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Till Death Do Us Part Rachel Floersch and Kayla Bilderback
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True Monogamy ● Monogamy will evolve as a strategy
for all members of a community if it’s in the best interest of both males and females
● Monogamy will be stable only when members of loyal partnerships have more surviving children than fickle creatures
● Examples: Black Vultures, Chinstrap Penguins, Kirk’s dik-diks, California Mouse
“True monogamy is rare. So rare that it is the most
deviant behaviors in biology.”
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Good Wife Theory Females cannot raise offspring alone.
“Female plot forced on males” - the females won’t cheat
because they don’t want to lose the help with the kids.
● Not always accurate: ○ sexual fidelity can be in the males interest ○ female’s need for male help does not
necessarily guarantee her fidelity
○ Not all species males help raise kids (dik-diks)
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Example: Fat-Tailed Dwarf Lemur
● Males and females live together in pairs ● Male “baby-sits” while female rests or gets food ● But genetic testing has shown infidelity is common
So let’s look at some more plausible theories!
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Danger Theory of Monogamy ● Females are hard to come by, so the male
needs to take what he can get ● Finding another female might entail a long or
dangerous journey ● “The greater the risk, the greater the incentive to
remain” ● Examples: Mantis Shrimp
○ Shrimp meet as adolescents and make burrow with mucus
○ Adults lose ability to produce mucus, can’t leave even if they wanted to
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Pop-Em-Out Theory of Monogamy
If female has a short brooding time, the female might be ready to breed again before the male would find another mate anyway.
Example: Djungarian Hamster ● Females live far apart from each other ● A single female can produce 18 litters a year
○ 1-9 pups per litter = up to 162 pups a year!
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Sociopath Theory of Monogamy ● Monogamy arises as a side effect of
aggression ● Aggressive towards any animal that is not
their partner/ members of their own sex ● Very Jealous ● Example: Banded Shrimp
○ Live in pairs, they feed each other and stand guard for each other while molting
○ Fight shrimp of own kind to the death
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Mutually Assured Destruction Theory of Monogamy Cheating or desertion by either partner results in total reproductive failure for both.
Example: Hornbills ● All 45 species ● Males and females completely co-
dependent
● Nest hole sealed entrance with hole for female’s beak only, she molts flight feathers
● Male feeds her, and once chicks hatch, he feeds the whole family
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Theory of Genetic Predisposition to Monogamy ● There could be a genetic predisposition ● Hard to test because it’s difficult to
pinpoint (Genes code for a lot) ● Example: Prairie Vole
○ Sex releases vasopressin which alters behavior ■ Makes males more aggressive after sex
“Consummate by
copulating anywhere
from 15- 30 times in 24
hours”
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Humans...
● As a whole, on the monogamous end of the spectrum ● Relatively little physical differences - same size mates usually monogamous ● Medium to small testicles = low to moderate risk of sperm competition
“Perhaps it will turn out, for example, that men with large testicles are prone to
seducing other men’s wives and have difficulty forming lasting bonds whereas men
with small testicles are prone to sexual fidelity and jealousy and turn all lovey-dovey
after sex.”
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Activity: Groups of 3 Good Wife Theory of Monogamy
Danger Theory of Monogamy
Pop-Em-Out Theory of Monogamy
Sociopath Theory of Monogamy
Mutually Assured Destruction Theory of Monogamy
Theory of Genetic Predisposition to Monogamy