Overview
This lecture covers the biology of sexual behavior, focusing on its evolutionary purpose, neurobiology, fixed action patterns, human-specific features, role of neurotransmitters and hormones, and the influence of sensory stimuli such as pheromones.
Course Structure & Approach
- The rest of the course will analyze behaviors (e.g., sexual behavior, aggression) across species using a timeline approach: immediate brain activity, hormonal influences, developmental, genetic, and evolutionary factors.
- Emphasis on the interaction between genetic, hormonal, neural, and environmental factors in shaping behavior.
Proximal vs. Distal Explanations
- Proximal explanations focus on immediate causes (e.g., pleasure from sex).
- Distal explanations focus on evolutionary reasons (e.g., passing on genes).
- Sexual behavior is primarily driven by proximal (pleasure/reward) mechanisms.
Fixed Action Patterns & Species Specificity
- Sexual behavior involves conserved, species-typical fixed action patterns (e.g., lordosis, pelvic thrusting).
- Species-specific cues ensure appropriate mating within species.
- Interplay ("chaining") between individuals' fixed action patterns and releasing stimuli creates species specificity.
Key Concepts & Terminology
- Proceptivity: active behaviors that initiate sexual interaction.
- Receptivity: willingness to engage in sexual activity.
- Attractivity: how attractive an individual is to potential mates.
- Motivation vs. performance: important distinction, especially in clinical settings.
Human Sexual Behavior: Unique & Shared Features
- Humans are not unique in non-reproductive sex, foreplay, homosexuality, or masturbation; many species exhibit similar behaviors.
- Humans uniquely often have sex in private, have egalitarian mating structures, and display serial monogamy.
- Social monogamy does not always equal sexual monogamy; paternity studies suggest high rates of nonpaternity.
- Romance is a relatively recent and culturally variable concept.
Neurobiology of Sexual Behavior
- The limbic system (esp. hypothalamus, amygdala) is central to sexual motivation and behavior.
- In females, the ventromedial hypothalamus (VMH) is key; in males, the medial preoptic area and amygdala are critical.
- Dopamine in the mesolimbic pathway drives sexual motivation and reinforcement.
- D1 and D2 dopamine receptors have distinct roles in pair bonding: D2 for bond formation, D1 for bond maintenance.
Hormonal Responses & Pair Bonding
- Sex increases progesterone and androgen levels in females, testosterone in males.
- Oxytocin (females) and vasopressin (males) promote pair bonding in monogamous species by acting in the brain.
- Genetic variations in vasopressin receptors relate to relationship stability in some species and possibly humans.
Sexual Orientation and the Brain
- Some hypothalamic structures differ in size between sexes and correlate with sexual orientation.
- Biological markers (e.g., finger ratio, otoacoustic emissions) suggest prenatal hormone influences on orientation.
- Transsexual individuals may have brain regions that match their experienced, not chromosomal, gender.
Sensory Triggers and Pheromones
- Species differ in primary sexual cues: visual (primates, humans), tactile, or olfactory (pheromonal) signals.
- Hormone levels affect both the production and perception of pheromones.
- Pheromones convey information about sex, health, reproductive status, and kinship.
Key Terms & Definitions
- Fixed Action Pattern — a stereotyped, species-specific behavior sequence triggered by specific stimuli.
- Proximal Causation — immediate, mechanistic causes of behavior.
- Distal Causation — evolutionary, ultimate causes of behavior.
- Mesolimbic Pathway — a dopamine system in the brain central to reward and motivation.
- Oxytocin — a hormone important for bonding and social attachment.
- Vasopressin — a hormone involved in social behavior and pair bonding, especially in males.
Action Items / Next Steps
- Review recommended chapters in the course books, especially those on sexual behavior and neurobiology.
- Check extended notes for details on research techniques and historical studies.
- Prepare for upcoming lectures on aggression, competition, cooperation, and language.