Lec 23

Course Overview

• Course Title: ANT 001: Human Evolutionary Biology

• Week Topic: Primate Reproduction

• Instructor: Prof. Brenna Henn

• Term: Winter Q 2025

Key Questions from Previous Class

• Milk & Holocene: Why was drinking milk beneficial during the Holocene?

• Gene Expression: How does the mechanism of gene expression differ between MCM6/LCT and AMY1?

• Lactase Persistence: Were most ancient Europeans lactase persistent 4,000 years ago?

Class Updates

• Reading Assignment: Chapter 6

• Canvas Quiz: Quiz #3 available until Wednesday at 10:30 am

• Writing Assignments:

  • Grades for Assignment #2 posted

  • Submit Assignment #3 by noon, March 19th (electronic version on Canvas)

Key Concepts of Primate Reproduction

• Reproductive Success Variance:

  • Greater variance in male reproductive success.

  • Female reproductive success influenced by:

    • Access to food

    • Early life experience

    • Social bonds

    • Dominance rank

• Menopause in Humans: Unique among primates with minimal post-reproductive lifespan in other species.

• Juvenile Survival: Closely tied to maternal care; primates have extended juvenile developmental periods.

• Mating Systems: Four types identified:

  • Pair-bonded

  • Cooperative

  • Polygynandry

  • Polygyny

• Sexual Selection: Occurs within and between sexes.

Human Diet Evolution

• Dietary Classification: Omnivores with diverse food intake.

• Food Preferences: Preference for fruits and sweet foods alongside leaves, seeds, tubers, nuts, honey, and insects.

• Meat Consumption: Transitioned from a minor component (3.4 Mya - 400 kya) to significant; aided by fire control and processing.

• Adaptation: Genetic changes in response to starch and milk consumption over the last few thousand years.

Evolution of Reproductive Strategies

• Mating Effort: Activities focused on conception (e.g., courtship, competition, territory establishment).

• Parenting Effort: Care activities post-conception (e.g., nursing, protecting juveniles).

  • Discuss Brad's lecture differences in investment between genders.

Female Primates' Reproductive Strategies

• Investment: High investment in offspring through physical reproduction and behaviors.

• Gestation: Long pregnancies, costly brain growth, and lactation periods.

• Behavioral aspects: Protection from predators, grooming, teaching behaviors, and limited offspring relative to other species.

Weaning in Female Primates

• Independence: As infants grow, they become independent and forage.

• Weaning Process: Gradual reduction of breastfeeding to free energy for new conception.

• Variability: Weaning duration varies among primate species, affecting maternal and offspring dynamics.

Female Reproductive Patterns

• Continuous Reproduction: Majority of female primates continuously reproduce except humans.

  • Young mothers face challenges like allocated energy to growth; lead to higher infant mortality.

  • Older mothers bring experience and social capital but face senescence.

• Influences on Success: Factors impacting female reproductive success include longevity, group size, dominance rank, and sociality.

Rank Implications for Reproductive Success

• Higher Ranking: Correlates with increased offspring among species like chimpanzees.

• Female Reproductive Variance: Many females do not reach reproductive age; graph analysis of baboons shows survival rates post-birth.

Selection Mechanisms

• Intrasexual Selection: Male competition leads to dominance and access to females.

• Intersexual Selection: Female choice influences reproductive outcomes.

Male Reproductive Tactics

• Mating & Parenting: Male strategies vary; competition among males is common.

• Linking Behavior: Mating strategies often connected with parenting behaviors.

Evolution of Male Reproductive Strategies

• Mate Finding vs. Offspring Care: Energy may shift toward either finding new mates or assisting offspring survival based on scenarios.

Pair-Bonding Species

• Definition: Male-female pairs co-raising juveniles.

• Characteristics: Mate guarding and enhanced parental investment; examples include titi monkeys, marmosets, gibbons, and siamangs.

Cooperative Breeding**

• Characteristics: Group members assist in raising offspring, increasing litter size.

• Examples: Marmosets and tamarins often have twins; helpers enhance energetic efficiency and fertility.

Mating System Dynamics

• Polygyny: One male, multifemale system leading to conflicts; examples include gray langurs and geladas.

• Polygynandry: Both males and females mate with multiple partners in multimale, multifemale groups with established dominance hierarchies.

Infanticide as Male Strategy**

• Concept: Male reproductive strategy; noted by Sarah Blaffer Hrdy; closely related to sexual selection.

  • Prevents mother’s lactational amenorrhea to expedite breeding.

• Observational Data: Late observations predict that new male takeovers coincide with high infant mortality rates.

Female Counterstrategies to Infanticide**

• Paternity Confusion: Method evolved by females to mitigate reproductive disadvantages.

Questions and Discussion

• Open Floor for Questions.

• Discussion on early hominin and modern human mating system characteristics.