Human The Mating System
Introduction and Male Strategies
What is a Mating System?
A mating system is the species-specific way in which matings are allocated among the members of a population Mating: the process leading to the fusion of haploid gametes (eggs and sperm) to form a new genetically unique zygote Zygote: a fertilized egg
Types of Mating Systems
Random Monogamy Polygyny: variance in male reproductive success > variance in female reproductive success Polyandry: variance in female reproductive success > variance in male reproductive success
What Factors Influence Mating System Evolution?
Mode of fertilization (internal vs. external: if internal, can the female store sperm?) Offspring ecology (how much parental care is required) Degree of asymmetry in parental investment in offspring
Mammal Reproductive Characteristics
Internal fertilization, with an intromittent organ, the penis No sperm storage Offspring have huge energy demands Asymmetry in parental investment is extreme: females provide essentially all of the energy (gestation and then lactation) to raise a zygote to independence
Mammal Mating Systems
Monogamy is rare and is scattered across a wide variety of mammal groups. It occurs in some primates (e.g., gibbons) In most mammals, the mating system is polygyny
Primate Mating Systems
Monogamy rare occurs in some newworld primates and in gibbons Polygyny common- occurs in most monkeys and in orangutans, gorillas, chimpanzees, bonobos
Why are Most Mammals Polygynous?
Precise male control of sperm transfer Huge energy cost of offspring Extreme asymmetry in parental investment in offspring females provide everything, males provide a sperm cell Especially in mammals, females are a reproductive resource over which males compete
Sexual Selection
A term coined by Charles Darwin Darwin viewed sexual selection as a force that could work in opposition to natural selection Sexual selection operates solely by differences among individuals in their ability to acquire mates Sexual selection is thus a strong force when the mating system is polygyny
How do differences in mating success among males arise?
Male male competition Female choice
Evolutionary Results of Male-Male Competition for Mates
Sexual size dimorphism Male weapons Fighting Male delayed maturation Increased male mortality rates Sperm competition Forced copulation Infant killing by males Behavioral emphasis on copulation
Sexual Size Dimorphism
In mammals, the degree of sexual dimorphism is correlated with the degree of polygyny
Variance in male RS/variance in female RS
Male Mass/Female Mass
Weapons
Male baboon
Female baboon
Fighting
Triangular relationships: 2 men compete over one woman
Human Sex-Specific Survivorship
100000
females males
Number Surviving
80000
60000
USA 1994
40000
20000
0 0 20 40 60 80 100 120
Age
Sperm Competition
Bellis, M. & Baker, R. 1995. Human sperm competition: copulation, masturbation and infidelity. London: Chapman & Hall
Study Couples used condoms each time they mated Immediately after mating, contents of the condom were placed in fixative, then delivered to the lab by the next day In lab, measure ejaculate volume and number of sperm ejaculated With each sample, the couple reported:
Time since previous mating Percent time apart during that interval
Bellis, M. & Baker, R. 1995. Prediction
The number of sperm ejaculated should vary with likelihood of sperm competition The number of sperm ejaculated should not vary with time since the previous mating
Bellis, & M. Baker, R. 1995. Results
Number of sperm ejaculated was independent of ejaculate volume Time since the last ejaculation was a weak predictor of number of sperm ejaculated Number of sperm ejaculated varied positively with partner body mass Number of sperm ejaculated varied inversely with percent time together since the last ejaculation
Bellis, M. & Baker, R. 1995
Infant Killing
Now clearly shown to be a male adaptation in several dozen species Occurs when a new male or a coalition of males takes over a female group By killing infants, new males are: Removing the offspring of competitor males Accelerating the return of the mother of the slain infant to estrus
Infant Killing
Infant killing in the adaptive pattern is wellknown in gorillas and in chimpanzees If it occurs in humans, it should happen when a mother and her young child live with a male who is not the child's sire.
Infant Killing
Daly & Wilson, 1988: Homicide
Behavioral Emphasis on Copulation
Buss, D. 1994. the evolution of desire. New York: Basic Books Anonymous questionnaires at elite American universities Sought to discover whether men and women have different attitudes about copulation
Buss,1994
How Strongly are you seeking a spouse? Women: Fairly Strongly Men: Fairly Strongly How Strongly are you seeking a one-night stand? Women: Not Strongly Men: Pretty Strongly
Buss, 1994, continued
How many sex partners do you want in the next month? Women: 0.8 Men: 2.0 How many sex partners do you want in the next two years? Women: 1.0 Men: 8.0 How many sex partners do you want in your lifetime? Women: 4-5 Men: 18
Buss, 1994, continued
Would you consider having sex with a desirable partner you have known for: 5 Years: Women: Probably yes Men: Yes 2 Years: Women: Probably yes Men: Yes 6 Months: Women Neutral Men: Yes 1 Week: Women: Definitely Not Men: Yes Less than 1 Week: Men: Yes
Buss, 1994
Question How strongly are you seeking a spouse? How strongly are you seeking a one-night stand? How many sex partners do you want in: The next month The next 2 years Your lifetime
Women Fairly strongly Not strongly
Men Fairly strongly Pretty ...