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.

Menopause in humans, killer whales and pilot whales.

Studies have found that only humans, killer whales and pilot whales have evolved menopause. Why have only these three species evolved this trait?

The evolutionary explanation for menopause has suggested a "grandmother" hypothesis which proposes that reproductive survival is improved by kin selection as well as the continuance of genes - including genes that are contributed to longevity (Johnstone & Cant 2010; Powledge 2008).

As discussed in the previous blogs it is known that female humans evolved menopause 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).

Killer Whales (Orca; Orcinus orca) have family groups of up to 40 individuals, with mothers giving birth every three to ten years after a 17-month pregnancy. These magnificent creatures can weigh up to 6 tons and life for an average of 50 to 80 years (National Geographic Society 2015).

Figure 1. Killer Whales. K.Spear (2010)

Short-finned Pilot Whales (Globicephala macrorhynchus) are found in groups of roughly 15-20 individuals, however some pods can be as large as 50 individuals. These beautiful creatures can live up to roughly 45 years and can weigh up to 3.5 ton (Anon 2015).

Figure 2. Short-finned pilot whale. J.D.Watt (2015)

These two species of whales are the only other two species known to have evolved menopause alongside humans. Lauren Brenton of the University of Exter's Centre for Research in Animal Behaviors found that post-reproductive females improved the survival of their kin through ecological knowledge transfer (Brent et al. 2015).  This helps to explain why they continue to live long after reproduction has been ceased. It has been found that these post-reproductive females lead groups when moving together in foraging grounds and in difficult years when there is a low food source, the leadership of these menopausal females is prominent (Brent et al. 2015). 

Undoubtedly, selection for a post-reproductive life span in humans, killer whales and pilot whales most likely evolved as a result of needed prolonged offspring care, prevention of large reproduction costs late in life, continuance of genes and knowledge transfer.

References:

Anonymous. (2015). Short Finned Pilot Whale (Globicephala macrorhynchus). http://us.whales.org/species-guide/short-finned-pilot-whale; Retrieved 04/04/2014.

Brent, L., Franks, D., Foster, E., Balcomb, K., Cant, M. & Croft. (2015). Current Biology:  Ecological knowledge, leadership and the evolution of menopause in killer whales. http://www.exeter.ac.uk/news/featurednews/title_438490_en.html; Retrieved 04/04/2015.

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.

National Geographic Society. (2015). Killer Whale (Orca). http://animals.nationalgeographic.com.au/animals/mammals/killer-whale/; Retrieved 04/04/2015.

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

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.

Spear, K. (2010). Killer Whales. http://phys.org/news187298115.html; Retrieved 04/04/2015.

Watt, J.D. (2015). Short-finned Pilot Whales. http://www.arkive.org/short-finned-pilot-whale/globicephala-macrorhynchus/; Retrieved 04/04/2015