The Problems of Parenting

1. The Problems of Parenting

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

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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. Abuse & Child Homicide • Stepchildren 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)

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

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

13. 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

14. 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% • 7.5-60% in children with serious illnesses

15. 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

16. 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

17. Interaction of Relatedness & Age Daly & Wilson (1988)

18. 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

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

20. A Word on Adaptation • Remember, adaptation is an onerous concept • Daly & Wilson argue that stepparental abuse & homicide is notadaptive, but a “reverse assay” of parental care • Performed irregularly & inefficiently • Great cost to perpetrator • No known direct benefits

21. 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

22. Differing Interests • Parental and offspring interests typically differ with relation to resources • Intrauterine conflicts • Extrauterine conflicts (e.g., weaning) • Sibling value, cooperation, & competition

23. 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

24. Intrauterine Conflict Fetal Fitness Om Of Nutrient Provided Williams (1997)

25. 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

26. 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

27. 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)

28. 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

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