Human Families Flashcards
What is parental investment
Any investment by a parent in offspring that increases the offspring’s likelihood of survival and reproduction at the cost of the parent’s ability to invest in/produce other offspring (Trivers 1972)
What are the different types of sex ratio
How can it be expressed
• BSR/ASR/OSR
=birth/adult (can be different from birth depending on mortality ratios) /operational
• Primary/secondary/tertiary
= fertilisation/BSR/ASR
Expressed as:
- ratio (number of males: 1 female)
- number of males per 100 females
- decimal (proportion of males)
What is the sex ratio in humans
Why may this be puzzling? Why what is the problem with this thinking?
The evolution of a 1:1 sex ratio,
approximately, usually
Wouldn’t it be more efficient to have a heavy female bias?
This is a group selection argument but we must remember evolution acts at the gene only
What is Fisher’s principle?
- Producing the rarer sex will be the best strategy for parents as the rarer sex has higher average reproductive success.
- Once the sex ratio is equal, the advantage disappears and the population is in equilibrium.
Automatically equilibrates
What are the assumptions of Fisher’s principle?
Assumptions:
- sexual reproduction
- random mating
- both sexes equally costly
Give 2 examples of animals which do not have an equal sex ratio
Woolly spider monkey - Sex ratio of 0.35
Western Tarsier - Ratio of 0.8?
Why may animals not have a balanced sex ratio
- Local Resource Competition (Clark 1978)
- Local Resource Enhancement (Komdeur et al. 1997)
Describe local resource competition
Given that individuals compete with others in the same group, if one sex is more likely to disperse, then this sex will be favoured
Females disperse so bias towards producing daughters in Woolly Spider Monkeys and vice versa for Western Tarsier
Describe local resource enhancement
When a certain sex increases resources in the environment so will be favoured
Some offspring birds remain in the nest to help further reproduction eg by guarding
How does the Seychelles warbler exemplify different reasons for an uneven sex ratio
- Local Resource Competition (Clark 1978)
- Local Resource Enhancement (Komdeur et al. 1997)
Depending on whether it was in a high quality resource location the driving force of the above 2 hypotheses change (in high quality places there is a bias towards daughters would remain local and help with cooperative breeding)
What is the Trivers & Willard Hypothesis
-male reproductive variance > female r.var
-high quality parents ought to produce more sons
(If you’re a winner its better to produce a son cos they will be a big winner than a daughter where there is less variance so it matters less )
• N.B. there are some species in which daughters
benefit more from maternal rank… eg in baboons
Why do baboons not fit the Trivers and Willard hypothesis
matrilineal inheritance
Mother ensures daughter is just below her in the hierarchy bc mother can outcompete anyone below her so can slot daughter in but son will disperse so are independent of mother rank
High ranking females will produce more daughters
Is there any evidence for adaptive sex determination in humans
Pastoralists with pronounced male skew
Society with very variable female health (1/4 the women have been estimated to have chronic energetic shortfalls)
Women with high BMI are much more likely to have a son (Mace and Gibson 2003) – fits with trivers wilard
Cameron and Dalerum (2009) – billionaires tend to have 60% sons, interestingly driven by male billionaires and not female billionaires – no clear explanation)
Mechanisms?
What are possible mechanisms for adaptive sex bias in humans
• Possibilities:
-male bias in production of x- vs y-bearing sperm.
-differential likelihood of successful fertilisation of x or y sperm
-differential likelihood of egg fertilised by x vs y sperm resulting
in completed pregnancy
• Numerous male and female hormonal influences of these processes see Navara 2013 e.g.
- F oestogen level > cervical mucosa viscosity > advantage for x sperm
- M testosterone:gonadotrophin > ratio of x:y sperm production
- Theories such as high ranking males have increased testosterone and increasing rank increases testosterone levels
• Glucose levels may inhibit female blastocyst development more so than males (Cameron 2008)…key driver of mechanism? - Increased energy intake prior to conception increases likelihood of male being born as glucose levels can inhibit female blastocyst development more than male
What is the tertiary sex ratio
Sex-ratio may also be altered after birth via parental behaviour e.g. infanticide/neglect/preferential treatment.
• Inuit female infanticide rates estimated at ~20% (Smith & Smith 1994). Male economic contribution and thus inclusive fitness contribution to parents much higher
“Life is short. We all want to be as prosperous as we can in the time we are alive. Therefore parents often consider that they cannot “afford” to waste several years nursing a girl. We get old so quickly, and so we must be quick and get a son. That is what we parents think, and in the same way we think for our children. If my daughter Quertilik had a girl child I would strangle it at once. If I did not, I think I would be a bad mother.” (Smith & Smith 1994)
Does the Triver Willard hypothesis hold up in India
what about china
Higher castes killed more girl babies (Dickermann 1979) bc triver hypothesis of high ranking males tend to produce sons bc they are bigger winners
But T-W predictions don’t consistently hold up:
-Lack of male infanticide in lower castes
-Among poor Chinese farmers majority of
infanticide directed at females,1y.o. SR reaching
375:100 (Hawkes 1981).
100 million missing women due to tertiary but now also due to sex specific abortions
No data on whether it is adaptive but unlikely bc there are surely many men who have no reproductive factors – short term proximal economic choice about labour etc
What are other causes of sex based infanticide
- dowry system
- sex differences in economic productivity
What did Dunbar find regarding biased sex investment in Hungary
• Empirical evidence that female biased PI is an adaptive strategy among Gypsies in Hungary due to hypergyny
(Bereczeki & Dunbar 1997).
• Daughters, particularly in urban population, produce more surviving grandchildren than sons because more likely to marry into wealthier native Hungarian population > lower child mortality risk.
• The gypsy population:
- F-biased sex-ratio at birth
- daughters breastfed for ~6 more months
- invest more in daughters’ education
Why is paternity uncertainty so important for male humans
• Paternity uncertainty has likely been an important selective force for men given the ubiquity of paternal investment in human populations.
Cuckoldry (where a male unwittingly invests in another male’s genetic child) is devastating for male fitness
Thus we would expect men to be endowed with some sort of psychological mechanism to protect against this by assessing paternity
Psychological experiments show that people tend to assure father’s far more than mothers that the baby looks like them
How common is cuckoldry
Non-paternity rates average ~3.3% - mixed sample, principally industrialised societies (Anderson 2006).
Paternity uncertainty may be higher in non-SIM social systems with partible paternity etc
Give an example of how putative fathers may use phenotypic matching to guide investment decisions and avoid cuckoldry.
- Photographs of same toddler morphed with five different adult faces including focal participant’s face.
- Self-resemblance had substantially more impact on male than female participants’ willingness to invest.
Platek et al. 2002
What did Alvergne 2009 find regarding paternity uncertainty
Took pictures of all men in the village and all children in the village then went to another village and showed pic of a child and 3 men, one of whom was the biological father. Did same with odours
Scored resemblance to the father based on how frequently correct child was chosen
Asked mothers about investment to create a paternal investment score
Investment score was positively correlated with odour and facial resemblance scores
Author tried to validate investment score by showing it was associated with child BMI and mid arm circumference
What is grandparental uncertainty predicted by (3)
- Number of uncertainty links (Differential
grandparental solicitude hypothesis; Euler &
Wizel 1996) - Number of uncertainty links compared to
other grandchildren (Preferential investment
hypothesis; Laham et al. 2005) - Marginal genetic relatedness due to shared
sex-chromosome (Sex chromosome selection
hypothesis; Chrastil et al. 2006)
Describe the uncertainty links in the differential grandparental solicitude hypothesis
Paternal grandfather uncertain by 2 degrees (not sure his son is bio son, and cant be sure grandchild is son’s bio child)
Paternal grandmother is 100% sure (no degrees of uncertainty because knows daughter is deffo hers and grandchild is deffo her daughter’s)
Therefore maternal grandmothers should invest most and paternal grandfather’s should invest least
Paternal grandmother may invest less than maternal grandmother bc there maybe other grandchildren who she can invest more in
Maternal grandfather more likely to invest than paternal grandmother (latter may have 100% sure grandchild, but former’s alternatives are even worse)
Think of alternative grandchildren
Which relative is most important for child survival
In a review of kin effects on child survival in 45 natural fertility populations, MGMs
found to be most important relative besides mother
Sear & Mace 2008
What did Marlowe find regarding how Hazda men interact with bio vs step children
Marlowe 1999
- Hadza men nurture, hold, communicate and play significantly more with biological children than stepchildren.
- Men living with stepchildren produce 877Kcal/day versus 1901Kcal/day of men living with only biological children.
How does being a step child affect your chances of survival
• Unequivocal evidence that stepchildren are at increased risk of physical abuse and neglect.
• National homicide archive of Canada, data from 1974-1990 indicate stepfathers more than 100X more likely than biological fathers to commit violent filicide. (Daly & Wilson 1996)
• Data from Australia indicate >300 fold increased risk (Strang 1996).
• Minimal confounding effects of other variables e.g. SES, family size, personality characteristics.
-When bio fathers kill resident children often associated with psych problems and followed by suicide whereas stepfathers kill without ether of these
Also see Marlowe’s 1999 finding that Hadza men play more and invest more calories in bio over step children
Summarise the lecture on biased parental investment
- Numerous adaptive reasons for biases in the BSR (LRC+LRE+TW). Some positive evidence for the TW hypothesis in humans.
- Female infanticide has produced substantially biased sex ratios and seems to be driven by economic interests of the family – labour contribution + dowry.
- Paternity certainty affects paternal and grandparental biases in investment based on cues of resemblance, number of uncertainty links and opportunity costs.
- Stepchildren at substantially higher risk of physical abuse and filicide.
What is Riet Ghok
(cattle talk) - Serious Business
When a youth wants to marry, the first question his sweetheart’s father asks him, or whoever is acting on his behalf, is: ‘What cattle have you got? ‘ although he generally knows the answer before asking it (Evans Pritchard 1946, p 256) [Nuer ethnography]
What is a marriage transaction
Economic exchange at point of marriage
Either between couple or their respective families
Marriage is usually a much longer, more complicated occurrence than in WEIRD societies
Describe the marriage transactions of the Nuer
Riet Ghok
Transfer of cattle from groom’s to bride’s family
Negotiate for many days/ weeks and many family members get in (pat. Uncles etc)
5-10 heads of cattle transferred -> initial official sign of path to marriage
More cattle brought at marriage and some cattle sacrificed bc both living and dead relatives are entitled of economic transactions
Marriage cannot be consummated until ~50% of total amount negotiated have been transferred
Even after this, bride will continue to stay with birth family until first child is born (sometimes around 50 cows can be transferred, around 1/3 of the male’s wealth has been transferred)
Once first child born then bride will move in
If childless there will be disputes about whether to dissolve to marriage and if cows should be returned etc
What are the different types of marriage transaction
How common is each
• Bridewealth – transfer of material resources from groom or his family to bride’s family.
-52% of SCCS/66% of Ethnographic Atlas
• Dowry – reversal of bridewealth
-6% of SCCS/3% of Ethnographic Atlas
• Brideservice – groom must provide labour/share products of labour with bride’s family for some
period of time.
-13% of SCCS
transaction is common in many small scale societies -
Most common in pastoralist societies
Usually some form of transaction
Sometimes equal transfer eg w1 bride from each family given to the other to cement alliance given the shared genetic interest (bidirectional)
Why is bridewealth expected to be common
what does it reflect
• Relative ubiquity of bridewealth expected since females are the limiting resource for male fitness not vice versa.
Bridewealth compensates family for loss of daughter and loss of additional worker
This occurs even where patrilocality isn’t the norm
• Among the Kipsigis higher brideprice is paid for females with higher reproductive value i.e. those reaching menarche earlier.
What is residual reproductive value
Residual reproductive value = number of future offspring female is expected to bear
How does brideservice work among the Kipsigis
Kipsigis pay bridewealth in cattle and cash and is based on reproductive value (translated to economic value)
Beginning of December, all girls who reach menarche undergo circumcision - then secluded so males can purchase one or multiple women as wives
Variable age of reaching menarche (12-20)
High payments biased towards those with earlier menarche (longer reproductive career) – difference of 8 years is significant = lots more children
What affects probability of dowry system emerging
Likelihood of dowry system emerging in stratified non-polygynous societies vs any other (including non stratified or stratified polygynous) is 50x more likely
PTM predicts differences in women’s access resources (held by men) largely cancelled out via polygyny.
Whether woman marries rich or poor will have a large impact on reproductive success
• If monogamy is imposed and there is substantial stratification in male wealth, women have to compete for wealthier husbands.
Gaulin and Boster, 1990
