Grandmother hypothesis
Encyclopedia
The grandmother hypothesis is a theory to explain why menopause
, rare in mammal species, arose in human evolution and how a long post-fertile period (up to one third of a female's life-span) could confer an evolutionary advantage.
Menopause should, by Darwinian Theory, reduce fecundity
, making menopause intriguing from an evolutionary perspective, as natural selection
will operate vigorously to delay the expression of harmful alleles. It is, therefore, advocated that there is a genetic ‘intention’ for postmenopausal females.
; when menopause begins, only 400 eggs would have actually matured. The intriguing question is why somatic cells decline at a slower rate and why humans invest more in somatic longevity
than other primates.
More important than the question of why longevity has been extended, however, is why selection has not adjusted female life-history to match; as it appears to have done with males. The most frequently cited adaptive causes for the menopause are variations on the ‘mother’, or ‘grandmother’ hypothesis. These theories advocate that the high costs attributed to female reproduction could prevail over the benefits of continuous propagation. It is true that with advancing age and decreasing fertility, there is also a corresponding increase of miscarriages and birth defects, such as Down’s syndrome. Age is less significant in the increased foetal abnormalities than is the number of the ova left in the ovarian follicular reserves.
A possible explanation is the rate of oocyte depletion. With ova numbers fixed, it is logical to think extension of fertility would require increasing the size of the ovaries, which may be a limiting factor. However, the rate of atresia accelerates before age 40; if it did not, oocyte depletion would not occur until age 70. Regardless, limited oocyte production and accelerated follicular atresia
only explains why fertility ends in females and not why they survive for many years in a post-reproductive state.
See also non-adaptive hypotheses for menopause
was the first to posit that menopause might be an adaptation. Williams suggested that at some point during evolution, it became advantageous for females to stop "dividing [their] declining faculties between the care of extant offspring and the production of new ones" (p. 408). Since a female's dependent offspring would die as soon as she did, he argued, older mothers should stop producing new babies and focus on the offspring they already had. In so doing, they would avoid the risk of dying during childbirth and thereby eliminate a potential threat to the continued survival of current offspring.
In addition, postmenopausal women can contribute knowledge and skills to other group members to enhance group fitness
. If the other group member receiving investment were kin, then this would increase the fitness of a post-menopausal woman.
This kin selection
emerged with climate-driven changes, around 1.8 –1.7 million years ago, in female foraging and food sharing practices. These adjustments increased juvenile dependency, forcing mothers to opt for a low-ranked, common food source (tuber
s) that required adult skill to harvest and process. Such demands constrained female birth intervals and consequently their fertility; thus providing an opportunity for selection to favour the grandmother hypothesis.
Quantitative models, however, showed increased fitness, but not significantly. One of the reasons given for low benefits is the inflexibility of the age at which menopause occurs. It could be that fertility behaviour is adjusted depending on long-term expected fitness. Such choice, however, is not available to an already infertile female – she cannot ‘choose’ to redirect investment.
Another problem concerning the grandmother hypothesis is that it requires a history of female philopatry
and yet present day evidence shows that the majority of hunter-gatherer societies are patriarchal. Others dispute the hypothesis, arguing that the grandmother herself will use up resources that could be used for new young.
In addition, all variations on the mother, or grandmother effect, fail to explain longevity
with continued spermatogenesis
in males. It also fails to explain the detrimental effects of losing ovarian follicular activity, such as osteoporosis
, osteoarthritis
, Alzheimer’s disease and coronary artery disease.
Alternatively, the debilitating symptoms that usually accompany menopause in Western cultures could be seen as a natural cull of non-reproductive members of a species. Hot flashes, loss of short term memory, decreased ability to concentrate and difficulty in the learning of new tasks would, in the wild, leave the sufferer at greater risk from predators and topographical dangers such as falls from a height. This natural cull would leave more food, usually in relatively short supply, for the reproductive members of a species whose youth could mean that they are less experienced at finding it. However, cross-cultural studies of menopause have found that menopausal symptoms are quite variable among different populations, and that some populations of females do not recognize, and may not even experience, these "symptoms". This high level of variability in menopausal symptoms across populations brings into question the plausibility of menopause as a sort of "culling
agent" to eliminate non-reproductive females from competition with younger, fertile members of the species.
Menopause
Menopause is a term used to describe the permanent cessation of the primary functions of the human ovaries: the ripening and release of ova and the release of hormones that cause both the creation of the uterine lining and the subsequent shedding of the uterine lining...
, rare in mammal species, arose in human evolution and how a long post-fertile period (up to one third of a female's life-span) could confer an evolutionary advantage.
Menopause should, by Darwinian Theory, reduce fecundity
Fecundity
Fecundity, derived from the word fecund, generally refers to the ability to reproduce. In demography, fecundity is the potential reproductive capacity of an individual or population. In biology, the definition is more equivalent to fertility, or the actual reproductive rate of an organism or...
, making menopause intriguing from an evolutionary perspective, as natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
will operate vigorously to delay the expression of harmful alleles. It is, therefore, advocated that there is a genetic ‘intention’ for postmenopausal females.
Background
In female placentals, the number of ovarian oocytes is fixed during embryonic development; possibly as an adaptation to reduce the accumulation of mutations. At birth there are, typically, one million ovaOvum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
; when menopause begins, only 400 eggs would have actually matured. The intriguing question is why somatic cells decline at a slower rate and why humans invest more in somatic longevity
Longevity
The word "longevity" is sometimes used as a synonym for "life expectancy" in demography or known as "long life", especially when it concerns someone or something lasting longer than expected ....
than other primates.
More important than the question of why longevity has been extended, however, is why selection has not adjusted female life-history to match; as it appears to have done with males. The most frequently cited adaptive causes for the menopause are variations on the ‘mother’, or ‘grandmother’ hypothesis. These theories advocate that the high costs attributed to female reproduction could prevail over the benefits of continuous propagation. It is true that with advancing age and decreasing fertility, there is also a corresponding increase of miscarriages and birth defects, such as Down’s syndrome. Age is less significant in the increased foetal abnormalities than is the number of the ova left in the ovarian follicular reserves.
A possible explanation is the rate of oocyte depletion. With ova numbers fixed, it is logical to think extension of fertility would require increasing the size of the ovaries, which may be a limiting factor. However, the rate of atresia accelerates before age 40; if it did not, oocyte depletion would not occur until age 70. Regardless, limited oocyte production and accelerated follicular atresia
Ovarian follicle atresia
Ovarian follicle atresia is the periodic process in which immature ovarian follicles degenerate and are subsequently re-absorbed during the follicular phase of the menstrual cycle. Typically around 20 follicles mature each month and only a single follicle is ovulated. The rest undergo atresia...
only explains why fertility ends in females and not why they survive for many years in a post-reproductive state.
See also non-adaptive hypotheses for menopause
The grandmother effect
G.C. WilliamsGeorge C. Williams
Professor George Christopher Williams was an American evolutionary biologist.Williams was a professor emeritus of biology at the State University of New York at Stony Brook. He was best known for his vigorous critique of group selection. The work of Williams in this area, along with W. D...
was the first to posit that menopause might be an adaptation. Williams suggested that at some point during evolution, it became advantageous for females to stop "dividing [their] declining faculties between the care of extant offspring and the production of new ones" (p. 408). Since a female's dependent offspring would die as soon as she did, he argued, older mothers should stop producing new babies and focus on the offspring they already had. In so doing, they would avoid the risk of dying during childbirth and thereby eliminate a potential threat to the continued survival of current offspring.
In addition, postmenopausal women can contribute knowledge and skills to other group members to enhance group fitness
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...
. If the other group member receiving investment were kin, then this would increase the fitness of a post-menopausal woman.
This kin selection
Kin selection
Kin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of group/kin selection...
emerged with climate-driven changes, around 1.8 –1.7 million years ago, in female foraging and food sharing practices. These adjustments increased juvenile dependency, forcing mothers to opt for a low-ranked, common food source (tuber
Tuber
Tubers are various types of modified plant structures that are enlarged to store nutrients. They are used by plants to survive the winter or dry months and provide energy and nutrients for regrowth during the next growing season and they are a means of asexual reproduction...
s) that required adult skill to harvest and process. Such demands constrained female birth intervals and consequently their fertility; thus providing an opportunity for selection to favour the grandmother hypothesis.
Evidence for
If the grandmother effect were true, post-menopausal women should work harder than their still-fertile counter-parts and share those proceeds with their kin. Studies of Hadza women have provided such evidence, but they have not shown that this behaviour actually increased the grandmother’s fitness by producing more grandchildren. Furthermore, the role of Hadza men, who contribute 96% of the mean daily intake of protein, was ignored. Other studies also demonstrated reservations about behavioural similarities between the Hadza and our ancestors.Quantitative models, however, showed increased fitness, but not significantly. One of the reasons given for low benefits is the inflexibility of the age at which menopause occurs. It could be that fertility behaviour is adjusted depending on long-term expected fitness. Such choice, however, is not available to an already infertile female – she cannot ‘choose’ to redirect investment.
The grandmother effect and longevity
It is said that the grandmother hypothesis “is a central determinant of our longevity”; yet available data suggests that the reverse may be so – our longevity is a central determinant of the grandmother hypothesis. Further analysis of the historical data found that the length of a female’s post-reproductive lifespan was reflected in the reproductive success of her offspring and the survival of her grandchildren.Maternal vs paternal grandmothers
Similar studies found comparative affects but only in the maternal grandmother – paternal grandmothers had a detrimental effect on infant mortality, as well as differing assistance strategies for maternal and paternal grandmothers. Maternal grandmothers concentrate on offspring survival, whereas paternal grandmothers increase birth rates. These finding are actually consistent with the grandmother hypothesis because of paternity uncertainty. Equally, a grandmother could be both a maternal and paternal grandmother and thus in division of resources, a daughter’s offspring should be favoured.Evidence against
Such historical studies are, however, unable to quantify grandmotherly assistance; they are merely correlations between infant mortality and the existence of a grandparent. Those studies that have calculated assistance to both offspring and grandchildren have not found appreciable affects to warrant termination of fertility as early as 50.Another problem concerning the grandmother hypothesis is that it requires a history of female philopatry
Philopatry
Broadly, philopatry is the behaviour of remaining in, or returning to, an individual's birthplace. More specifically, in ecology philopatry is the behaviour of elder offspring sharing the parental burden in the upbringing of their siblings, a classic example of kin selection...
and yet present day evidence shows that the majority of hunter-gatherer societies are patriarchal. Others dispute the hypothesis, arguing that the grandmother herself will use up resources that could be used for new young.
In addition, all variations on the mother, or grandmother effect, fail to explain longevity
Longevity
The word "longevity" is sometimes used as a synonym for "life expectancy" in demography or known as "long life", especially when it concerns someone or something lasting longer than expected ....
with continued spermatogenesis
Spermatogenesis
Spermatogenesis is the process by which male primary germ cells undergo division, and produce a number of cells termed spermatogonia, from which the primary spermatocytes are derived. Each primary spermatocyte divides into two secondary spermatocytes, and each secondary spermatocyte into two...
in males. It also fails to explain the detrimental effects of losing ovarian follicular activity, such as osteoporosis
Osteoporosis
Osteoporosis is a disease of bones that leads to an increased risk of fracture. In osteoporosis the bone mineral density is reduced, bone microarchitecture is deteriorating, and the amount and variety of proteins in bone is altered...
, osteoarthritis
Osteoarthritis
Osteoarthritis also known as degenerative arthritis or degenerative joint disease, is a group of mechanical abnormalities involving degradation of joints, including articular cartilage and subchondral bone. Symptoms may include joint pain, tenderness, stiffness, locking, and sometimes an effusion...
, Alzheimer’s disease and coronary artery disease.
Alternatively, the debilitating symptoms that usually accompany menopause in Western cultures could be seen as a natural cull of non-reproductive members of a species. Hot flashes, loss of short term memory, decreased ability to concentrate and difficulty in the learning of new tasks would, in the wild, leave the sufferer at greater risk from predators and topographical dangers such as falls from a height. This natural cull would leave more food, usually in relatively short supply, for the reproductive members of a species whose youth could mean that they are less experienced at finding it. However, cross-cultural studies of menopause have found that menopausal symptoms are quite variable among different populations, and that some populations of females do not recognize, and may not even experience, these "symptoms". This high level of variability in menopausal symptoms across populations brings into question the plausibility of menopause as a sort of "culling
Culling
Culling is the process of removing animals from a group based on specific criteria. This is done either to reinforce certain desirable characteristics or to remove certain undesirable characteristics from the group...
agent" to eliminate non-reproductive females from competition with younger, fertile members of the species.