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quality. However, adaptations will be influenced by costs as well as benefits: surely
once sperm are inside the female's body, she will have better control over the outcome?
Bill Eberhard (1996), in particular, championed the potential for female control and
the possibility of female sperm choice (cryptic female choice). There is now growing
evidence for this view.
A dramatic example of female control is provided by feral fowl Gallus gallus . Females
prefer to copulate with dominant males. However, subordinate males can sometimes
force matings despite female resistance. In these cases, the female retaliates immediately
after mating by cloacal contractions which eject the subordinate male's sperm (Pizzari
& Birkhead, 2000). If more than one male succeeds in inseminating a female, could she
then later bias fertilization success by sperm choice? Experiments with Drosophila have
shown that the relative success of two males in sperm competition depends not only on
male genotype but also on female genotype (Clark et al ., 1999). This shows that females
are not simply passive providers of a reproductive tract for male-male sperm competition,
but rather have some control over the outcome.
Recent experiments by Tom Tregenza, Nina Wedell and their colleagues have provide
convincing evidence for female sperm choice after mating. They studied field crickets
( Gryllus bimaculatus ), where females readily mate with more than one male (relatives or
non-relatives) both in the field and in the laboratory. In laboratory experiments, females
were mated successively with two males: two of the female's siblings, or two non-
siblings, or a sibling and a non-sibling (the order was varied). Male crickets transfer a
spermatophore to the female during mating and they prepare this in readiness before
they encounter a female, so siblings and non-siblings transferred equal numbers of
sperm. There was also no difference between treatments in the numbers of eggs laid.
However, females mated with two of her siblings had decreased hatching success
compared to those mated with two non-siblings (Fig. 7.22), which is the typical outcome
of inbreeding depression arising from homozygosity for deleterious recessive alleles. If
sperm from the two males mixed inside the female reproductive tract, then in the sibling
plus non-sibling treatment we would expect, on average, half of the eggs to be fertilized
by the sibling, so these females should have had an intermediate reproductive success.
However, they did just as well as those mated to two non-siblings (Fig. 7.22). This
suggests that a female can bias fertilization in favour of the ejaculate of unrelated males.
By using DNA markers to identify sperm from different males, it was shown that a
female preferentially stored sperm from the unrelated males inside her spermathecae,
and it was this biased storage that enabled females in the sibling and non-sibling
treatment to avoid the costs of inbreeding (Bretman et al ., 2009).
A key question for future research is whether male versus female control of fertilization
varies between species and, if so, why? In general, we might expect most female control
post-copulation in species where females have least control over which males mate.
Cryptic female
choice
Females can
control sperm
storage
Sperm choice in
female field
crickets
Sexual conflict: who wins?
If each sex evolves suites of sexually antagonistic adaptations that bias the outcome
towards their own interests, who will win the conflict? Theoretical models suggest that
the outcome is often a never-ending evolutionary chase leading to rapid evolutionary
change by both parties (Parker, 1979; Chapman et al ., 2003). For example, male
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