evolution of menopause. Reduced paternal investment may have been linked with increasing maternal age for two reasons: men’s defection from their middle-aged mates and men’s relatively earlier death. The absent father hypothesis is not an alternative to the grandmother hypothesis but rather a complement. It outlines an additional cost—reduced paternal investment—associated with continued reproduction by ancestral middle-aged women that could have been an additional impetus for the evolution of menopause.

One selective pressure associated with the high level of paternal investment in humans compared to other primates is the relatively helpless nature of human infants. Without prolonged investment from both mothers and fathers or related kin, infants and young children were more likely to die before they reached reproductive age. Children, whose fathers die, for example, suffer a generally higher death rate than children with fathers.

Reproductive Conflict Hypothesis

A different approach suggests that humans might in part have evolved this strategy in order to decrease competition between generations of reproducing women in one family and increase child survival in times when resources for childrearing were scarce.

Previous research had hinted that competition for resources, such as food, time and help with childcare between generations living under the same roof may have been a key factor in the evolution of menopause—a theory called the Reproductive Conflict Hypothesis. Based on the same Finnish data, mentioned earlier, researchers noted on potential reduced competition between the older women and their daughters-in-law, along with childcare provided for grandchildren, the fact which may explain the evolutionary benefits of "menopause".

In fact, more often during this time period, mothers and daughters-in-law lived under the same roof and thus shared the same resources, according to Lummaa. Daughters, however, typically married off and moved to live with a husband and his family.

Supporting their idea further, the researchers found that children born to older woman who were pregnant at the same time as a daughter-in-law were 50 percent less likely to survive to age 15. Meanwhile, children born to younger women pregnant at the same time as a mother-in-law, were 66 percent less likely to survive to adolescence. The researchers suggest competition for resources may explain this dip in offspring survival. However, simultaneous pregnancies between mothers and daughters had no significant effect on child survival.  

Patriarch Hypothesis 

The Patriarch Hypothesis is one more theory to explain the occurrence of menopause in human females as substantial evolutionary advantage. It is an alternative theory to the grandmother hypothesis which tends to ignore male benefits of continued spermatogenesis and their roles in assistance.

Patriarch hypothesis suggests selection pressure on male longevity extended the female lifespan; whose adjustment of life history has been constrained by the size of the ovaries – resulting in human females surviving beyond the age at which they can reproduce. Frank Marlowe first put forward the patriarch hypothesis, suggesting that if women survive beyond an age at which they can reproduce and men continue spermatogenesis, then old males stand to benefit greatly if they can copulate with younger females. It is theorized that increased use of tools and weapons compensates for the decline in natural fighting ability with age. This serves to produce a more stable male hierarchy, where attainment of high social status and reproductive access is less reliant on physical strength.

With such a scenario older males are able to retain a competitive ability with younger males, thereby asserting a selection pressure on extending longevity in males that could retain social status. Higher ranking males may also be a more attractive mate choice. One mechanism that could extend the lifespan is delaying the age at maturity. Offspring with a slower life history would exhibit a protracted period of dependence. If depletion of oocytes occurs at age 50, females should selectively counter this as it reduces their fecundity.

Recruitment of help from kin and husbands may compensate by enabling females to reduce birth intervals by weaning offspring at an earlier age. In addition, by passing on longevity to her sons, a female would stand to gain inclusive fitness.

Sources and Additional Information:

Related Posts Plugin for WordPress, Blogger...