Biology Reference
In-Depth Information
Male traits
Female traits
Table 7.4 Sexual
conflict: summary
of some male and
female adaptations
and counter-
adaptations
Enforced copulation.
Resistance.
Intromittant organs which enhance mating
success.
Elaborate reproductive tracts which
pose obstacle courses for sperm.
Mate guarding, frequent copulation, strategic
allocation of sperm.
Seek extra-pair copulations.
Remove or displace sperm of rival males.
Sperm ejection.
Copulatory plugs and anti-aphrodisiacs to
discourage matings with other males.
Sperm choice.
Accessory gland proteins to manipulate female.
Chemical defence.
secretion or a dead insect (this is very similar to the Hylobittacus described earlier). The
female feeds on the gift during copulation and turns the food into eggs. However,
sometimes a male enforces copulation: he grasps her with a special abdominal organ
(the notal organ) without offering a gift (Thornhill, 1980). Enforced copulation appears
to be a case of sexual conflict. The female loses because she obtains no food for her eggs
and has to search for food herself, while the male benefits because he avoids the risky
business of finding a nuptial gift. Scorpion flies feed on insects in spiders' webs and quite
often get caught up in the web themselves, so foraging is certainly risky (65% of adults
die this way). Why do not all males enforce copulations? The exact balance of costs and
benefits is not known, but it appears that it results in a very low success rate in fertilizing
females, so perhaps males adopt this strategy only when they cannot find prey or make
enough saliva to attract a female.
Studies of water striders (Gerridae) by Goran Arnqvist and Locke Rowe (2002a,
2002b) provide particularly good evidence for how conflicts over mating lead to
antagonistic coevolution between the two sexes. These insects can often be seen skating
over the surface of ponds and streams in search of food or mates. A male pounces on
top of a female and tries to secure a mating by grasping her. However, superfluous
matings are costly for females; with a male on her back, the female has reduced
mobility, which reduces feeding success and increases predation, so females who have
already mated struggle to avoid males. Comparison across species shows correlated
evolution of male morphology to increase grasping (elongation of grasping genitalia)
and female morphology to resist (elongation of abdominal spines, which help thwart
the male) (Fig. 7.18). Thus, adaptations in one sex are matched by counter-adaptations
in the other sex. Differences in the opportunity for matings and the costs of armaments
may explain why different species are at different equilibrium armament levels.
In other cases the sexual conflict goes in the other direction: it pays a female to gain
extra-pair matings but it pays her social mate to prevent her from doing so, to guard his
paternity. In many socially monogamous birds, the social male follows the female closely
throughout the week or so that she is fertile and chases off any intruding males. For her
Water striders:
coevolution of
male grasping
and female
resistance
Mate guarding
and frequent
copulation for
paternity
assurance
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