Biology
Adaptation unto infidelity
Natural Selection
Traits that confer a reproductive advantage will increase in frequency
What about traits that directly concern reproduction?
(e.g. mating opportunities)
- Maternity is always certain, but paternity is not
- Because of female multiple mating, males have to contend with uncertain paternity (i.e., being cuckolded)
- Profound selection on male adaptations to maximize the chance of paternity from matings
But it doesn’t end there…
…not just mating, but also paternity
*
Natural selection is about reproduction!
Sexual selection is a special case of natural selection
Maternity vs Paternity:
Due to multiple mating, just because you mated doesn’t mean the baby is yours
Why commit infidelity?
Direct benefits
(resources)
Indirect benefits
(good genes)
Human female adaptations for infidelity
- Most likely to commit infidelity when ovulating
- Attracted masculine features
- Less likely to use contraception when committing infidelity
- Proportion of males and females in committed relationships who report experiencing more intense orgasms* as a result of having sex with someone else:
Males 15.4%
Females 47.9%
*increases the likelihood of conception
*
Greater than zero
Infidelity certainly happens
*
Counter-adaptations
Males are good at guessing, because they should be
*
Has Competition for Paternity Affected Genital Morphology?
*
- Reproductive traits do not evolve randomly
- If there are differences between species, they are likely the product of selection
*
Comparative Reproductive Features of Male Great Apes
Body Wt. Testes Wt. Sperm Ct. Penis L.
Gorilla 169 kg 29.6 g 50 x 107 3 cm
Orangutan 75 kg 35.3 g 70 x 107 4 cm
Chimpanzee 44 kg 118.8 g 600 x 107 8 cm
Human 66 kg 40.5 g 250 x 107 13 cm*
Does morphology infer mechanisms to prevent cuckoldry?
*Only ape with coronal ridge
*
Is the human male genitalia shaped to displace rival male’s semen from the reproductive tract?
Artificial Genitals
*
Semen displacement trial
*
Semen displacement as a function of phallus type
Coronal ridge
Coronal ridge
As control, later removed coronal ridge of D and functioned the same as C
*
Semen displacement as a function of depth of thrusting
r = .927, p<.0001
*
Behavioral Implications
Does the prospect of female infidelity impact the way men use their penis?
- Allegations of female infidelity
- Periods of separation
*
Depth of thrusting following allegations of female infidelity (N=336)
*
Speed of thrusting following allegations of female infidelity
*
Depth of thrusting following periods of separation (N=286)
*
Speed of thrusting following periods of separation
*
Objections to the Semen Displacement Hypothesis
- What about the prospect of self-semen displacement?
*
- Penis hypersensitivity
- Loss of erection
- Refractory period
- Diminished depth and vigor of thrusting?
Adaptations to Self-Semen Displacement: Post-Ejaculatory Changes
*
Grace was impressed that a rooster could mate all day long.
Told the farmer to tell her husband that…
But is it always with the same hen?
Ratings of Post-Ejaculatory Thrusting
*
Further Objections to the Semen Displacement Hypothesis
What is the likelihood of finding semen from two or more males in a woman’s reproductive tract?
Were there recurrent situations during human evolution where females had sex with multiple males in sufficiently close temporal proximity to one another to make semen displacement adaptive?
*
Contemporary Evidence for Double Mating
Survey of SUNY at Albany and Oswego Coeds (N = 335)
Proportion of females reporting heterosexual intercourse:
87.5%
Number of reported sex partners:
Mean = 5.47, Standard Deviation = 5.99
Proportion of females reporting one or more extra-pair copulations (infidelity) while being in a committed relationship:
26.1%
Proportion of females who reported having sex with more that one male within a 24 hour period:
13. 4%
Proportion of females reporting multiple concurrent male sexual partners:
Threesomes 8.3%
Group sex 2.9%
*
Heteropaternal Superfecundation:
Heteropaternal Twins
*
528.bin
529.bin
530.bin
Allegations of infidelity
- Predict sex differences in the proclivity of engaging in sexual intercourse following allegations of female infidelity
- If female infidelity is an attempt to cuckold her current partner, what differences should be present?
- Fertilization is most likely within 48hrs of mating
*
The Intra-Pair Copulation Proclivity Model of Female Infidelity
*
“Following an instance of infidelity, how long do you wait to have sex with your committed partner?”
*
531.bin
I love someone else OR I had sex with someone else
What about adaptations to semen?
Spurt order effects
The fact that depression scores among females who were not having sex did not differ from those who were using condoms demonstrates that it is not sexual activity per se that antagonizes depression.
In those that do not use condoms, breakups are more devastating and sexual activity with another mate begins sooner.
What mechanisms exist post birth?
Detect paternity and invest accordingly
Domestic Violence
“In several countries, stepparents beat very young children to death at per capita rates that are more than 100 times higher than the corresponding rates for genetic parents”
Daly, M., Wilson, M. (1994). The “Cinderella effect”: Elevated mistreatment of stepchildren in comparison to those living with genetic parents.
0
10
20
30
40
50
60
70
80
90
100
IncreaseNo ChangeDecrease
Depth and Vigor of Thrusting
Percent
Male
Female
P1: GVH/hgi P2: HAA
Archives of Sexual Behavior PP457-371756 April 10, 2002 12:51 Style file version July 26, 1999
Archives of Sexual Behavior, Vol. 31, No. 3, June 2002, pp. 289–293 ( C⃝ 2002)
Does Semen Have Antidepressant Properties?
Gordon G. Gallup, Jr., Ph.D.,1,2 Rebecca L. Burch, B.S.,1 and Steven M. Platek, B.A.1
Received December 4, 2000; revision received April 20, 2001; accepted July 3, 2001
In a sample of sexually active college females, condom use, as an indirect measure of the presence of semeninthereproductive tract,wasrelatedtoscoresontheBeckDepressionInventory.Notonlywere females who were having sex without condoms less depressed, but depressive symptoms and suicide attempts among females who used condoms were proportional to the consistency of condom use. For females who did not use condoms, depression scores went up as the amount of time since their last sexual encounter increased. These data are consistent with the possibility that semen may antagonize depressive symptoms and evidence which shows that the vagina absorbs a number of components of semen that can be detected in the bloodstream within a few hours of administration.
KEY WORDS: semen; depressive symptoms; vaginal absorption; condom use; sexual activity.
INTRODUCTION
When it comes to mental and emotional problems, one of the most consistent sex differences involves the prevalence of depression. Females are more prone to de- velopdepressivedisordersthanmales(Shanfield&Swain, 1984; Thomas & Striegel, 1994; Vance, Boyle, Najman, & Thearle, 1995). The incidence of clinical depression in females exceeds that shown by males by a factor of three tofive times (Culbertson,1997). In females, depression is often associated with different reproductive outcomes such as death of a child, miscarriage, and menopause (Suarez & Gallup, 1985).
In reviewing the literature on vaginal absorption of seminal products, Ney (1986) hypothesized that semen may have an effect on mood in women. Hormones in seminal plasma include testosterone, estrogen, follicle stimulating hormone and luteinizing hormone, prolactin, and a number of different prostaglandins. Many of the compounds present in human semen can be absorbed through the vagina (Benziger & Edleson, 1983). Both testosteroneandestrogenareabsorbedthroughvaginalep- ithelium(Rigg,Milanes,Villanueva,&Yen,1977;Schiff,
1Department of Psychology, State University of New York at Albany, Albany, New York. 2To whom correspondence should be addressed at Department of Psychology,StateUniversityofNewYorkatAlbany,1400Washington Avenue Albany, New York 12222; e-mail: gallup@csc.albany.edu.
Tulchinsky, & Ryan, 1977; Wester, Noonan, & Maibach, 1980).Although little researchhasbeenconductedon the vaginal absorption of prolactin, the absorption and subse- quent rise in estrogen levels triggers an increase in pro- lactinaswell (Keller,Riedmann,Fischer,&Gerber,1981; Yamazaki, 1984). Some prostaglandins have been shown to be absorbed rapidly from the vagina, namely E1, E2, and F2 α (Eliasson & Posse, 1965; Sandberg, Ingelman- Sundbery, Ryden, & Joelsson, 1968) and testosterone is absorbed more quickly through the vagina than through the skin (Wester et al., 1980).
To test Ney’s hypothesis, we measured depressive symptoms in college females as a function of sexual ac- tivity and condom use. Consistency of condom use was used to index the presence of semen in the female repro- ductive tract.
METHODS
Participants were recruited as volunteers from upper division undergraduate courses at the State University of NewYorkatAlbany.Thestudywasapprovedby the local institutional review board and subject participation was strictly optional. A sample of 293 college females agreed tofilloutananonymous,writtenquestionnairedesignedto measure various aspects of their sexual behavior, includ- ing frequencyof sexual intercourse, numberofdays since their last sexual encounter, and types of contraceptives
289 0004-0002/02/0600-0289/0 C⃝ 2002 Plenum Publishing Corporation