Sex Differences I: Evolutionary Theory and Sexual Selection

Animal model systems

• Use animals to investigate biological principles.

• Animals are studied not as themselves, but for what they reveal about biology.

• Findings are often extrapolated to humans.

• Help show possible solutions to survival and reproduction problems.

Animal role models

• The idea of using animals as moral or behavioral guides for humans.

• Explicitly rejected in this course.

• We do not judge animals as “better” or “worse” models.

• Example: marmoset vs baboon paternal care is not a value judgment.

Naturalistic fallacy

• The belief that what is natural is good, right, or inevitable.

• A logical error.

• Commonly misused to justify human behavior.

• Must be avoided when interpreting animal behavior.

Zuk’s solution to the naturalistic fallacy

• “Nature is witless.”

• Nature is not moral, cruel, kind, or purposeful.

• Behaviors are solutions to environmental and reproductive problems.

• Nature has no agenda.

Darwin’s theory of evolution by natural selection

• Individuals face environmental challenges.

• Populations show variation in traits.

• Some individuals survive and reproduce more successfully.

• Their traits become more common in future generations.

• Evolution = differential reproduction.

Reproductive success

• Defined as the number of offspring an individual produces.

• The currency of success in evolutionary theory.

• Not about happiness, strength, or morality.

Proximate explanation

• Refers to immediate causes or triggers.

• Answers “How does this happen?”

• Example: hormonal or environmental cues.

• Works alongside ultimate explanations.

Ultimate explanation

• Always concerns reproductive success.

• Answers “Why did this trait evolve?”

• Focuses on evolutionary function.

• Must be consistent with proximate explanations.

Genetic determinism

• The belief that behavior is solely caused by genes.

• Assumes biology overrides environment and learning.

• Implies behavior is fixed and unchangeable.

Evolutionary theory and sex differences

• Predicts relatively few differences between males and females.

• Both sexes must survive.

• Expect similar survival traits.

• Major differences appear mainly in mating and reproduction.

Two levels of potential sex differences

1. Differences in body form (visible differences).

2. Differences in behavior, especially:

• Within a mating context.

• Less clear outside reproduction.

Sexual dimorphism

• Refers to differences in body form between males and females.

• Humans are dimorphic, but in limited ways.

• Explains less than commonly assumed.

Secondary sexual characteristics

• Traits that differ between sexes.

• Not required for the physical act of reproduction.

• Often found more on males than females.

• Examples: antlers, large canines, bright plumage.

Darwin and secondary sexual characteristics

• These traits:

  • Can be costly.

  • Can reduce survival.

• Therefore, they cannot be explained by natural selection alone.

• Led Darwin to propose sexual selection.

Sexual selection

• Explains how sex differences evolve.

• Focuses on mating success, not survival.

• Similar to but distinct from natural selection.

• Acts through competition and choice.

Natural selection vs sexual selection

• Natural selection:

  • Traits that improve survival.

• Sexual selection:

  • Traits that improve mating success.

Intrasexual competition

• Competition within the same sex.

• Usually male–male competition.

• Males compete to:

  • Exclude rivals.

  • Mate more often.

• Produces weapons or large body size.

Intersexual selection

• Selection between sexes.

• Usually female choice.

• Females prefer certain traits in males.

• Produces ornaments or displays.

Male-male competition outcomes

• Larger body size.

• Weapons (e.g., antlers, large canines).

• Physical dominance.

• Seen in species like elephant seals.

Female choice

• Females select mates based on preferred traits.

• Preferences influence which traits evolve.

• Can be more important than physical combat.

• Drives ornamental traits.

Courtship displays

• Exaggerated behaviors or traits.

• Often costly and time-consuming.

• Function to attract females.

• Examples: sage grouse dances, bowerbird bowers.

Sage grouse mating system

• Males gather in leks.

• Females choose based on visual and acoustic displays.

• Only a few males mate.

• Selection driven by female choice, not fighting.

Bowerbirds and sexual selection

• Males build elaborate bowers.

• Collect objects (e.g., blue items).

• Females prefer certain designs.

• Displays are sexually selected traits.

“Good genes” hypothesis

• Exaggerated traits signal good health.

• Displays are honest signals.

• Only healthy males can afford costly displays.

• Females choosing these males may get healthier offspring.