Transcript Sexual Dimorphism.
Evolutionary Psychology, Lecture 5. Human Short-Term Mating Behaviours.
Learning Outcomes.
At the end of this session you should be able to:
1 . Discuss the adaptive benefits of short-term mating in males.
2 . Discuss the adaptive benefits of short-term mating in females.
3 . Explain certain human sexual behaviours from an evolutionary perspective.
Thoughts for the day
“ Men are like soup - you always want to have one on the back burner ” (Anon).
“..
everywhere sex is understood to be something females have that males want ” (Symons, 1979).
“I love men, even though they are lying cheating scumbags” Gwyneth Paltrow.
“ Woman wants monogamy, man delights in novelty (Dorothy Parker). ” “ Sex is an antisocial force in evolution. Bonds are formed between individuals in spite of sex not because of it ” . (E.O. Wilson).
1. Adaptive benefits of short-term mating for males.
Ancestral males would have benefited in terms of reproductive ‘fitness’ by always copulating with available and willing fertile females.
We would expect natural selection to favour males who: Had low thresholds for sexual arousal.
Would seek out casual sex.
Who found 'new' females sexually attractive.
Had lower standards for a short-term mate .
Evidence for Short-Term Mating Adaptations in Males.
a) Testicle size : Large testes evolve as a consequence of sperm competition - where sperm from several males occupy the female reproductive tract, such competition exerts a selection pressure for the male to produce more ejaculate and hence larger testes are required to make the increased sperm required.
Preston et al., (2002) sheep, a species characterised by male competition studied Soay and female promiscuity.
Sperm competition is intense and males have testes 4 times larger than predicted for their body mass.
Testicular size was positively associated with copulatory activity and subsequent paternity success.
Penis and Testicle Size in Primates.
Males who dominate a harem of females have small testes (gorillas).
Polygamous males (chimp’s) actively compete for fertile females, and have larger testes ( Short & Balaban, 1994 ). On the basis of testes size Short (1979) argued that ancestral humans were a polygamous primate who tended towards serial monogamy.
From Cartwright, (2000), p 223
b) Ejaculate Content.
Baker & Bellis (1995) analysed ejaculate content and flowback following sexual intercourse in 35 couples who had been separated for variable amounts of time.
Sperm count rose as a function of time the couples had been apart since their last sexual encounter.
Males separated from their partners ejaculated nearly twice as much sperm than males who spent all their time with their partner.
Males separated from their partner ejaculated a much higher quantity of 'blocker sperm' which are infertile and act to block rival sperm from another male.
Ejaculate Amount and Partner Separation.
From Barrett et al., (2002), p 115
Adaptations of the Human Penis.
Gallup et al., (2003) argue that the coronal ridge evolved to displace semen from the female reproductive tract, deposited by other males.
Displacement of artificial semen in simulated vaginas using a variety of penis models showed that those with a coronal ridge were indeed more successful at semen displacement.
In surveys of college students, they reported that males separated from their partners, or following suspicions of infidelity, engaged in more vigorous thrusting during sexual intercourse.
Coronal ridge
c) Psychological Adaptations to Sperm Competition.
Shackelford et al., (2002) responses to the risk of sperm competition in 194 males in committed sexual relationships.
investigated psychological Males who spent more time away from their partners found their partners more attractive, and showed greater interest in having sex with them.
Such feelings are psychological adaptations to the problem of sperm competition.
A male equipped with such psychological mechanisms would have reduced the possibility of being cuckolded.
d) Desire for Multiple Partners.
Buss & Schmitt (1993) sexual partners they would like to have within certain time periods.
asked males and females how many From Buss & Schmitt (1993), p211.
e) Rapidity of Intercourse.
The less time that elapses between meeting a new female and intercourse is a clear benefit to a male.
Buss & Schmitt (1993) asked participants to rate how likely they would consent to sex with an attractive member of the opposite sex depending on different time criteria.
At almost every interval males were much more likely to consent to sexual intercourse.
Clarke & Hatfield (1989) hired a male and female to approach members of the opposite sex, to say that they found the person attractive and ask them to have sex.
Around 75% of males approached by the female agreed to have sex, but not one of the women approached by a male consented.
Likelihood of Intercourse as a Function of Time Known.
From Buss & Schmitt (1993), p211.
f) Lowering of Standards.
While both sexes have fairly high standards for a potential long-term mating partner, standards for a casual sex partner are dramatically reduced in males.
Buss & Schmitt (1993) found that: Females tended to maintain their standards across all conditions.
Males were much more likely to accept minimum criteria in a casual sex partner.
The only criteria males maintained were in relation to negative female characteristics such as 'low sex drive', 'prudishness', and 'need for commitment'.
Characteristics such as 'highly promiscuous' were much valued by males in a potential casual sex partner, but not so for a potential long-term partner .
Preferred Intelligence In a Mate.
From Kenrick (1994) p 97
The ‘Closing Time Phenomenon’.
Pennebaker et al., (1979) interviewed males and females in a bar at 9pm, 10.30pm and midnight.
Ratings of the opposite sex increased over time while ratings of the same sex did not alter.
Madey et al., (1996) relationship status.
replicated the study but controlled for Males and females rated opposite and same sex patrons in a bar at 10pm, 12pm and 1.30am.
Ratings of the opposite sex did increase over time but only in participants not currently in a relationship.
Alcohol consumption may be important here.
(2003) Jones et al showed that attractiveness ratings of the opposite sex were positively related to the amount of alcohol consumed.
Closing Time Phenomenon.
From Barrett et al., (2002), p 123.
g) Sexual Fantasies/Behaviours.
Ellis & Symons (1990) asked 307 students to complete a questionnaire on sexual fantasy and arousal. They found: Males reported having sexual fantasies at least once per day and of becoming sexually aroused at least once a day.
Male sexual fantasies involved multiple partners, strangers, and no emotional commitment.
Female fantasies generally involved a single partner whom they were currently or had been romantically/sexually involved.
Male fantasies were highly visual and always included genitals/breasts; female ones involved more personal characteristics, and had an emotional context.
Male fantasies rapidly moved to an explicit sexual encounter whereas female ones built slowly and did not always involve explicit genital sexual activity .
Pornography Versus Romance.
According to Symons (1979) the pornography industry: Is exclusively aimed at males.
Is overwhelmingly visual.
Involves different youthful and attractive partners.
Provides instant gratification without commitment.
Romance novels are aimed directly at women and are about mate selection, emotional commitment, and typically do not contain explicit sex scenes.
Prostitution occurs in all recorded human cultures with males overwhelmingly being the consumers.
Within a relationship males often trade gifts or resources for sexual favours (as do chimpanzees).
Other aspects of the sex industry (e.g strippers and escorts) are almost exclusively male-driven. ( Symons, 1979 ).
h) Extramarital Affairs.
Males pursue extramarital sex more often than their wives.
Surveys typically reveal that women have to very dissatisfied with their marriage before they consider an affair.
Cheating males generally report that they are not all that dissatisfied with their current relationship.
Males often cite sexual variety as the reason, but females cite the desire for more meaningful interpersonal relationships.
2. Adaptive Benefits of Short-
Term Mating in Females.
Females have little to gain (and much to lose) from a short term mating encounter: Rejection by her current mate, which may also entail an increased risk of violence from him.
Loss of resources and protection by her partner.
Gaining a 'bad' social reputation of promiscuity which can affect their desirability as a potential long-term mate.
Increased risk of unplanned pregnancy.
Increased risk of sexually transmitted diseases .
However, for every male who is engaging in a short-term relationship, there must of course be a female who is doing the same.
Short-term relationships must also have some adaptive benefits to females.
a) Female Orgasm and Conception.
An orgasm primarily serves to draw the sperm into the cervical canal and uterus, thus increasing the chance of fertilisation.
Baker (1996) have an orgasm when engaged in an extrapair copulation (EPC).
reported that women are much more likely to When having an affair, women appear to unconsciously time their copulation's to coincide with the phase of maximum fertility, this does not happen in intercourse with their regular partner ( Bellis & Baker, 1990 ).
Fertility and Unfaithfulness.
Most fertile Least fertile From Barrett et al., (2002), p 134
b) Changes in Behaviour.
Grammer et al., (1997) found that women were more likely to visit singles bars and night-clubs without their current partner when in their fertile phase.
Gangstead et al., (2002) reported that women's sexual attraction to, and fantasy about males other than their partner increased when they were in their most fertile phase. Males increased their vigilance behaviours when their partner is in her most fertile phase.
However, such evidence is disputed by (1998) Chavanne & Gallup who asked female college students to complete a risk assessment questionnaire over their menstrual cycle.
A significant decrease in risk-taking behaviours during the most fertile phase in females not taking the contraceptive pill was found.
c) Concealed Ovulation.
In most mammals estrus is highly visible but in human females ovulation is concealed, and so must have some adaptive significance. Two main theories: i) Infanticide prevention : Males who find a single female will kill her offspring to bring her into estrus. By concealing ovulation, male parenthood is uncertain, and so infanticide carries a possible risk for the male.
ii) 'Daddy at home' : Concealed ovulation may divert males from a sexual strategy of low investment polygamy towards high investment monogamy.
For a man to be assured of his parenthood, he needs to remain with his partner when she is most fertile (just before ovulation). If he cannot tell for certain when ovulation is, then he will have to stay with his partner most of the time ( Alexander & Noonan, 1979 ).
Evaluation of the Theories.
Sillen-Tullberg & Møller (1993) argued that concealed ovulation began as a strategy to allow females to mate with several males and avoid the danger of infanticide.
Once concealed ovulation was established, a female could choose a caring male with good access to resources and use concealed ovulation to ensure his continued commitment.
She could also seek matings with other males confident that her partner would raise any subsequent offspring as his own.
This may also explain why human females have permanently enlarged breasts.
If a females' breasts are constantly enlarged, then it is difficult for a male to assess when she has ceased lactation and is ovulating.
d) Genetic Benefits of Mate-Switching.
A short-term sexual relationship may confer direct genetic benefits as it may provide superior genes compared with a female's regular partner.
By having an affair with a very attractive male, a woman might bear a son who shares these attributes and produces more grandchildren ( 'sexy son's hypothesis' ).
A female may also gain improved health for her offspring by mating with a male who possesses a different MHC.
e) Economic Benefits of Mate Switching.
If a woman's regular long-term partner stops bringing in resources (unemployment), or becomes abusive or violent to her and her children, then pursuing short-term mates with the potential of gaining another long-term partner has clear adaptive benefits.
Greiling & Buss affair: analysed perceived benefits from having an Women reported that having an affair made it easier to break up with her current partner.
Having an affair made it more likely that they would find a more desirable partner, particularly in terms of compatibility and resource provision (sexual gratification did not appear to be that important).
Having an abusive partner, or one who had also been unfaithful were key factors in the woman seeking an affair .
Contextual Influences.
There are contextual influences on whether males and females decide to engage in short-term mating strategies.
Cashdan (1993) pointed out that if a female is in an environment where males are few (in prison) or are of obviously poor quality (drug addicts, unemployed) then she may have to form alliances with several males in order to ensure a supply of resources for her children.
To do this she will have to act in a sexually provocative manner, and have several short-term relationships.
Females who are surrounded by plentiful males of good quality can instead emphasis chastity and fidelity and engage in long-term relationships (with perhaps the occasional short-term relationship).