2nd half Lecture 6
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Slide 1 Male Battles and the Evolution of Sneaky Sex:
Assessing the social environment & Phenotypic Plasticity Selective maintenance of alternative
Beetles are 22% of multicellular organisms!
Male Battles and the Evolution of Sneaky Sex
Scarabs roll dung into sphere to lay their eggs into it!
Onthophagus acuminatus -> most species use dung.
• Central American, tropical rain forest species that uses monkey dung
• Beetles burrow under dung pat rather than rolling dung
• Dung varies (seasonally/daily/hourly) in amount -> can’t predict.
• Dung varies seasonally in quality. When fruits are available to monkeys, dung is high quality.
However, when only leaves are available, dung is of low quality
• Two male types: if larger than most rivals, guard female; if smaller than most rivals, sneak
Dung beetle biology
There is only a main burrow entrance that the resident male guards.
But, sneakers can enter by digging a side burrow
Should we expect structural differences between the two male types?
Weaponry associated with male size: males either have large horn or tiny horn.
Males beetle larvae monitor their social environment while they are still in the dung to choose
whether to grow big or small horns depending on size of their rivals! Assess environment by
chewing on the dung! Small beetles will be sneakers and they develop because they don’t have
nutritional food to develop. Large beetles will defend females. Maintain the lesser fit phenotype
because making the best of a bad situation.
How are two morphs maintained in species with variable ways of obtaining mates?
Other examples of polymorphism in males include:
• throat colour variation in side-blotched lizards
• “jacks” (small) vs big males in Pacific salmon
• Parental vs female mimics vs satellite males in bluegill sunfish
• Shuster’s marine isopods
A pattern: There are two “horn morphs”
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