BIO318Y5 Lecture Notes - Parental Investment

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14 May 2011
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Lecture 4
• The pattern: sexual dimorphism
• Details on the pattern: usually males sport weapons and ornamented bodies (including
genitalia); and these structures vary greatly between closely related species
• A cause: sexual selection as a powerful form of selection from an animals social environment:
competitors and potential mates (alternatives?)
• 3 potential mechanisms of sexual selection:
predator conflict
sexual conflict
female choice
• All 3 concern sexual competition in the broad sense
m-m
m-f
f-f -> rare
Why are Female Battles Rare? (Why Males are more Gorgeous)
Tests Using Insects
Long Tailed Dance Flies: females form swarms shaped like a disk. Females breathe air to inflate
their abdomen to look like they have lots of eggs. Male are smaller than females. Males come
from under the females to select the females who have the most eggs (from the size of their
abdomen) Females eat the insects which the males bring to them. Therefore, females are being
strongly sexually selected on.
Balancing selection:
Sexual selection -> bigger devises.
Natural selection -> smaller devices
But being big is not so anti-survival because although females are more likely to be caught in
spider webs than males, bigger females are also more likely to escape!
Sexual Differences in:
• Ornaments & weapons (usually males)
• Mate choice (usually females)
• Sexual competitiveness (usually males)
• Risky mating behaviour (usually males)
• Mating mistakes (males) (toads, beetles, Sorocow the New Zealand parrot)
• Territorial + aggressive (usually males)
• Provide parental care + choosy (usually females)
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