Parental Investment

Parental Investment

1. DEFINITION

   • Trivers
   • investment from the parent that benefits the offspring, but at some cost to parent's future reproductive sucess
   • Examples...
     • Intrauterine investment in foetus  Abortion / Infanticide Breast-feeding + interval before next birth Nutrition during childhood Education, health care... Marriage costs (dowry, bridewealth)

2. KIN SELECTION

   • = natural selection of an apparently disadvantageous trait that increases in the population due to increased survival of those genetically related to those who possess it. 
   • explains majority of SOCIAL evolution
     • e.g. altruistic behaviour
   • Hamilton's Rule
     • rb>c
     • inclusive fitness
       • Haldane: would save 2 brothers or 8 cousins
   • explains why...
     • parents care for offspring
     • members of wider kin group help rear
     • parents + offspring sometimes in conflict

3. PARENT-OFFSPRING CONFLICT

   • Offspring concerned with own fitness, not future reproductive sucess of parent -> differences in optimal investment
   • mother related to offspring 0.5
   • WEANING
   • STEP-PARENTS
     • Daly & Wilson 1988
       • Many more cases of criminal chidl abuse in homes of 1 step-parent and 1 natural parent, than with 2 natural parents. Especially 0-4 yr old range.
     • have more interactions involving conflict than do natural parents
     • historically, step-mothers (Cinderella)
       • Past: high maternal mortality. Investment from mother more - so step-mother effect more significant
     • Lawson & Mace 2010 : Father Presence and Child Height
       • More height dificit in children with new father figure than in those with no father. Especially in boys.
   • POLYANDRY
     • = more than one husband to every female young man baby-sitting child of another man to whose marriage he hopes to add himself
     • unrelated father investment -> increased reproductive success with mother
     • Yanomamo
   • ADOPTION
     • Oceania studies: bias toward inclusive fitness strategies
       • Adoption nearly always between kin

4. PATERNITY UNCERTAINTY

   • RESEMBLANCE
     • Platek et.al. 2002,2004
       • Are men's desired to invest in children more influenced (than are women's) by resemblance of children to themselves?
     • faces of babies blended with own face -> "which one of these babies would you adopt?"
       • Men: influenced
       • Women: described all babies as sweet
     • ANONYMOUS BABY FACE
       • Babies equally similar to mother or father - but hard to link with either "Aww, it looks so much like its father!"
       • Facial resemblance not so present in new-born
       • protection from infanticide
         • not outweighed by potential benefit from paternal investment
   • PATRILINEAL KIN
     • reduced r -> less altruistic to children of mat kin
     • Fathers
       • Why they devote less effort to child-care than mothers Other reasons: division of labour, alternative oppurtunities open to men
     • Grand-parents
       • patrilineal grandparents less likely to visit and give presents to grandchildren than matrilineal ones
       • Euler et.al. 1995
     • Aunts+Uncles
       • Gaulin et.al. 1997

5. EVOLUTIONARY COSTS+BENEFITS determined by...

   • ABORTION
     • Lycett & Dunbar 1999
       • Factors influencing parental investment also influence decision to abort pregnancy - a definite reproductive decision 
     • Influence: female age + marital status
     • key variable: liklihood of future marriage
     • Single: probability decreases with age Married: increases with age
       • also: increases with probability of future marriage
   • BENEFITS
     • parents relatedness to offspring
     • Age of offspring
     • Vitality of offspring
     • Sex-specific potential for future reproduction
       • e.g. opputunities to marry up If poor -> better to have daughter that you can marry up to rich husband. Bridewealth: husband -> bride family. If have a son -> cannot compete, may not get bride
     • offspring contribution to family
   • COSTS
     • Economic/Ecological fluctuation
     • Economic opp costs
     • future fertility
     • Parental age
     • Economic constraints
       • dowry/bridewealth payments, competition between siblings If poor and in mongomous society with dowry -> expensive to have daughters -> sister competition If have son -> marry one woman. dowry: bride's family -> newly weds

6. DIVISION OF LABOUR

   • COOPERATION
     • Humans: long childhood, multiple dependent offspring whilst still reproducing
       • Other coopertaive breeders: younger adults (waiting for mating oppurtunities) help Humans: older females/males help post-reproduction
       • Able to maintain unusual life history due to cooperation?
         • Other unique characteristics: Rapid reproductive rate Female menopause Low adult mortality BIG BRAIN - these features may be relates (directly or indirectly) to big brain
       • In terms of net production: humans only break even around 20 yr old
         • Kaplan & Lancaster 2007: data on Ache
   • RAPID REPRODUCTIVE RATE
     • major contribution from?
       • FATHER
         • of Paraguay
         • Ache
         • DAD OR CAD?
           • Male can choose to desert partner in search of new one - costs and benefits. Trade-off of quality (stay) or quantity (desert)
         • Monogomy v polygamy
           • Hunter-gatherer (no marital wealth): marriages fluid
             • Ache: partible paternity beliefs
               • Primary and secondary fathers based on probability of paternity.  Primary: mostly his. Secondary: partly his.
           • Hurtado & Hill 1992
             • Ache: -more females to males -higher female fertility -higher mating reward (success if go and find other mate - high if decreased danger of being on own) -higher child survival IF father present (secondary fathers) Dislike of orphans, pre-contact infanticide common
             • remarriage in Ache (poly) + Hiwi (mono)
               • reward higher for Ache
           • ECOLOGICAL VARIATION
             • Arctic Inuits
               • Women cannot go outside in winter, hunting dangerous -> Men cannot hunt enough to support more than one wife
                 • Ecologically imposed monogomy
             • Hadza of Tanzania
               • Mothers + grandmothers provision (tubers). Men make little difference - hunting season not associated with better nourishment
                 • males hunting for other reason?
             • determines dependency on men
       • Grandmother: Hadza
         • of Tanzania

7. MENOPAUSE

   • Grandmother Hypothesis
     • Better to invest in daughter's offspring than own after certain age - increasing inclusive fitness
     • Weight of children in rural Gambia
       • Mothers make largest contribution Mat grandmothers also significant Decline in weight when grandmother is still reproducing.
   • Mother Hypothesis
     • Better to invest in existing children than risk maternal mortality in later births
   • Physical Constraint
     • Increased risk of maternal mortality with age due to large babies, high reproductive rate + mean high wear+tear Cab't exceed 'chimp' reproductive span / mammalian eggs cannot survive past 50 yrs
   • Avoiding Reproductive Competition
     • human generations not breeding at same time
     • Cant & Jonstone 2008: more likely to happen when there is FEMALE DISPERSAL
       • CONFLICT: young woman unrelated to mother-in-law and wins
       • MENOPAUSAL SOLUTION: mother-in-law related to young woman's offspring -> best to help and increase inclusive fitness

8. SEX-BIASED INVESTMENT: heritable resources

   • SEX RATIO
     • Everybody has 1 mother 1 father -> reproductive success equal Rarer sex at advantage -> selection pull ratio back to 50:50
     • actually parental investment should be equal (not number)
       • If one sex more costly + investment equal -> ratios not 50:50 e.g. males more costly -> lower survival rate Ratio evens out once investment stops
       • Sex-biased investment to account for asymmetries in costs/reproductive returns
   • RESOURCE AVAILABILITY
     • If large body size more important in reproductive success to males, mothers in poor conditions should favour daughters -> NET GAIN from smaller daughters higher (than from small sons)
     • Effects in womb: Sex ratio at birth by mother's BMI
       • Higher %live male births with increasing BMI of mother
     • After birth: nutrition, education, medical care, inheritance pattern, infanticide
     • INFANTICIDE
       • Ache: female infanticide
         • Hunting important - males pay back later?
       • Arctic Inuits: female infanticide
         • male mortality high -> risk of girls not finding husband
           • rates of female infanticide increase the more northerly you get
   • MARITAL INHERITANCE
     • POLYGYNY
       • Resource Holding Polygyny (RHP)
         • Polygynous marriages usually only exist because males can secure ownership of farmland/herds Women choose who to marry based on resource availability -> rich men get many wives, poor men get non Under RHP women frequently inherited by younger brother if husband dies
       • MALE variance in reproductive success
       • Resources: greater impact on males
       • Male biased wealth transmission
         • Bias positively correlated with degree of polygyny
       • BRIDEWEALTH
         • resource competition for wives
           • from grrom's to bride's family
         • male-biased
           • Daughter cheap - can make profit. Males competing Spend money on son
         • males compete
     • MONOGAMY
       • LITTLE variance in reproductive success
       • Resources: husband cannot invest wealth recieved from wife in additional wives
       • male wealth not diluted between multiple wives
       • DOWRY
         • resource competition for exclusive access to wealthiest males' resources between families
           • from bride's family to bride/newly wed family
         • female-biased
           • Females competing. Spend money on getting daughter good marriage
         • females compete
           • sister cometition
             • female infanticide in some Asian countries where dowry is costly