The Problems of Parenting

Offspring “Vehicles” transporting copied genes to succeeding generations Offspring reproductive success is the major determinant of fitness Parenting may be selected to optimise fitness

Sex Differences in Parenting Hypotheses explaining why human fathers invest less parental effort than mothers  Paternity uncertainty  Abandonability  Mating opportunity costs

Paternity Uncertainty Due 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

Abandonability The first parent that can abandon the offspring should be more likely to do so  Greater prevalence should be found in species with internal fertilisation However  This relates back to paternity certainty  In simultaneous gamete release, a 50/50 split should occur, but does not

Mating Opportunity Costs By 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 Effort A 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 Solicitude 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 Relatedness Resemblance of offspring is crucial to father  One year-olds tend to resemble fathers Men 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 Homicide Stepchildren are 40 times more likely to be abused than genetically-related children Stepchildren are 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 )

Risk of Child Abuse Adapted from Daly & Wilson (1988)

Risk of Child Homicide Adapted from Daly & Wilson (1988)

Conversion of Parental Care For parental care to be selected, it must increase offspring reproductive success Most likely affected by two factors  Birth abnormalities  Child age

Congenital Abnormalities Children with congenital abnormalities probably have decreased reproductive value Large proportion of children with serious illnesses are institutionalised Abuse rates  Base rate of 1.5%  % in children with serious illnesses

Infant Health Mann (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 bias Discriminative parental solicitude does not imply that parents will only invest in healthy children

Child Age Reproductive 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 & Age Daly & Wilson (1988)

Alternative Use of Resources Maternal age  Cost of missed reproductive opportunities increase with age Maternal marital status  Single mothers have less resources than married ones

Effects of Age & Marital Status Daly & Wilson (1988)

A Word on Adaptation Remember, adaptation is an onerous concept Daly & 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 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 Interests Parental and offspring interests typically differ with relation to resources  Intrauterine conflicts  Extrauterine conflicts (e.g., weaning)  Sibling value, cooperation, & competition

Abnormal Zygotes A 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 Conflict OmOm OfOf Williams (1997) Fetal Fitness Nutrient Provided

Genomic Imprinting Kinship 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 Example The 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 Prevails The “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-Up Offspring as genetic vehicles Sex differences in parental investment Discriminative parental solicitude  Stepparenting as a risk factor  Health & age of offspring  Age & marital status of mother Parent-offspring conflict & genomic imprinting

Things to Come Kinship  Hamilton’s rule  Evidence of inclusive fitness applications  Grandparental investment  Sex differences  Evolution of the family unit