PSYC 2850 - Human Sexuality - Class 2

Course Code
Psychology 2800
Thisis P S Y C2850- Human Sexuality N O T P S Y C2800

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January 26, 2011
Chapter 11
Pg. 358-363
Read for class 4 (next week)
Sex Education and Sexual Attitudes Continued
Sexual selection will be most intense as the potential for differential reproduction
among males increases
In monogamous primates, males reproduce success is relatively similar, so sexual
selection is not intense
In polygynous species, male reproductive success varies greatly, so sexual selection
is intense
Sexually selected traits vary as a function of mating system
Why do males compete?
Why do females choose?
To explain why this sex difference existed, Robert Trivers formulated his “Theory of
Parental Investment
Trivers noted that in “typical” species, female provide the overwhelming bulk of
parental care for offspring
Females are responsible for gestation, parturition, lactation, nurturing, protecting,
and feeding their young
Female lifetime reproductive output relatively low and more or less equivalent
Offspring survival important for females
Male lifetime reproductive success potentially high much higher than that of
Unlike females, the potential exists for enormous variation in reproductive success
among males
Moulay Ishmail “The Blood Thirsty”
888 -1042 children
Largest female documentation is 69 children, peasant in Russia in the 17th
Females represent valuable reproductive “resources” for males
As such, males will compete intra-sexually for female mates, and will copulate in a
relatively indiscriminate manner, whereas, females will carefully select the male with
which they will mate
Parental investment strategies determine mating strategies
In sex role reversed species, the opposite pattern holds true: male invest more in
offspring and are choosy whereas, females compete intra-sexually for males
Example: Jacana
If males and females invest more equally in reproduction sexual selection may not
lead to marked differences between the sexes
What type of males should females choose?
Males that contribute to the female’s reproductive success
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January 26, 2011
Choose males that are willing to offering some sort of parental investment when
Example: Barbary Macaque
What kind of males should females choose if males make no parental investment?
Choose males that are health and have “good genes” (i.e., genes that promote the
reproductive success female and their offspring
How do females assess “good genes” in males?
Females choose males who are perform impressive courtship displays
Example: Satin Bowerbird
Females choose males who are symmetrical
Fluctuating asymmetry: difference in the left and right side of the body that
result from random perturbations during developments
Females choose males who have exaggerated secondary sexual
characteristics (“handicaps”)
Example: iberian deer
Why do females choose these types of males?
When females mate with males that exhibit these characteristics, they
produce healthier offspring. (Iberian deer)
When females mate with males that exhibit these characteristics, they
produce sons who experience increased reproductive success (Bowerbird)
Most elaborate secondary characteristic = most elaborate
Example: peacock, lots of eye spots and symmetrical
Males can be choosy sometimes. Why?
Investing in mating is costly
Investing in offspring is costly
Females sometimes compete intra-sexually for male mates. Why?
Males vary in their quality (example: Moorhens)
Females can also court males
Proceptive behaviours
Actively looking for sex
Receptive behaviours
Example: Chimpanzee
In some species, females mate promiscuously. Why?
Obtain resources from multiple males
Food for sex exchange in chimpanzees
Obtain protection from multiple males (example: Baboons)
Obtain sperm from high quality males (example: Black-capped chickadee)
Infanticide avoidance (example: Hanuman Langurs)
When females are promiscuous, it poses a reproductive problem for males and post-
copulatory competition occurs. Why? So that a male can ensure only his sperm is
deposited with a female’s reproductive tract
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