Scientists
believe the shape of the penis may have evolved to help men
remove the semen of love rivals during sex.
Tests
led a team of US researchers, headed by Professor Gordon Gallup,
to conclude that the penis acts as a "semen displacement device"
and its shape has evolved in part to displace another man's
semen.
The team from the State University of New York believe the
thrust of the penis during sex may help to clear a woman's
reproductive system of a previous lover's semen.
They tested their theory in experiments using latex phalluses,
an artificial vagina and a mixture of starch and water.
New Scientist magazine reports they found the coronal ridge
of the penis, found where the glans, or head, meets the shaft,
could scoop out more than 90% of the cornstarch mixture with
just one thrust.
"The
human penis may enable males to substitute their semen for
the semen of their competitors."
Professor Gordon Gallup
A phallus
with no coronal ridge only managed to remove 35%.
They found the depth of thrusting was also important. A three-quarter
thrust was found to clear out less than 40% of the viscous
mixture.
Sex survey
They also say they have evidence, from surveys of students,
to indicate that sex tends to be more energetic if the women
is suspected of cheating, or if the couple has been apart
- suggesting a sub-conscious desire on the part of the man
to rid his partner of any trace of another's semen.
Lead researcher Professor Gordon Gallup told BBC News Online:
"We theorize that as a consequence of competition for paternity,
human males evolved uniquely configured penises that function
to displace semen from the female vagina left by other males.
"Thus, the human penis may enable males to substitute their
semen for the semen of their competitors.
"Not only did we obtain data derived from artificial genital
models that were consistent with this hypothesis, but we also
found that under conditions that raise the threat of female
infidelity, college students reported changes in sexual behavior
that would serve to purge the vagina of foreign semen prior
to ejaculation."
'Far fetched' theory
Mr Derek Machin, a urologist at University Hospital, Aintree,
said the theory seemed "far fetched".
"The research might very well be accurate, but I'm not convinced
that just because the penis does something like this it was
necessarily designed to have that effect."
Dr Colm O'Mahony, chairman of the UK Association for Genito-Urinary
Medicine, said the theory seemed flawed.
"If the man continues to thrust after ejaculation he would
simply be scooping out his own semen.
"Also does the sexual position matter - I imagine gravity
has some role here. Maybe those missionaries knew something
about position after all?"
The research is published in the journal Evolution and Human
Behaviour.
Tests
led a team of US researchers, headed by Professor Gordon Gallup,
to conclude that the penis acts as a "semen displacement device"
and its shape has evolved in part to displace another man's
semen. 
