Why Sex? Mating. Disadvantages of Sex. Advantages of Sex. What Would We Expect? Sex Differences in Parental Investment

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1 Mating Cost of males Red Queen hypothesis Reproductive value Waist-hip ratio (WHR) Body mass index (BMI) Parental investment (PI) Why Sex? Most genes have a 5% chance of being replicated in sexually produced offspring. All genes have a 1% chance of getting into asexually produced offspring. The advantages of sexual reproduction must outweigh this large and obvious disadvantage, plus others. Disadvantages of Sex Disrupting adaptive combinations. Cost of finding a mate and mating. Individual gene s chances of replication are halved (cost of males). Advantages of Sex Genetic diversity of offspring Reduce competition between offspring Some may be successful if the environment changes Immune system diversity to keep ahead of parasites and pathogens (Red Queen Hypothesis) Sex Differences in Parental Investment Minimum (obligatory) Investment Males - sperm Females - egg and 9 months gestation Typical Investment Males - resources, some child care Females - lactation, child care, some resources What Would We Expect? What do men want from women? Many partners Fertile mates Healthy mates Parental investment Fidelity (paternity certainty) What do women want from men? Resources Parental investment Healthy mates 1

2 Men Desire More Partners and are Less Choosy Sex with a Stranger I ve noticed you around and I find you very attractive a) would you go out with me tonight? b) would you come over to my apartment tonight? c) would you go to bed with me tonight? Question Women Men a 5% 5% b 6% 69% c % 75% (Buss & Schmidt 1993) (Clark & Hatfield 1989) Female Reproductive Value Female Fertility Declines with Age Age-specific RV = S (probability of attaining age) * (age-specific fertility) 3 Reproductive Value Age in Years Male Fertility Declines Less Canadian Age-Specific Fertility Rate children per 1 women age (Dunson, Colombo & Baird 22) 2

3 Signs of Fertility in the Face Androgens cause: Wider and longer lower jaw Wider nose More pronounced brow ridges Thicker eyebrows Estrogens cause: Fuller lips Fat deposits on hips Fat deposits on breasts Cultural Variance in WHR Preference The Matsigenka people of Peru prefer.9 WHR over.7. The Hazda of Tanzania prefer women with higher weight and do not differentiate based on WHR. WHR versus BMI Waist-Hip Ratio (WHR) and Fertility Much research has found that a low (.7) waist-hip ratio is most attractive. WHR is influenced by levels of androgens. This could signal non-pregnancy and fertility. Women with a WHR greater than.8 have a more difficult time getting pregnant through IVF. Body Mass Index (BMI) and Fertility Body mass index = kg/m2 BMI under 19 and over 25 have been shown to reduce fertility. Are Face and Body Signals the Same? In a study where undergraduate men rated pictures of women s bodies, both WHR and BMI significantly pedicted attractiveness ratings. BMI accounted for 73.5% of the variance while WHR accounted for only 1.8%. Waist-bust and bust-hip ratios were not correlated with attractiveness. (Toveé et al. 1998) (Thornhill & Grammer 1999) 3

4 Skin Colour Women have lighter skin than do men. People become darker with age, but women s skin gets lighter at puberty. Women s skin is darker during pregnancy and during infertile phases of the menstrual cycle. There is a preference for lighter skin colour in 47 of the 51 HRAF societies with relevant information. 3 mention this preference only for females, while 4 mention it only for males. Light skin may be a sign of youth and fertility. Resources Women rate earning potential as a more important factor in a mate than do men. Both women and men rate earning capacity as more important for longterm than short-term relationships. (van den Berghe & Frost 1986) Facial Dominance Facial Dominance and Rank Attainment Rated facial dominance of photographs of the West Point class of 195 was significantly correlated with rank attainment. (Mueller & Mazur 1997) Parental Investment Male Body Type and Attractiveness Women rate men with lower waist-chest ratios and intermediate BMI to be most attractive. Women find the man interacting positively with the baby considerably more attractive. Comparable photos of women produced no effect on men s judgments of attractiveness (La Cerra 1994) BMI Waist-Chest Ratio (Maisey et al. 1999) 4

5 Sperm Competition Competition between sperm from different males for fertilization of eggs produced by a single female. 1) How much opportunity is there for sperm competition in humans? 2) What evidence is there that sperm competition affects human sexual response and behaviour? 3) How might women choose which sperm fertilize their eggs? Do Women Double Mate? How long do sperm last in the female reproductive tract? How Long Do Sperm Live? Vagina - only after 12 hours (too acidic) Cervix - up to 1 days Fertilization can occur 5 days after copulation. Women must mate with two different men within about 5 days for sperm competition to occur. Percentage of females Double Matings Lifetime copulations >3 Min days bet. men 5% of extra-pair copulations (EPCs) involved copulation with different partners within five days. 1 5 Behavioural Evidence The greater the risk of sperm competition, the more sperm should be inseminated during copulation 35 couples collected semen in condoms as part of an on-going diary study. They measured the number of sperm inseminated (concentration times volume), the time between episodes of intercourse, and also the amount of time partners spent together. (Baker & Bellis 1989, 1993) Males Ejaculate More the Less Time a Couple Spends Together Number of sperm inseminated (millions) Proportion of time together Hrs since last ejacn <24 h h > 72 h 5

6 Women might have evolved various devices to bias sperm competition in favour of certain partners, such as short-term extra-pair males (because they have different genes than the main partner). In a survey study, 2546 females stated when they last copulated, whether it was with the main partner or with a lover, and the day of their menstrual cycle on which it occurred. Phase I (days 1-5) Phase II (days 6-14) most likely to conceive Phase III (days 15-28) Female Choice (Baker & Bellis 199) Number copulations/day Women Bias EPCs in Phase II I II III PHASE I II III PHASE Intra-pair copulations Extra-pair copulations Number of copulations/day 6