Male Sexual Behavior, Sex Drive & Performance

• Proximate bases of male sexual behavior is the definition and evolutionary basis of all behaviors necessary to deliver sperm to female gametes
• Sex steroid hormones regulate behaviors that ensure successful reproduction
• Nematode worm (C.elegans) exhibits complex mate - searching behavior regulated by neurotransmitters (serotonin, insulin) and genes)

Sex Drive, Sexual Performance, and Arousal

• Sex drive is the motivational force that compels males to seek sexual union and is primitive and strong in most species
• Male C. elegans exhibit sex drive when they leave food-rich areas to search for mates.
• Sexual drive is influenced by hormonal and environmental factors
• Sexual performance is the execution of behaviors leading to copulation and is distinct from sex drive but influenced by similar hormones
• Sexual arousal is difficult to define precisely, especially in nonhuman animals
• Historically sexual motivation was equated with motivation (Beach, 1942)
• Newer definitions of sexual motivation focuses on genital blood flow or presence of penile erection in a sexual context
• Erectile dysfunction complicates the definition of sexual motivation

Hormonal Influence on Male Sexual Behavior

• Testosterone and other androgens are critical for sexual motivation and behavior
• Puberty onset coincides with increased sexual interest
• Castration leads to reduced sexual behavior
• Androgen replacement therapy restores sexual behavior
• Androgens lower the threshold for sexual responses, but do not directly "cause" sexual behavior
• High testosterone increases likelihood of copulation in the presence of appropriate stimuli
• Hormones interact with experience and social context to shape behavior
• Androgens affect neurotransmitter levels (e.g., dopamine, serotonin), availability of neurotransmitter receptors, and other proteins that regulate neural activity

Species Differences in Hormonal Control of Sexual Behavior

• Rodents are strongly dependent on hormones for sexual behavior
• Primates (including humans) are more influenced by socialization and learning
• Rats require testosterone for mating, while primates can engage in sexual behavior despite hormonal fluctuations
• Different species exhibit varying degrees of hormonal dependence
• Red-sided garter snake exhibits mating behavior independent of hormones
• Rodents are widely used to study sexual behavior due to their hormonal dependence -Studies helps in understanding the neural and endocrine basis of sexual motivation and performance
• Comparative studies highlight evolutionary adaptations in reproductive behavior

Early Observations of Castration and Sexual Behavior

• Castration has been known to eliminate mating behavior in many species since ancient times
• Latin testiculus (diminutive of testis, meaning "witness") in the etymology of "Testicle" and "Testify"
• Ancient Roman witnesses swore oaths by placing their hands on their testes, symbolizing the link between virility and truth
• In the 19th-century it was believed seminal vesicle swelling activated male sexual behavior and was based on discovery of neural stretch receptors in the 1880s
• Experiments refuted that seminal vesicles were responsible for sexual behavior and showed persistence even when seminal vesicles were emptied
• Eugen Steinach's research emphasized sensory information's importance in regulating sexual behavior
• Berthold's studies demonstrated that testicular hormones, rather than neural inputs, drive sexual motivation

Impact of Experience on Postcastration Sexual Behavior

• Sexual experience affects responses to castration
• Sexually inexperienced male rats stop mating within two weeks of castration
• Sexually experienced males show variation and some stop mating quickly, others continue for weeks
• Tomcats neutered after sexual experience may still attempt copulation for months
• Aristotle observed that castrated bulls sometimes continued mating
• 19th-century records indicate that some men who lost their testes due to injury or disease maintained sexual function indefinitely
• Pfüger suggested that human persistence in sexual behavior post-castration could be explained by unique cognitive faculties (e.g., imagination, memory) in 1877

Brown-Séquard and the Hormonal Rejuvenation Myth

• Charles Edouard Brown-Séquard claimed that testicular extracts had rejuvenating effects
• Brown-Séquard injected himself with a solution from dog and guinea pig testes
• Found that his method was ineffective, as androgens are lipid-soluble and not water-soluble
• Charlatans marketed endocrine extract "treatments" in commercial exploitation of Brown-Séquard's claims
• The Brown-Séquard controversy spurred scientific interest in sex steroid hormones

Early Quantification of Male Sexual Behavior

• Calvin P. Stone studied the age at which male rats first engaged in copulation
• First copulation occurred around 50 days of age
• Showed variation in how long sexual behaviour persisted post-castration
• Drive theory shaped North American psychology in the 1920s
• Key research questions explored prioritized sex drive over hunger or thirst and the effort or pain an individual will endure to gain access to a mate and satisfy sexual motivation
• C. elegans are currently used as a model organism for genetic studies on sex drive regulation

Discovery of Testosterone and Its Role in Sexual Behavior

• Discovery that ovarian estrus-inducing hormone was lipid-soluble led researchers to test lipid-based extracts from testes in search of the active component in the testes
• A bull testis extract dissolved in oil rapidly restored comb growth in capons
• Pure crystalline testosterone was identified in 1935
• Synthetic testosterone was soon produced from cholesterol
• Beach & Holz-Tucker experiments in 1949 measured post-castration androgen replacement in rats
• 100+ µg/day testosterone maintained sexual behavior at precastration levels
• Testosterone treatment maintains sexual behavior after castration
• Mating behavior declines after castration in male rats but can be maintained by testosterone treatment
• Maintenance of sexual behavior by testosterone is dose-dependent
• In the study, only rats receiving testosterone doses of 100 µg/day or greater displayed sexual behavior at precastration levels

Challenges to the "One Hormone, One Behavior" Hypothesis

• Early research assumed specific hormones directly caused distinct behaviors
• Later studies showed both estrogens and androgens could restore male copulatory behavior

Challenges and Sex Drive

• Challenges in Studying Human Sexual Behavior due to limited research on human sex drive and performance
• Separate scientific literatures for human and nonhuman sexual behavior with few interactions between researchers in both fields
• Literature often presents sex drive as passion or emotional imperative
• Romeo's obsessive desire for Juliet as an example of extreme sexual motivation
• Romeo's actions is driven by passion despite social and physical obstacles, and the story reflects the intensity of male sexual motivation in real life

The Strength of the Sex Drive in Human Males

• Ford & Beach (1951) conducted the patterns of sexual behavior
• Patterns of Sexual Behavior was one of the first comprehensive studies of human and nonhuman sexual behavior
• The study combined three major sources of data to reach a conclusion
• Data used to formulate the Pattern of Sexual Behavior: Anthropological research on non-Western societies, Kinsey reports on American sexual habits, Comparative studies on animal sexual behavior
• Premarital sex is universal
• In every culture studied, young people engaged in sexual activity despite risks
• Severe punishments did not deter from sexual activity in some cultures
• Gilbert Islands exhibited people still engaged in sex, even if someone was caught having premarital sex they were immediately executed
• Some were willing to risk death or from a woman's husband or male relatives to pursue sex
• This demonstrated that the biological drive for sex often overrides fear and rational decision-making

Risk-Taking and Ineffectiveness of Abstinence-Only Education

• Sex drive often overrides fear of consequences

• Syphilis before antibiotics is an historical example, as is the Roaring Twenties sexual liberation despite deadly venereal disease with mercury and arsenic treatments

• The use of mercury and arsenic treatments had severe side effects

• Many people discount risks (e.g., STDs, pregnancy) during sexual arousal

• Testosterone may increase or decrease risky behaviour

• High testosterone linked to both risky behaviors and safe sex attitudes

• Regarding the ineffectiveness of abstinence-only education approximately $1 billion (1997–2008) spent on abstinence programs

• Studies show no difference in sexual behavior vs. traditional sex ed

• Virginity pledges had no impact on age of first sex, number of partners, or STD rates

• 82% of pledgers denied making a pledge after 5 year

• Testosterone influences sexual motivation and risk-taking

• Higher testosterone is associated with increased sexual activity

• Men with high testosterone levels often display greater impulsivity and riskier behavior

• Some studies suggest men with higher testosterone actually have stronger attitudes toward safe sex

• High testosterone may increase sexual confidence, leading to better decision-making about protection

Motivations for sex

• People do not engage in sex only for pleasure or reproduction
• A study identified 237 different motivations for having sex, including: physical pleasure, emotional connection, revenge, social status, altruism and spiritual motivations
• Sexual motivation in humans is multi-dimensional and influenced by hormones (e.g., testosterone, oxytocin, dopamine), social and cultural norms, and individual psychological factors
• Physical attraction, physical pleasure, fun, affection, love, expressing love, arousal, and desire are the most commonly reported reasons to engage in sex
• Love, Intimacy, and emotional closeness are slightly significant reasons to engage in sex
• Opportunity, being in the moment, and romantic settings are all contributing motivations to engage in sex
• Some people engage in sex for experimentation and skill-building
• Personality traits have the lowest impact on someone's interest in sex

Human Male Sexual Behavior: A Description

• Introduction to the importance of describing human and nonhuman primate male sexual behavior before you can understand physiological mechanisms

• Several scientific descriptions exist, including studies by Kinsey et al. (201), Masters and Johnson (207), and Money (42), but they have limitations

• Unlike animal research, where behaviors can be observed in controlled environments, human sex research does not involve placing couples in controlled settings with direct observation

• Human male sexual behavior, like rodent mating behavior, can be divided into two key components: sex drive (appetitive phase, libido, or sexual motivation) and sexual performance (consummatory phase, or potency)

• Both components are necessary for normal male sexual behavior, and dysfunction in one or both can lead to sexual disorders

• Hormones play a critical role in maintaining both components, although further research is needed

• Humans engage in various sexual positions unlike most other mammals

• In most mammals, mating occurs dorsal-central with the male mounting the female from behind,

• Dogs, cats, and most other mammals engage in rear-entry mating, which is dictated by female receptivity and male mounting behavior

Hormonal Correlates of Primate Male Sexual Behavior

• Prepubescent boys do not engage in sexual activity outside of play contexts
• After puberty there us the emergence of sexual behaviors
• Average age of first masturbation: ~13.5 years
• At 14 years 62.6% reported masturbation, and that increased to 80% when they were 17
• In a recent increase in masturbation: 42.9% (age 14) → 67.6% (age 17) a similar trend was witnessed
• The First intercourse before 19 has increased from 45% (1940s) to 61% (2003)
• Sexual activity peaks in the 20s and declines gradually with age
• Testosterone levels peak in late teens and 20s and that is also shown in Decline in testosterone in the 60s and 70s which mirrors reduced fertility
• Andropause - debated in medical circles regarding hormone replacement therapy

Individual Differences in Sexual Behavior

• There is vast variability in someone's: Age of onset of sexual activity and frequency of sexual behaviors
• There are also varied effects of castration in males; 50% cease sexual activity quickly, 25% experience gradual decline, and 10% show no change
• There is a factor, that Age at castration influences decline rate

Hormonal Manipulation and Sexual Behavior

• Leuprolide acetate (Lupron) suppresses testosterone -> reduces sexual activity (Hypogonadism)
• Testosterone treatment restores sexual behavior
• Key example on using leuprolide is that it's used in the treatment of sexual offenders
• Stanford Hypogonadal Men Experiment - 6 men with testosterone <3 ng/ml, and that double-blind study with three treatments: 100 mg testosterone enanthate, 400 mg testosterone enanthate, Placebo (oil vehicle)
• High dose peak erections ~1 week after injection
• Testosterone increased sexual activity in dose-dependent manner
• Placebo had no effect

Androgens and Sexual Response

• Key Findings illustrate that sexual fantasies and arousal show dose-dependent response to androgens
• Men still respond to erotic stimuli and it's related to Hypogonadal
• Hypothesis: POA dopamine may mediate sexual response in castrated individuals
• This is similar to studies comparing animal models and how had similar results: Similar effects observed in castrated rats with dopamine elevation

Brain Mechanisms of Primate Male Sexual Behavior

• Electrical stimulation of the MPOA induces penile erection in socially isolated male rhesus monkeys
• Different stimulation sites produce varied responses with Erections, Mounting and even Mounting with erections
• There is no specific "sex center" and stimulation likely increases general arousal
• Unlike rodents, chemosensory cues are not critical for primate mating
• Olfactory bulb removal does not affect rhesus monkey sexual behavior

Amygdala and Sexual Behavior

• Amygdala lesions yield mixed results in the experiment
• Klüver-Bucy syndrome (bilateral temporal lobe removal) leads to, Docility and fearlessness, Hypersexuality (indiscriminate mounting) and Ingestion of inedible material
• Overall Suggests that limbic structures regulate sexual behavior

Steroid Hormone Binding in Primate Brain

• Androgen, estrogen, and progestin receptors concentrated in: Hypothalamus, Amygdala, Hippocampus
• Pattern of steroid receptors is conserved across vertebrates
• Neural steroid metabolism differs among species
• Early primate research lacked sufficient sample sizes
• DHT binding data: one male rhesus monkey and Testosterone binding data: two males
• Electrode recordings in testosterone-treated males had: Slow, synchronized electrical activity in hypothalamus, mammillary bodies, amygdala, and hippocampus and that Synchronization intensified after castration
• Androgens affect neural transmission speed and firing patterns

Imaging studies related to sex

• fMRI in male marmosets exposed to ovulatory vs nonovulatory female odors showed activity in the Preoptic area (POA), Anterior hypothalamus, Striatum, Hippocampus, Septum, Periaqueductal gray and the Cerebellum
• The brain regions in general are involved in sexual behavior, emotion, and reward
• There was also fMRI studies in young men when watching fishing documentaries vs pornography and identified; Automatic activation of pars opercularis (mirror neuron region) before penile erection and that Mirror neurons likely play a command role in sexual response
• Ventral tegmental area (VTA) will show the activation during your PET scans show
• The VTA is part of the brain's reward system, also activated by heroin and that there are Similar findings in rodents during ejaculation