Human Reproduction Flashcards
Conflict and cooperation in human reproduction
Menopause
Cooperative breeding
Mating systems
Social evolution
Evolutionary psychology (aka sociobiology) Evolutionary theory can be applied to human reproductive behaviour and mating systems just as in other animals, although this is a controversial area of research because of cultural influences and the ethical difficulties of conducting experiments to test hypotheses.
The study of the effects of genes on human behaviour Evolutionary psychology (aka sociobiology) raises hackles in some quarters because it has been used to highlight differences between groups and to justify prejudice. The idea of genetic determinism often contrasts with anthropological interpretations, where culture is thought to be paramount. Another frequent criticism of evolutionary pyschology is that it is easy to propose simplistic explanations, but hard to test them rigorously.
Menopause in humans
1. Menopause
Life history theory suggests that senescence of somatic and reproductive functions should be synchronized so that individuals have the capacity to reproduce throughout life. The evolution of menopause, in which there is a loss of reproductive function and long post-reproductive life, therefore presents a major puzzle for evolutionary biologists. Moreover, menopause is extremely rare, being known to exist only in humans and some species of toothed whales. N.B. a long post-reproductive life in women is not simply a function of modern healthcare, but was true historically and is currently found in surviving hunter-gatherer societies. We will look at two hypotheses for the evolution of menopause
Life history theory: natural selection should synchronize senescence of somatic and reproductive functions
Menopause
Loss of reproductive potential and long post-reproductive life
In natural fertility human populations:
- median age of last birth c. 38 - menopause c. 50 - post-reproductive life 20 + years
In pilot whales:
- age of last birth c. 36
- post-reproductive life c. 30 years
In killer whales:
- age of last birth c. 48
- lifespan up to 90
Menopause in humans
Hypotheses: (a) Mother hypothesis Avoid risky reproduction and mortality in later life and ensure survival of existing offspring
(b) Grandmother hypothesis Kin selection – gain inclusive fitness by helping existing offspring to reproduce
Mother hypothesis
Evidence suggests that beyond weaning, offspring fitness was not affected by maternal death
- compensated for by other family members?
e.g. Lahdenperä et al. (2011)
Life history data on pre-modern Finns & Canadians
(a) Mother hypothesis - Proposes that mothers avoid risky reproduction later in life so that they can ensure survival of their existing offspring. But, life history data on pre-modern Finns and Canadians shows that beyond the age of weaning, offspring fitness was not adversely affected by maternal death, presumably due to compensation by remaining family members (Lahdenperä et al. 2011 Evolution 65: 476-489).
(b) Grandmother hypothesis
e. g. Lahdenperä et al. (2004)
Living granny associated with:
- higher fecundity higher LRS more kids IF LOCAL earlier reproduction shorter birth intervals increased survival to 15
Number of grandchildren is correlated with post-reproductive lifespan
2 extra grandchildren for every 10 years post- menopause
(b) Grandmother hypothesis - Proposes that post-reproductive females gain fitness indirectly (via kin selection) by helping existing offspring to survive and reproduce. Life history data on pre-modern Finns and Canadians shows that the number of grand-offspring a woman has is correlated with her post-reproductive lifespan. The presence of a mother is associated with higher fecundity and higher Lifetime Reproductive Success of sons and daughters via earlier reproduction of offspring, shorter birth intervals and better survival of grandoffspring. These effects are most apparent when the grandmother is local (<20km away) (Lahdenperä et al. 2004 Nature 428: 178-181). There is no equivalent effect of grandfathers (Lahdenperä et al. 2007 Proc R Soc Lond B 274: 2437-2444), although this is unsurprising because men do not go through menopause, even though there is some senescence in reproductive function. But, the Grandmother Hypothesis may not provide the complete answer because Cant & Johnstone (2008 Proc Nat Acad Sci USA 105: 5332-5336) argued that the kin-selected benefits of helping as a granny do not outweigh the benefits of continued reproduction. Instead they suggest that menopause is a consequence of reproductive competition with daughters and daughters-in-law, an idea supported by another study of Finns (Lahdenperä et al. 2012 Ecology Letters 15: 1283-1290).
What about grandfathers?
e.g. Lahdenperä et al. (2007)
Little or no effect…
BUT, Granny Hypothesis may not be complete answer
e.g. Cant & Johnstone (2008):
Kin-selected benefits of ‘helping’ don’t outweigh benefits of continued reproduction
…suggest menopause ALSO driven by reproductive competition between mother and daughters (in-law)
Cooperative breeding in humans?
Are siblings helpers?
Nitsch et al. (2013)
The positive effect of grannies on grand-offspring production (i.e. as helpers) has led to the idea that humans are cooperative breeders. The potential role of siblings and uncles and aunts as ‘helpers’ has also been investigated, although there is little evidence of any net survival or reproductive benefits from their presence (Nitsch et al. 2013 Proc R Soc Lond B 280: 20122313; Nitsch et al. 2014 Behavioral Ecology 25: 424-433). Furthermore, a recent study indicates that there are costs of closely overlapping reproduction of ‘joint-breeding’ wives of brothers living in the same household (Pettay et al. 2016 Behavioral Ecology 27: 1601-1608).
Positive effect of elder siblings on survival to sexual maturity
But negative effect of same-sex elder siblings on number of offspring
Overall, fitness increased by older opposite-sex siblings, but reduced by older same-sex siblings
Are siblings helpers?
Are aunts and uncles helpers?
Are co-breeding wives helpers?
NO Nitsch et al. (2013)
NO Nitsch et al. (2014)
NO Pettay et al. (2016)
Mating systems
Males have higher reproductive potential than females
Mating System Diversity
849 societies….
0.5% polyandrous
16% monogamous
84% polygynous
Polyandry
e.g. Kashmir & Tibet
Environment and culture dictates mating system
Kinship & age hierarchy reduce male conflict
(a) Polyandry - In Tibetan Buddhists, polyandry is ascribed to severe constraints on agricultural land. When resource base changes, polyandry is abandoned. Male conflict is reduced by kinship and age hierarchy among co-husbands.
Usually 2-3 co-husbands per wife
33% of males aged 10-59 were in polyandrous marriages
31% of females aged 20-59 were unmarried
- 7 children per unmarried woman
- 3 children per married woman
