The Problems of The Problems of ParentingParenting

OffspringOffspring“Vehicles” transporting copied genes to

succeeding generationsOffspring reproductive success is the

major determinant of fitnessParenting may be selected to optimise

fitness

Sex Differences in ParentingSex Differences in ParentingHypotheses explaining why human

fathers invest less parental effort than mothers Paternity uncertainty Abandonability Mating opportunity costs

Paternity UncertaintyPaternity UncertaintyDue to internal fertilisation, men cannot

be certain of their offspring’s relatedness

Cuckolding is very costly to men, so men will be reluctant to invest

For paternal care to exist: Benefits of other investment <

benefits of raising own offspring

AbandonabilityAbandonabilityThe first parent that can abandon the

offspring should be more likely to do so Greater prevalence should be found

in species with internal fertilisationHowever

This relates back to paternity certainty

In simultaneous gamete release, a 50/50 split should occur, but does not

Mating Opportunity CostsMating Opportunity CostsBy investing in offspring, males miss

more mating opportunities (minimum investment)

When mating opportunity costs are high, paternal care should be low Sex ratio bias

Mating & Parental EffortMating & Parental EffortA reminder

Mating effort: % of reproductive effort invested in acquiring & maintaining mates

Parental effort: % of reproductive effort invested in ensuring survival of offspring

Sex differences in parental effort Partner variety benefits men Paternity uncertainty

Discriminative Parental Discriminative Parental SolicitudeSolicitude

Basically, the idea of parental favouritism

Based on three factors Genetic relatedness of offspring Conversion of parental care to fitness Alternative use of resources

Genetic RelatednessGenetic RelatednessResemblance of offspring is crucial to

father One year-olds tend to resemble

fathersMen tend to invest less $ in education

of stepchildren than genetic children Investment in current stepchildren

appears to be a form of mating effort

Abuse & Child HomicideAbuse & Child HomicideStepchildren are 40 times more likely

to be abused than genetically-related children

Stepchildren are 40 - 100 times more likely to be killed

“Stepparenthood per se remains the single most powerful risk factor for child abuse that has yet been identified” (Daly & Wilson, 1988, p. 87-88)

Risk of Child AbuseRisk of Child Abuse

0

2

4

6

8

10

12

14

0-4 5-10 11-16

Age of Child

Child V

icti

ms

per

Thousa

nd

Natural ParentsOne Stepparent

Adapted from Daly & Wilson (1988)

Risk of Child HomicideRisk of Child Homicide

0

100

200

300

400

500

600

700

0-2 3-5 6-8 9-11 12-14 15-17

Age of Child

Child V

icti

ms

per

Million

Natural ParentPerpetratorStepparentPerpetrator

Adapted from Daly & Wilson (1988)

Conversion of Parental CareConversion of Parental CareFor parental care to be selected, it must

increase offspring reproductive successMost likely affected by two factors

Birth abnormalities Child age

Congenital AbnormalitiesCongenital AbnormalitiesChildren with congenital abnormalities

probably have decreased reproductive value

Large proportion of children with serious illnesses are institutionalised

Abuse rates Base rate of 1.5% 7.5-60% in children with serious

illnesses

Infant HealthInfant HealthMann (1992)

Healthy-unhealthy twin dyads At four months, 50% of mothers

showed positive bias to healthy twin At eight months, 100% of mothers

showed positive biasDiscriminative parental solicitude does

not imply that parents will only invest in healthy children

Child AgeChild AgeReproductive value changes with age

Infants are low in reproductive value, because of high mortality rate

Infants killed if birth interval too short or family size too large

As child increases in age, reproductive value increases

Interaction of Relatedness & Interaction of Relatedness & AgeAge

0

5

10

15

20

25

30

35

40

Age of Victim

Child V

icti

ms

per

Million

Natural ParentNonrelative

Daly & Wilson (1988)

Alternative Use of ResourcesAlternative Use of ResourcesMaternal age

Cost of missed reproductive opportunities increase with age

Maternal marital status Single mothers have less resources

than married ones

Effects of Age & Marital Effects of Age & Marital StatusStatus

0

20

40

60

80

100

120

140

160

<19 20-24 25-29 30-34 >35

Maternal Age

Infa

nti

cides

per

Million

MarriedUnmarried

Daly & Wilson (1988)

A Word on AdaptationA Word on AdaptationRemember, adaptation is an onerous

conceptDaly & Wilson argue that stepparental

abuse & homicide is not adaptive, but a “reverse assay” of parental care Performed irregularly & inefficiently Great cost to perpetrator No known direct benefits

Parent-Offspring ConflictParent-Offspring Conflict If offspring are vehicles for parental

genes, aren’t parent & offspring interests the same?

No, they are not Human offspring may share 50%

unique genes with parents, but they also differ by 50%

Hence, interests will not always coincide

Differing InterestsDiffering InterestsParental and offspring interests

typically differ with relation to resources Intrauterine conflicts Extrauterine conflicts (e.g., weaning) Sibling value, cooperation, &

competition

Abnormal ZygotesAbnormal ZygotesA woman has several chances to get

pregnant, but a zygote has only one chance to be born Up to 78% of all fertilised eggs fail to

implant or are spontaneously aborted, likely due to abnormalities

Women appear to have developed a fetal screening mechanism

Intrauterine ConflictIntrauterine Conflict

Om Of

Williams (1997)

Feta

l Fi

tness

Nutrient Provided

Genomic ImprintingGenomic ImprintingKinship theory of imprinting (Haig)

Whether an allele entered a zygote by sperm or by egg affects the relatedness of the zygote to the parent at that locus

A locus will converge on either symmetric or asymmetric evolutionarily stable strategies (ESS)

Cost/benefit is usually maternal investment

Imprinting: An ExampleImprinting: An ExampleThe case of insulin-like growth factor II

(Igf2; Constância et al, 2002) Igf2 is paternally-expressed Controls placental growth Studies on mice with placental Igf2

deletions demonstrate reduced placental growth and smaller offspring

Loudest Voice PrevailsLoudest Voice PrevailsThe “loudest-voice-prevails” principle

refers to the escalating competition between fetus genes & mother

May have health implications (e.g., gestational diabetes, spontaneous abortions, preeclampsia)

The Wrap-UpThe Wrap-UpOffspring as genetic vehiclesSex differences in parental investmentDiscriminative parental solicitude

Stepparenting as a risk factor Health & age of offspring Age & marital status of mother

Parent-offspring conflict & genomic imprinting

Things to ComeThings to ComeKinship

Hamilton’s rule Evidence of inclusive fitness

applications Grandparental investment Sex differences Evolution of the family unit