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Lecture 22

PSYC 3100 Lecture Notes - Lecture 22: Sperm Competition, Parental Investment, Sexual Selection

Course Code
PSYC 3100
Pat Barclay

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Evolutionary Psychology Lecture 22 Between-Sex Conflict
Sperm Competition
Sexual selection and competition for mates does not always end at mating
If multiple males’ mate with a female, only one sperm will fertilize each egg
Sperm competition increasing the likelihood that one’s sperm will fertilize the eggs
o Number of ways to do this: put out more sperm, invest more time in sperm
Investment in sperm
o Nontrivial costs of sperm production including “depletion”
if you use all your sperm on one person, you won’t have enough for the
next person that comes around
o Across spices: rates of polyandry (risk of sperm competition) correlate with
testes size (relative to body size)
larger testes can store more sperm
in species low in polyandry, testes are smaller because you don’t need a
lot of sperm for multiple females
Male responses to cues of sperm competitions
o Preventative tactics: mate guarding & coercion
if there is evidence that a female as mating a male, the male with
compete with another male’s sperm- ejaculating more sperm somehow
into her, trying to get her pregnant by him, than other guy
o Sperm competition tactics: mate more often release more sperm per mating,
remove competitors’ sperm
some species have hooks on the end of their penises so remove another
male’s sperm from the female’s tract
o Differential parental investment: reduce investment in the presence of cues of
Sperm competition and arousal
o Hypothesis: cues of sperm competition leads to higher arousal (need for more
sperm relatively soon)
o non-humans: let a male mate with a female after observing her mate with
another male, measure sperm
o humans: measure sperm responses to images involving sperm competition
males will produce more sperm if there is a picture if there are two guys
and a girl in the image than an image of two girls- example of sperm
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