“Ghost of reproductive competition past”.

Cant and Johnstones complementing hypothesis that early reproductive cessation reflects “the ghost of reproductive competition past” (Hrdy 2009 and Cant and Johnstone 2008) predicts that there will be obvious costs to females that are breeding alongside a reproductive grandmother. This hypothesis does not imply that older females who have experienced menopause should not assist daughters if the dispersal system becomes less female-biased or mothers are able to maintain kin ties to their daughters.

Variation amongst individuals or populations in factors that change the intensity or timing of reproductive competition from the next generation is predicted to associate with the age at last reproduction as well as reproductive overlap. Social system variation shown by modern humans also demonstrates the physiological processes underlying the species-wide trait of rapid reproductive senescence (a cell that is no longer capable of dividing but still alive and metabolically active) compared with the possible flexibility of behavior that leads to menopause.

The decision to help daughters outside the family increases the relatedness asymmetry which favours younger females in within-family conflict, as older females are able to help produce grand offspring that are related by ¼.

However, assuming that parenthood uncertainty is extensively accepted as a factor favouring maternal over paternal grand mothering, it is helpful to compare the magnitude of this effect with the magnitude of the relatedness asymmetry between older and younger females.

The kin-selected benefits of helping can explain post reproductive survival, but not why women cease reproduction so early in the first place. A model incorporating reproductive competition can help to account for this trait and for the particular timing of reproductive cessation in human females. Cant and Johnstone suggest that a combined model that takes into account both the inclusive fitness benefits of helping and the potential inclusive fitness costs of reproduction suggestions an improved understanding of the evolution of menopause.

References:

Hrdy, S.B. (2009). Will the Real Pleistocene Family Please Step Forward? Anon, Mothers and Others: The Evolutionary Origins of Mutual Understanding (pp. 207). United States of America: Anon. 

Bereczkei, T., & Dunbar, R.I.M. (1997). Female-biased reproductive strategies in a Hungarian Gypsy population. Proc R Soc London Ser B 264:17–22

Johnstone, R.A., & Cant, M.A. (2008). Reproductive conflict and the separation of reproductive generations in humans, 105(14):5332-5336. Doi:10.1073/;pnas.0711911105.

Sear R., Mace R., & McGregor I.A. (2000). Maternal grandmothers improve nutritional status and survival of children in rural Gambia. Proc R Soc London Ser B 267:1641–1647

Voland E., & Beise J. (2002). Opposite effects of maternal and paternal grandmothers on infant survival in rural Krummhörn. Behav Ecol Sociobiol 52:435–443. 

Follicular release in women.

From birth, a woman contains a lifetime supply of follicles (envelopes of cells that contain immature eggs; Derry, 2006) which gradually lessen as menstrual cycles occur, however this process accelerates powerfully around the age of 38, in a way that doesn’t happen in chimps, monkeys or rodents (Cant and Johnstone, 2008). If this acceleration never evolved, menopause would occur around the age of 70 due to the earlier and slower rate of follicle loss, which is predicted by trends in other primates of the same age. 

What happens to accelerate this process of follicular release in women? Once puberty is reached, the ovary contains about an average of 200,000 follicles (Block E, 1952). During a reproductive lifespan, a gradual decrease in the original follicle pool is caused by continuing growth of primordial and primary follicles into secondary and larger follicles. The primordial follicle pool could also decrease due to apoptosis (a tightly regulated cell suicide process that destroys cells that are no longer needed or of threat) of resting follicles.

Figure 1. Development of an Ovarian Follicle. M.H. Ross (2015).

About 10 years before menopause, increasing percentages of follicles are lost from the resting pool (Richardson et al. 1987, Faddy & Gosden 1996, Faddy et al. 1992, Gougeon et al. 1994) of cells due to association with increased serum follicle-stimulating hormone (FSH) and decreases in circulating inhibin hormones. The decreasing follicle reserve serves as a ticking clock to time the commencement of menopause.

References:

Block E 1952 Quantitative morphological investigations of the follicular system in women. Variations at different ages. Acta Anat (Basel) 14:108–123.

Derry, P.S. (2006). A Lifespan biological theory of menopause. Sex Roles. 54, 393-399.

Faddy MJ, Gosden RG, Gougeon A, Richardson SJ, Nelson JF 1992 Accelerated disappearance of ovarian follicles in mid-life: implications for forecasting menopause. Hum Reprod 7:1342–1346.

2015

Faddy MJ, Gosden RG 1996 A model conforming the decline in follicle numbers to the age of menopause in women. Hum Reprod 11:1484–1486.

Gougeon A, Echochard R, Thalabard JC 1994 Age-related changes of the population of human ovarian follicles: increase in the disappearance rate of non-growing and early-growing follicles in aging women. Biol Reprod 50:653–663.

Richardson SJ, Senikas V, Nelson JF 1987 Follicular depletion during the menopausal transition: evidence for accelerated loss and ultimate exhaustion. J Clin Endocrinol Metab 65:1231–1237.

 Ross, M.H. (2015). Schematic drawing of the stages in the development of ovarian follicles, beginning with the primordial follice and ending with the pre-ovulatory of Grannfian. History A Text and Atlas 3rd Ed. p. 684 Fig. 22.3. 

Reproductive conflict between successive generations.

A new model proposed by Michael Cant and Rufus Johnstone in 2008, suggests that reproductive conflicts between successive generations of women is reduced due to the help of fertility loss. The grandmother hypothesis also helps to support this and according to Hamilton’s rule of ‘kin selection’, you are always more closely related to your own offspring than your grand offspring. So a question that was posed to me that I am wanting to answer is, is there any evidence that reproductive conflict between successive generations is actually high in humans?

Despite enormous differences in social systems as well as access to resources, the universality of menopause in modern humans illustrates the flexibility of behavior compared to the physiological process underling reproductive sequences.

The model of reproductive conflict does not suggest that older females should not help daughters or younger females if the social system becomes less female-biased, or maintaining kin ties between mothers and their daughters. But, if and when given a choice of helping either daughter or son, mothers should direct there help towards the daughters since grandchildren through sons may have been fathered by extra-pair males (for example (of an extra-pair male), a female may mate with one male, but rely on another male for resources).

Bereczkei & Dunbar (1997) and Sear et al. (2000, 2002) reported that the presences of a paternal grandmother has relatively little effect on offspring survival compared to that of a maternal grandmother from a number of studies.

For example: data obtained from the eighteenth and nineteenth centuries in the Krummhorn region of Germany show that the chance for a daughter-in-law to have a stillbirth was increased by 35% if the paternal grandmother was present in the household, and at the start of the daughter-in-laws marriage, these mortality costs are predominantly high (Beise & Voland, 2002, Voland and Beise, 2005).

Figure 1. Map of Krummhorn in Germany. Anon (2015).

This is data shows evidence that supports there is reproductive conflicts between successive generations in human families, as the assumption of a female-biased dispersal system has a positive impact on maternal grandmother.

References:

Anonymous. (2015) Map of Krummhorn in Germany. http://www.krummhoern.de/uploads/images/zoom/krummhoern_karte.jpg; Retrieved 29/05/2015.

Beise, J. & Voland, E. (2002). A multilevel event history analysis of the effects of grandmothers on child mortality in a historical German population (Krummhorn, Ostfriesland, 1720-1874). Demographic Research, 7, 469-497.

Bereczkei, T. & Dunbar, R.I.M. (1997). Female-biased reproductive stratigies in a Hungarian Gypsy population. Proceedings of the Royal Society of London B, 264, 17-22.

Johnstone, R.A., & Cant, M.A. (2008). Reproductive conflict and the separation of reproductive generations in humans, 105(14):5332-5336. Doi:10.1073/;pnas.0711911105.

Sear, R., Mace, R., & McGregor, I.A. (2000). Maternal grandmothers improve nutritional status and survival of children in rural Gambia. Proceedings of the Royal Society of LondonB, 267, 1641-1647.

Sear, R., Steel, F., & McGregor,I.A. (2002).  The effects of kin on child mortality in rural Gambia. Demography, 39, 43-63.

Old vs. new - menopause

As older and younger women experience reproductive encounters, why has the older generation become infertile and not the younger one?  After all, it is the other way around for most mammals that collaborate to raise offspring – the older generation continues to breed and repress the fertility of the younger generation. It is believed that it all comes down to how our ancestors mingled with one another in their social groups (Johnstone & Cant, 2008). 

A Layman's Study of Human Evolution. M.McClintock (2014).

Members are switched within social groups to some extents and it is mostly the males that strike out. However, for ancestral humans, evidence suggests that females were most likely to leave and find new groups. Supporting this idea is the evolutionary behaviour of hunter-gatherers as well as our neighbouring relatives, the chimps and bonobos, along with hereditary evidence.

This changes the behaviour of how associated females are towards other members of their group, thus shifting the stability of conflict in favour of the young new group members. A young female entering a group is better off raising her own offspring as she is completely unrelated to the existing members of the group and obtains no indirect benefits from helping to raise their offspring. However, older females benefits by either obtaining more offspring herself or assisting to raise any offspring that her sons father with the young new group members.

Unequal benefits were established, by Cant and Johnstone, which skew the results of competition towards the younger females by using a simple model to mimic these interactions (Johnstone & Cant, 2008). If reproduction is ceased in older females as the younger ones become sexually mature, then the competition becomes stably resolved.

Cant and Johnstone declare further testing is needed for their hypothesis and they have suggest ways of executing this. They have stated that if their theory is accurate, scientists ought to be able to provide that young mothers experience disadvantages if they raise offspring in conjunction with older females who are still doing the same, as is the case in some polygamous societies.

To conclude, Cant and Johnstone announce that their new hypothesis is not an alternative to existing ones, but a complement to them. In hope that it will help to gain a full understanding in the origins of menopause if it is viewed as a reflection of the “ghost of reproductive competition past”.

Finally, Cant and Johnstone note that their new hypothesis is not meant to be an alternative to existing ones, but a complement to them. They hope that it will help us to more fully understand the evolution behind the origins of menopause if we view it as a reflection of the “ghost of reproductive competition past” (Hrdy, 2009).

References:

Hrdy, S.B. (2009). Will the Real Pleistocene Family Please Step Forward? Anon, Mothers and Others: The Evolutionary Origins of Mutual Understanding (pp. 207). United States of America: Anon.

Johnstone, R.A., & Cant, M.A. (2008). Reproductive conflict and the separation of reproductive generations in humans, 105(14):5332-5336. Doi:10.1073/;pnas.0711911105.

McClintock, M. (2014) A Layman's Study of Human Evolution. http://www.frontrangeforum.org/courses/a-laymans-study-of-human-evolution/; Retrieved: 07/05/2015.

Menopause - A new model

Previous studies have allowed for the current picture to be incomplete, according to Cant and Johnstone, as they have disregarded the fact that it is not only the mothers that are affected by new offspring, but other members of the community too. In any group of fertile women, the offspring draw upon the same resources, nourishment and attention from other adults, which leads to a form of “reproductive rivalry” between mothers. 

Menopause is proposed, by Cant and Johnstone, to assist towards minimising this conflict as well as to cite the timing of menopause as evidence for their theory. Within humans, different generations show minimal overlap between reproductive periods, while in hunter-gatherer civilizations, the mothers fertility tends to cease at more or less the same time as their daughters reach sexual maturity.

Amongst other primates, this degree of separation is extraordinary, which, as the graph below shows, most offspring develop fertility while their mothers are still more than capable of conceiving. For example, roughly 12 years of Japanese macaque’s fertility periods overlap, which is around 70% of their total reproductive lifespan. Based on the trends presented by other primates, human women would be expected to keep on reproducing till the age of 70, rather than the earlier cut-offs in their 50s. 

Figure 1. Reproductive overlap in humans and other primates. (A) Maximum reproductive versus maximum generational overlap in 12primate species recently classed as exhibiting a post reproductive life span (24). (B) Pattern of overlap for these four species. R.A. Johnstone & M.A. Cant (2008).

From birth, a woman contains a lifetime supply of follicles (envelopes of cells that contain immature eggs; Derry, 2006) which gradually lessen as menstrual cycles occur, however this process accelerates powerfully around the age of 38, in a way that doesn’t happen in chimps, monkeys or rodents (Cant and Johnstone, 2008). If this acceleration never evolved, menopause would occur around the age of 70 due to the earlier and slower rate of follicle loss, which is predicted by trends in other primates of the same age. 

References:

Brenner, R.M., & Slayden, O.D. (2012). Molecular and functional aspects of menstruation in the macaque. 10.1007/s11154-012-9225-5.

Derry, P.S. (2006). A Lifespan biological theory of menopause. Sex Roles. 54, 393-399.

Johnstone, R.A., & Cant, M.A. (2008). Reproductive conflict and the separation of reproductive generations in humans, 105(14):5332-5336. Doi:10.1073/;pnas.0711911105.

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.

Menopause - a new insight

You could argue that life is all about survival and the contribution to the evolution of species through sexual reproduction. Human reproduction can be very mystifying as our reproductive potential has evolved to terminate early. At an average age of 38, women start to become rapidly less fertile only to permanently lose the ability to reproduce some 10 years later during menopause.This decline seems uncanny from an evolutionary point of view. Fertility remains until close to the end of the lives of other long-lived animals like elephants as they continue breeding until their 60’s. In contrast, a human woman loses her child-bearing potential years or decades before she passes away.  Even in hunter-gatherer societies that lack access to technology and modern medicine, women who pass through menopause are expected to live well into their sixties.


Figure 1. Native Tribes of South America. Anonymous (2015).

A new model has been proposed by Michael Cant (University of Exeter) and Rufus Johnstone (University of Cambridge), in 2008, to explain the origins of menopause. They suggest that reproductive conflicts between successive generations of women is reduced due to the help of fertility loss.


Figure 2. Four Generations of Women. A.Prucha (2015)

To resolve this conundrum, a few theories have been put forward; with one of the most popular being the “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). With some studies to back this up, this basic idea it’s clearly not the whole story. Some analyses of hunter-gatherer populations have found that the potential benefits of having more children yourself is not outweighed by the indirect advantages of helping your family. Alone, women continuing to live past menopause is explained by the grandmother hypothesis, however what isn’t explained is why women have evolved to go through it in the first place.

References:

Anonymous. (2015) Native Tribes of South America, http://www.angelhills.org/Pages/satribes.aspx; Retrieved 20/04/2015.

Johnstone, R.A., & Cant, M.A. (2008). Reproductive conflict and the separation of reproductive generations in humans, 105(14):5332-5336. Doi:10.1073/;pnas.0711911105.

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.

Prucha, A. (2015). Four Generations of Women, http://www.ardenprucha.com/2012/04/four-generations-of-women/; Retrieved 20/04/2015. 

An elephant never forgets

Animals almost always continue to reproduce until the day they pass away. However, there are three exceptions that have been discovered so far: humans, killer whales and short-finned pilot whales (Johnstone et al. 2010; Peccei, 2001; Flinn, 2013). The females of all three species live in large family groups, have matriarchs that transmit information about resources and demonstrate prolonged offspring care, however they lose the ability to have children, but continue to live for decades afterwards, thus signifying menopause.

Elephants provide their herd with the same benefits as humans, killer whales and short-finned pilot whales, so why has menopause not evolved within this species? “They are better at recognising welcoming faces and know the best anti-lion moves” (Yong 2015). Elephants differ from the other species such as the killer whales in one critical respect; in killer whales their daughters and sons remain within the same groups in which they were born. This means that’s when a female grows older, her pod becomes full of her own children and grandchildren. Over time, the female becomes increasingly related to her neighbours, and more genes are shared with her neighbours. This then creates a powerful impetus to shift her efforts away from having more offspring and towards helping existing descendants. 

Figure 1. Nowadays, only two original regions of elephants.Cr. Pinterest (2015)

This impetus does not exist in elephants as the sons eventually leave the birth group to discover new ones. Therefore, females become less related to their group-mates over time which leaves a matriarch’s to carry on reproducing until she passes away (Brent et al. 2015). “So, it’s not just about the dispersal patterns, but also the role that old females can play in the group,” says Croft.

https://www.youtube.com/watch?v=OrHiEYGqnEk

                          

References:

Brent, L.J.N., Franks, D.W., Cant, M.A. & Croft, D.P. (2015). Ecological Knowledge, Leadership, and the Evolution of Menopause in Killer Whales. Current Biology 25, 746-750. http://dx.doi.org/10.1016/j.cub.2015.01.037; Retrieved 12/04/2015.

Flinn, M.V. (2013). Are humans the only species that go through menopause? http://www.researchgate.net/post/Are_humans_the_only_species_that_go_through_menopause; Retrieved 27/03/2015.

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.

Peccei, J.S. (2001). Menopause: Adaptation or Epiphenomenon. Evolutionary Anthropology, 10:43-57.

Pinterest, C. (2015). Howadays, only two original regions of elephants. http://www.mthai.com/en/lifestyle/10426.html; Retrieved 12/04/2015.

Powledge, T.M. (2008). The Origin of Menopause: Why Do Women Outlive Fertility? Scientific American, a division of Nature America, Inc.http://www.scientificamerican.com/article/the-origin-of-menopause/; Retrieved 04/04/2015.

Yong. E. (2015). Why Killer Whales Go Through Menopause But Elephants Don’t. http://phenomena.nationalgeographic.com/2015/03/05/why-killer-whales-go-through-menopause-but-elephants-dont/; Retrieve3d 12/04/2015.