Jumping Spider Dances: Different
potential reproductive rates lead to different levels of sexual activity
By definition, having lots of offspring = "success" or fitness
in the natural-selection sense.
The male can easily make thousands of sperm. Since sperm are cheap/easy
to make, males will maximize their fitness by trying to have as many matings
with as many different females as possible. He goes for quantity of matings,
not quality of mates.
However, the female is quite limited in how many eggs she can make, because
each egg depletes her of a significant amount of resources. Mating with
lots of different males will not necessarily increase the females's fitness,
because one mating can provide her with all the sperm she needs to fertilize
her eggs (however, there can be exceptions to this rule).
For the female, the best way to maximize her fitness is to make each
egg count, by selecting the best possible father or fathers for her eggs.
She goes for quality, not quantity.
Relative
fertility as a function of number of mates, for two groups of Drosophila
melanogaster (fruit flies). X-axis is number
of mates, Y-axis is relative fertility. Solid lines are males, dashed
lines are females. From Bateman, 1948.
With each additional mating, the male can father more and more offspring.
But the fitness of the female maxes out at one mating.
Reference: Bateman, A. J. 1948. Intra-sexual selection in
Drosophila. Heredity 2: 349-368.
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