Biology Reference
In-Depth Information
successful reproduction (breeding sites, parental care, social rank) rather than for male
sperm. For males, whose reproductive success is most often limited by mates, the
competition will usually be for access to females, either directly (through force or charm)
or indirectly (by monopolizing resources that females need).
Sperm competition
The process of sexual selection as envisaged by Darwin is only half the story. A century
after Darwin first proposed the theory Geoff Parker (1970c) came up with a novel insight.
He had been observing dung flies (
Scatophaga stercoraria
) competing for mates around
cowpats. He noted firstly that a female often mated with more than one male, and secondly
that females stored sperm in organs called spermathecae. He then realized that sexual
selection must continue after the act of mating, as the ejaculates from different males
compete for fertilizations inside the female tract. He called this process 'sperm competition'.
This is now recognized to be a powerful selection pressure on reproductive behaviour
throughout the animal kingdom (Birkhead & Moller, 1998; Parker, 2006). Just as with
sexual selection before mating, there is the potential for two processes, namely competition
between sperm from rival males (analogous to male combat) and female sperm choice
(often called 'cryptic female choice', analogous to female mate choice; Table 7.2). And just
as with Darwin's theory, the evidence for the first process was readily accepted whereas it
is only recently that the importance of female sperm choice has been recognized.
Geoff Parker
shows that sexual
selection
continues after
mating
Why do females copulate with more than one male?
As we saw with Bateman's classic experiments (Fig. 7.3), the advantage to a male from
copulating with several females is clear - he fathers more offspring. But why should a
female copulate with more than one male, especially as in many species (like Bateman's
Drosophila
) a single insemination provides sufficient sperm for a female to fertilize all her
ova? We shall consider three hypotheses.
Costs of resistance exceed the costs of acquiescence
In some cases it may pay a female to accept an extra mating even though she might gain
no benefit. For example, a female dung fly has to go to a cowpat in order to lay her eggs.
Here she meets an army of eager males who attempt to grab her (Chapter 5; Fig. 5.3).
After one male has mated, another may displace him and mate too. In some cases the
Three hypotheses
Two processes
Table 7.2
Sexual
selection operates
both before and
after mating
Sexual selection
Male-male competition
Female choice
Before copulation (Darwin, 1871)
Between rival males
Of mates
After copulation (Parker, 1970c)
Between rival sperm
Of sperm
