Biology Reference
In-Depth Information
part, the female may attempt to sneak off to achieve extra-pair matings. In cases where
males cannot guard their mates closely because one partner has to defend the nest site
while the other goes off to forage (many seabirds and birds of prey) the social male
engages in frequent copulations to swamp the sperm of rivals, sometimes copulating
hundreds of times per clutch (Birkhead & Møller, 1992).
Sexual conflict after mating
Male adaptations
Males employ a diverse array of tactics to increase their success in sperm competition.
The invertebrates provide the most extraordinary examples, and we can only give a
taster here (Birkhead & Møller, 1998; Simmons, 2001).
Sperm removal
. In many insects, females store sperm in special sacs called
spermathecae. Jonathan Waage (1979) was the first to show that male damselflies and
dragonflies may remove sperm deposited by rival males before inserting their own. The
penis of male
Calopteryx maculata
has two specialized scoops at the end which are used
to scrape out of the female any sperm left by previous males, before he injects new sperm
into the female's sperm stores (Figs 7.19c and 7.19d). In other species, males use an
inflatable penis with horn-like appendages to pack sperm of previous males into the
corners of the spermatheca, so that their own sperm will have first access to the
fertilization ducts (last in - first out; Figs 7.19a and 7.19b).
Sperm displacement
. In other insects, a male's insemination flushes out inseminations
of previous males. In yellow dung flies
Scatophaga stercoraria
females have three
spermathecae and a single mating fills all three of them. When two males mate in
succession, the second male gains 80% of the paternity (Chapter 3). Leigh Simmons
et al
. (1999a) used amino acids labelled with stable isotopes to identify sperm from two
males inside the female's reproductive tract. They found that the second male's sperm
displaced the first male's sperm out of the spermathecae, but this flushing action was
not 100% effective because it led to some sperm mixing (hence the first male achieved
some success). The two male's share of paternity was then predicted by their relative
sperm proportions in the spermathecae (in other words the result of a 'fair raffle').
Copulatory plugs
. In some invertebrates (especially insects) the male cements up the
female's genital opening after copulation to prevent other males from fertilizing her. The
males of
Moniliformes dubius
, a parasitic acanthocephalan worm in the intestine of rats,
produces a chastity belt of this kind but, in addition to sealing up the female after
copulation, the male sometimes mounts rival males and applies cement to their genital
region to prevent them from mating again (Abele & Gilchrist, 1977). No less remarkable
are the habits of the hemipteran insect
Xylocoris maculipennis
. In normal copulation of
this species the male simply pierces the body wall of the female and injects sperm, which
then swim around inside the female until they encounter and fertilize her eggs. As with
the acanthocephalan worms, males sometimes engage in homosexual 'copulation'.
A male
Xylocoris
may inject his sperm into a rival male. The sperm then swim inside the
body to the victim's testes, where they wait to be passed on to a female next time the
victim mates (Carayon, 1974).
Anti-aphrodisiacs
. Larry Gilbert (1976) noticed that female
Heliconius erato
butterflies
have a peculiar odour after they have mated. He showed experimentally that the scent
Male tricks for
increasing
paternity after
mating
Removing rival
sperm
Reducing the
chance that
females mate
again
