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his effort in chick feeding (compared to control female removals outside the female's
fertile period) (Sheldon et al ., 1997). Thus a female may have to trade-off benefits from
extra-pair fertilizations with loss of help in raising the chicks.
A second hypothesis is that different sires are best for sons and daughters. This could
arise because of sexually antagonistic genes, which have a beneficial effect in one sex
but a harmful effect in the other sex. Imagine, for example, a gene in our hominid
ancestors that caused an increase in hip width. This might be good for daughters (easier
for them to give birth) but bad for sons (reduces locomotion efficiency). Selection for
sex-limited gene expression could result in independent evolution in the two sexes, so
enabling each sex to reach its optimal outcome. Nevertheless, selection experiments
with Drosophila have shown that selection for genes that have beneficial effects in one
sex can lead to reduced fitness in the other sex (Chippindale et al ., 2001). There is also
evidence for sexually antagonistic genetic effects in natural populations. In red deer
Cervus elaphus on the isle of Rum, Scotland, males with high lifetime reproductive
success fathered, on average, daughters with low fitness (Foerster et al ., 2007). In
crickets, breeding experiments reveal that more attractive males in the wild sire higher
quality sons but poorer quality daughters than do less attractive males (Fedorka &
Mousseau, 2004). This could impact on mate choice. A female might temper her extra-
pair matings because her social mate is a better sire for offspring of one sex while an
extra-pair male is a better sire for the other sex.
Different sires
best for sons
versus daughters
Sexual conflict
We now come to the third main theme of this chapter. We began with the origin of
anisogamy as the outcome of a primeval conflict in which microgametes evolved to
exploit the resources of macrogametes. We then showed that the greater parental
investment by females before mating often combines with great investment after mating,
leading on the one hand to intense competition among males for mates, and on the
other to female choice among males for better access to resources or for genetic benefits.
This will often result in sexual conflict, both over the act of mating and over sperm use
after mating (Table 7.4). Sexual conflict occurs whenever the optimal outcome is
different for the male and female. In theory, it should lead to each sex evolving
adaptations that bias the outcome towards its own interests, leading to sexually
antagonistic coevolution between traits in males and females (Parker, 1979; Chapman
et al ., 2003). We now consider the evidence for such sexual conflict: firstly over mating
itself, and secondly concerning fertilization of ova after mating. In the next chapter we
explore sexual conflict concerning parental investment.
Sexual conflict over mating
We have seen that a male's reproductive success is often more limited by access to mates
than is a female's reproductive success. Thus, for a given encounter it will often pay a
male to mate but a female to resist. An extreme manifestation of this conflict is enforced
copulation, as exemplified by scorpion flies ( Panorpa spp.). Male scorpion flies usually
acquire a mate by presenting her with a nuptial gift in the form of a special salivary
Sexually
antagonistic
coevolution
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