Lec 24

Week Overview

• Course: ANT 001: Human Evolutionary Biology

• Topic: Human Reproduction & Behavior

• Professor: Brenna Henn

• Term: Winter Q 2025

Key Questions from Previous Class

• Rank among female primates: Higher-ranking females have higher fitness as illustrated by ______.

• Mating system definition: A mating system characterized by 1 male per group and multiple females is known as ___.

• Female baboons: The primary determinant of variance in the number of offspring among female baboons?

Class Updates

• Chapters for reading: Chapters 6 and 15.

• Canvas Quiz #3: Closed this morning; class average ~85%.

• Study guide is available on Canvas.

• Writing Assignment #3: Due by noon, Mar. 19th, electronic submission required.

Key Concepts in Human Reproductive Behavior

• Main Mating Systems:

  • Pair-bonded

  • Cooperative

  • Polygyny

• Sexual Dimorphism: Reduced sexual dimorphism (in terms of canines and body size) indicates decreased sexual selection pressure among human males.

• New Terminology: Includes terms like minor marriage, endogamy, consanguinity, infanticide, and recessive traits.

• Inbreeding: Can be assessed through the fraction of the genome that is homozygous, which can lead to negative effects, such as increased fertility issues. Inbreeding avoidance is a universal practice among humans.

• The Himba Population: Used as a case study for understanding inbreeding avoidance, paternity, various marriage types, and familial jealousy.

Infanticide as a Reproductive Strategy

• Definition: Infanticide is seen as a male reproductive strategy, as posited by Sarah Blaffer Hrdy.

• Related Concepts:

  • Sexual selection infanticide hypothesis.

  • Lactational amenorrhea: The period during which lactating females do not resume ovulation, and how infant death can speed up return to fertility.

Observations and Predictions on Infanticide

• Changes in male residence may correlate with increased incidences of infanticide.

• Infanticide predicted behaviors: Males may kill infants that lead to a female's resumption of her reproductive cycle, often targeting the infants of others rather than their own.

• Data Insights:

  • 85% of deaths coincide with takeover by new males.

  • Unweaned infants are the primary targets, with incidences low amongst males currently mating within the group.

  • 45% - 70% of cases have males mating with the same female after the occurrence of infanticide.

Female Counterstrategies to Infanticide

• Infanticide is not beneficial for females from a reproductive standpoint, leading them to evolve counterstratergies.

• Paternity Confusion: This strategy helps females guard against infanticide.

Mating Systems and Hominin Evolution

• Discussing which mating systems characterized early hominins and modern humans.

Polygyny in Contemporary Cultures

• Polygyny Frequency:

  • Rare in many societies, with exceptions mainly in Central Asia and central/west Africa.

  • Cultural tolerance varies significantly across regions.

• Polygyny Threshold Model: Predicts that women will enter polygynous marriages only when they perceive equal or greater benefits than in monogamous relationships (considering factors like male wealth, food sharing, and child-care).

Polygyny in Hunter-Gatherer Societies

• Among hunter-gatherers, polygyny exists mostly among a minority of men (typically 2-3 wives), correlating with higher social capital.

• Partnerships are commonly structured as serial pair-bonding with an average duration of 7 years.

• Divorce is permitted and remarriage occurs within 2 years as per common practice.

• In polygamous societies, family hierarchy is often standardized.

Psychology of Reproductive Behavior

• Mate Preferences: Predictive patterns are complicated by historical norms.

• Female Limiting Factors: Focus on resources and caregiving; thus, women often prefer males with superior resource distribution or child-rearing capabilities.

• Male Limiting Factors: Males tend to favor healthy, fertile females.

Jealousy as an Evolutionary Function of Mate Guarding

• Jealousy emerges as a behavioral tactic to mitigate the risk of infidelity.

• Risks of resource diversion due to infidelity lead to a fitness cost for males.

• Distinction between sexual and emotional infidelity; cultural emphasis on infidelity correlates with the level of male resource investment in children.

Inbreeding Avoidance: An Evolutionary Behavior

• Many diseases in humans are homozygous recessive in nature.

• Average individuals carry 2-5 potentially fatal alleles, heightening risk through familial relations.

• Inbreeding leads to increased homozygosity and the potential for negative genetic phenotypes.

• Many primate species exhibit dispersal behaviors upon reaching sexual maturity to avoid inbreeding.

Homozygosity and Phenotypic Effects

• Homozygous Genotype: Characterized by identical alleles (e.g., T T).

• Increased homozygosity linked with multiple phenotypic outcomes in various species:

  • Increased mortality in desert tortoises (Scott et al., 2020).

  • Decreased fitness in Florida scrub-jays (Chen et al., 2016).

  • Association with cardiometabolic traits in humans (Ceballos et al., 2020).

  • Reduced height in human populations (McQuillan et al., 2012).

  • Effects attributed to recessive alleles manifesting within runs of homozygosity.

Understanding Khoisan Populations and Homozygosity

• Concept of FROH: Runs of homozygosity in genetic studies of Khoisan populations.

Insights on the Himba Population

• The Himba are semi-nomadic, Bantu-speaking agro-pastoralists located in Northwestern Namibia.

• Various partnership models include monogamous, polygamous, and concurrent partners.

• Practice of arranged marriages with preferences for first cousins among minors; concurrent relationships accepted.

• Encouragement of natural fertility and pro-natal ideologies in partnerships.

Paternity Testing Among the Himba

• Paternity tests revealed a 52% rate of biological paternity in social marriages, with 48% instances involving non-biological social fathers due to concurrent relationships.

• Low biological inheritance impacts due to cattle inheritance practices.

Consanguinity Practices Among the Himba

• Minor marriages may occur involving minors that are not consummated.

• Arranged marriages can include cousins, showing a close-knit cultural structure.

• Evidence suggests high FROH levels arise from endogamy and population bottlenecks.

• The mechanisms of inbreeding avoidance among the Himba appear effective.

Psychological and Biological Mechanisms of Inbreeding Avoidance

• Inbreeding avoidance manifests universally in humans through psychological adaptations.

• Westermarck Effect: Suggests individuals are not attracted to those they grow up alongside.

• Chemical preference for dissimilar partners based on HLA genes suggests a biological basis for aversion.

Consequences of Homozygosity on Fertility

• Recent studies tested the link between inbreeding (homozygosity) and lower fertility among Himba women.

• Factors affecting fertility included:

  1. Year of birth (cohort effects like drought or antibiotics)

  2. Number of marriages experienced by the woman

  3. The fraction of the genome that is homozygous (FROH).

• Demonstrated that FROH correlated with a 13% reduction in the number of surviving children.

Impact of FROH on Height

• Height is influenced by both genetic and environmental variables, with about 80% heritability but limited gene identification (20-50% known).

• The aggregate effects of recessive alleles captured by FROH potentially influence height.

• No significant evidence that FROH affected height among adult Himba populations.

Minor Marriage in Taiwan: Inbreeding Avoidance Cases

• Graphical data showcasing divorce rates correlated with age at adoption and duration of marriage.

Questions and Discussions

• Space for questions and clarifications from students.