Sexually antagonistic coevolution (or SAC) is the biological
phenomenon in which one
sex of a species first develops an antagonistic adaptation toward the
other, to which the latter sex responds by evolving a resistant
counter-adaptation. If the two sexes continue evolving and
compensating for one another, the species is said to be
locked in an
evolutionary arms race.
As there can be rapid developments over the course of generations,
particularly
in the sexual functions of a species, there are a number of
circumstances which
can induce this coevolution. Male seed beetles, for
example, have been found to
possess spiny genitalia, a sexual
characteristic that is harmful to the female.
The spines, by helping the male to more firmly anchor itself within
the female,
allow the male to transfer more sperm. This initial difference
between
reproductive priorities is known as a sexual conflict; though the
male's spiny
genitalia causes injury and a loss of fitness in the female, it
provides the
male with a competitive edge over its rivals, and so
optimizes the male's
potential for fathering offspring. However, despite the potential
benefit of
the male's adaptation, were the female's genes not to competitively
coevolve,
the consequence would be a decrease in female fitness, possibly endangering the survival of the species. And so, as a responsive
counter-adaptation, the female
seed beetle develops a thicker tissue along her reproductive tract;
the
predictable result being that as the male and female genitalia
competitively
evolve, neither sex loses substantial fitness, and the species
population is
maintained.
The duck is another animal recently associated with SAC, though
the
circumstances differ substantially from those of the seed beetle.
Because 97% of all birds species lack a phallus at all, the duck was already considered
sexually
unusual by the scientific community; and scientific curiosity was only
compounded further by the considerable lengths these phalluses could
reach—the
largest known phallus (relative to size) belonged to an Argentine Lake Duck,
which at 17 inches was twice the size of its body. The initial
explanation was
similar to the seed beetle, that the duck's large phallus was a
competitive
evolutionary response to other male rivals. That was until recently;
new
research into the size and shape of female duck genitalia suggests
that the
duck's phallus evolved not as an adaptation but as a
counter-adaptation.
Similar in shape to the long, corkscrew phallus, the female duck's
oviduct was
discovered to be a length of spirals and dead ends. Additionally, by
examining
different duck species, a precise correlation was found between the
lengths of
the male and female genitalia. Dr. Patricia Brennen, the ecologist
responsible
for the discovery, suggests the theory that male sexual aggression
caused the
female oviduct's elaborate development. Male ducks are known to rape
females,
sometimes injuring or even killing them in the process. Therefore, the
complexity and length of the oviduct, Brennen reasons, is most likely
an
anatomical check against undesirable mates; the
dead ends and pockets of the
oviduct potentially allow the female duck to collect and then eject
unwanted
sperm. Brennen supported her theory by pointing out previous studies
which
claim that while 1/3rd of duck matings are forced, those encounters
only
account for 3% of total duck offspring. The relative failure of forced
mating
suggests that the
evolution of the oviduct necessitated a counter-development
in the entire male population for mating, forced or otherwise.
And then we have the African bat bug. Like their relatives the
bed bugs, male bat
bugs are known for their sharp, hypodermic genitalia. Ignoring the
female's reproductive tract entirely, the male bat bug instead pierces directly into her
abdomen, a process called traumatic insemination. Though the sperm
continues to
find its way to the ovaries, the damage to the female bat bug is
considerable.
And so, as an evolutionary response, the female bat bugs have developed
paragenitalia (known as the "spermalege") on their abdomen. This
paragenitalia prevents injury by
guiding the male organ into reservoirs of spongy tissue. However,
while this is strange, researchers were more surprised to find the
effect it
had on the male bat bug population. Because male bat bugs would
commonly pierce
the abdomens of other males as well, male bat bugs began adapting after
the
example set by the female bat bugs, developing
similar paragenital defenses. And stranger still, in some populations,
the female bat bugs were observed to have mimicked the male
paragenitalia, resulting in greater efficiency and less stabbings.
Sources
- "Evolutionary arms race" Wikipedia
- "Sexual conflict" Wikipedia
- "Coevolution between harmful male genitalia and female resistance in seed beetles" by Johanna Rönn, Mari Katvala & Göran Arnqvist, Proceedings of the National Academy of Sciences (May 14th, 2007)
- "In Ducks, War of the Sexes Plays Out in the Evolution of Genitalia" by Carl Zimmer, the New York Times (May 1st, 2007)
- "The Largest Penis in the World" by Stefan Anitei, Softpedia (January 5th, 2007)
- "Bat Bugs Evolved Fake Genitals to Avoid Sex Injuries" by Anne Minard, National Geographic News (September 25th, 2007)
- "The Most Bizarre Sex: Abdominal Stabbing Penetration, Sperm Through Blood, Defensive Para-Genitalia and Female-to-Male Transsexuality" by Stefan Anitei, Softpedia (September 22nd, 2007)
- "Traumatic insemination in the plant bug genus Coridromius Signoret (Heteroptera: Miridae)" by Nikolai J Tatarnic, Gerasimos Cassis & Dieter F Hochuli, Biology Letters (November 2nd, 2005)
- "Afrocimex constrictus" Wikipedia