the problems of parenting. offspring “vehicles” transporting copied genes to succeeding...
TRANSCRIPT
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