Anthropology 173 The Evolution of Human Sexuality Lecture Seven: Part One Monogamy and Concealed Ovulation Are Humans “Naturally” Monogamous? ◼ The classic argument (Desmond Morris): ❑ Human children require investment from two parents ❑ The pair-bond evolved to facilitate parental investment ❑ Ovulation was concealed to keep males around ❑ Constant sex maintains pair-bond, depletes male sperm To Test This We Need to Know: ◼ What people do: Are they behaviorally monogamous? ◼ What selective pressures do our anatomy appears designed for: Are we shaped like monogamous creatures? ◼ Is ovulation actually “concealed”? ◼ Paternal investment (next week) Human Mating Systems ◼ Like primates in general, human cultures show variety in mating systems ◼ There are costs and benefits of each for men and women ◼ 83.4% of human societies allow polygyny (as either a usual or occasional marital arrangement) but, the majority of human marriages are monogamous… Marriage ◼ ◼ Shared interest in genetic offspring requires expectation of exclusive sexual access Not just about reproduction (directly): ❑ ❑ Marriages bind alliances, economic relations between groups and families Allows for division of labor Monogamy ◼ Monogamy can be life long or serial, that is one monogamous relationship after another ◼ Even in monogamous societies, males are more likely to remarry and have a second family than females ◼ When males remarry, they often marry a younger female Monogamy vs. Polygyny ◼ ◼ Women Costs ❑ ❑ ◼ Less ability to have the best available male as a mate No shared parenting with co-wives Benefits ❑ ❑ High male investment Less competition with cowives   Men Costs ⚫  Limited reproductive success Benefits ⚫ ⚫ ⚫ ⚫ Offspring quality (rather than quantity) Paternity certainty Lower male-male competition Higher quality mate Promiscuity in Humans ◼ There is a distinction between marriage and mating ◼ Similarly, mating systems are idealized versions and do not always reflect what people actually do The Human Penis ◼ Physical adaptations provide evidence about the kinds of conditions they evolved in Loss of the Baculum ◼ Humans have no baculum (os penis) Walrus ◼ Boner is a misnomer Dog Hypotheses about the Loss of the Baculum ◼ God took it out of Adam to make Eve (the Hebrew word for rib can also mean the trunk of a tree or the supports on a door or house) ◼ Encourage extended foreplay, which strengthens the pair bond (monogamy) ◼ Byproduct of penis elongation for sperm competition (promiscuity) ◼ Extended copulation produced an increased chance of phallus injury, which could be minimized by baculum loss Monogamous Primate Penises ◼ Monogamous primates have relatively unadorned penises Multi-male Primate Penises Human Penises ◼ Humans have relatively simple but very large penises ◼ Humans have relatively large testes The Well Hung Humans ◼ Hypotheses for the large penis: ❑ Female choice ◼ Visual ◼ Tactile ❑ Male-male competition ❑ Sperm competition Female Preference Penis-sheaths for scaring other males? Male-male Competition ◼ No evidence that human males actually use erect penises in competition ◼ Doesn’t explain loss of baculum ◼ Males might have evolved the ability to use penis size to assess sperm competition threat Anthropology 173 The Evolution of Human Sexuality Lecture Seven: Part Two Monogamy and Concealed Ovulation Ribbed for her pleasure? ◼ In fact, the coronal ridge appears to facilitate sperm displacement ◼ In an artificial simulation, penises with realistic ridges and deep thrusting displaced the most semen from a fake vagina ◼ Gallup, et al 2003 Behavioral Changes after Threat of Infidelity ◼ Males and females report greater depth and vigor of thrusting when there is some threat of infidelity ◼ Threats of infidelity increase male arousal, but decrease female arousal ◼ An analysis of Internet porn showed men downloaded images with more than one male and a single female more often than other images Opportunity for Sperm Competition ◼ 479 undergraduates in New York “Have you had sex with more than one male within a 24 hour period?” ❑ 13% of females said yes ❑ 8% had participated in a ménage à trois with two males ❑ 25% of females reported having had sex within someone else while in a relationship ❑ 15% of males Self-semen Displacement ◼ After ejaculation ❑ Loss of erection (head may shrink first) ❑ Shallower thrusting after ejaculation ❑ Change in penile sensitivity ◼ ❑ Continued thrusting can become unpleasant following ejaculation The refractory period ◼ Usually 30 minutes to 24 hours depending on age ◼ The Coolidge effect can shorten the refractory period Semen Coagulation ◼ Semen coagulates within seconds after ejaculation and then liquefies 15-30 minutes later ◼ Viscous semen is more difficult to displace ◼ Many species of primates form copulatory plugs Flowback: Sperm Retention ◼ Flowback (the wet spot) is sperm and vaginal fluids that leak back out of the vagina after intercourse, or are expelled by urination ◼ By measuring ejaculate size (with a condom) and flowback size (by having females catch it) researches can figure out how much sperm females retain Bipedalism ◼ ◼ ◼ Problem with bipedalism: standing up causes flowback Face to face copulation encourages a horizontal orientation of female reproductive track Mechanisms for maintaining a horizontal posture ❑ ❑ ❑ Post-copulatory cuddling Nocturnal copulation Sedative like effects of orgasm Males Adjust Sperm Number and Quality ◼ Baker and Bellis Female Orgasm ◼ Female orgasm may serve to “suck” sperm up ◼ The cervix dips into the sperm pool with vaginal contractions ◼ Female orgasm may be a way for women to exercise cryptic female choice Circumcision ◼ ◼ Reduces penile sensitivity Increases sperm displacement ❑ ◼ Circumcised men thrust deeper and harder Why do men have a foreskin? O’Hara and O’Hara 1999 ◼ Women were more than four times less likely to have an orgasm with a circumcised male ◼ Circumcised males were almost twice as likely to have a premature ejaculation ◼ Twenty times more likely to report vaginal discomfort with a circumcised partner Foreskin and the Pair Bond ◼ Women felt more intimacy with unaltered partners ◼ Circumcised: partner cared little about me, glad it’s over, discomfort, he’s working hard for an orgasm, disinterested, pumping until it hurts me ◼ Uncircumcised: relaxed, “complete as a woman”, afterglow, “gee that was great”, “what a lover” ◼ “When the anatomically complete penis thrusts in the vagina, it does not slide, but rather glides on its own ‘bedding’ of movable skin, in much the same way that a turtle’s neck glides in and out on the folded layers of skin surrounding it.” STDs and Circumcision ◼ HIV, HPV, & Herpes-All reduced Anthropology 173 The Evolution of Human Sexuality Lecture Seven: Part Three Monogamy and Concealed Ovulation Is Ovulation Concealed? ◼ Ovulation is relatively concealed in humans ◼ Many primates mate only at estrus Sexual Swellings ◼ Many primate females advertise oestrus with bright perineal swellings Sexual Swellings ◼ A graded signal that encourages mating with all males during fertile period ◼ A defense against infanticide ◼ Gradual swelling makes mating with the most dominant male most likely to actually conceive (Dominant males mate at peak swelling, subordinate males, outside peak.) In Humans: ◼ Continuous sexuality ◼ Hypotheses about concealed ovulation ❑ Evolved to facilitate monogamy ❑ Evolved to confuse paternity ❑ Evolved to keep women from choosing not to have children ❑ Didn’t evolve at all (this is the ancestral state) ◼ Jared Diamond Birth-Control Hypothesis How Concealed is Human Ovulation? ◼ Meredith Small: undergraduate students in Anthropology courses: ❑ Females: 74% said they knew when they ovulated (related to regular cycles, and knowing process) ❑ Males with girlfriends: 49% said they knew when she ovulated Ways Human Females Advertise Ovulation ◼ Males find partner body odors (saliva, vagina, underarms, and loin) most pleasant when from ovulatory phase of cycle ◼ Male strangers rated body odor of women from T–shirts sexier when women in fertile phase ◼ The skin is lightest, smoothest, and most free of blemishes around ovulation ◼ Men are able to distinguish fertile post–pubescent females from infertile pre–pubescent females on the basis of their skin condition ◼ Breasts may swell and become more symmetrical Anth 173 Non-advertised Does not Mean Concealed - Havlicek et al 2006 Pipitone and Gallup 2008 13 Other Ways to Advertise Ovulation ◼ Women who are ovulating wear skimpier clothing to nightclubs ◼ Women are more likely to engage in EPCs when ovulating Other Menstrual Cycle Effects ◼ ◼ As short term mates, fertile women prefer: ❑ Symmetrical men ❑ The scent of symmetrical men ❑ Deeper male voices ❑ Creativity over wealth Differences are not seen for long-term mate preferences ◼ Preference for the scent of symmetrical men ◼ Garver-Apgar et al. 2008
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