WE don't often question why body parts look a certain way.
But the penis is one which people are fascinated by, wondering if there is a reason the tip is mushroom-shaped.
It turns out, there is a theory for why penises have a mushroom-like look.
And scientists from the State University of New York put it to the test in 2003 with fascinating results.
Why is the penis mushroom-shaped?
Some experts believe the penis is shaped the way it is due to the "semen displacement theory".
The theory is quite simple – in order for men to ensure they are the one who carry on their lineage and impregnate a woman, their penis acts as a "tool".
The penis has a ridge in order to scoop out the semen of love rivals from the vagina, or so the theory says.
As the semen from another male is removed, it is replaced with that of a new partner, increasing the chances of him becoming a father if the woman has had sex with multiple partners in a short amount of time.
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Nowadays, having a mushroom-shaped penis doesn't seem entirely necessary.
But from a natural-science perspective, it makes sense, as humans biologically want offspring.
The 'scooping out' mechanism would have been much more useful hundreds of thousands of years ago when humans were considerably more occupied with procreating.
Put to the test
Professor Gordon Gallup led a team at State University of New York to look into this theory, publishing their findings in the Journal of Evolutionary Psychology.
They found the thrust of the penis – and the ridge that separates the head from the shaft – could indeed help clear a woman's vagina and reproductive system of a previous lover's sperm.
They tested their theory in the lab, with latex penises and model vaginas.
Mixing starch and water together to make a solution similar in consistency to semen, they found the ridge around the top of a man's penis could scoop out more than 90 per cent of the "semen" substitute, in just one thrust.
But a fake penis without a ridge only managed to remove 35 per cent of the "semen".
Depth of thrusting was also important for clearing the vagina.
Professor Gallup explained: "We theorise that as a consequence of competition for paternity, human males evolved uniquely configured penises that function to displace semen from the vagina left by other males.
"Thus, the human penis may enable males to substitute their semen for the semen of their competitors."
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With surveys, the team found evidence that couples tend to have more energetic sex if the woman has been suspected of cheating, or if a couple has been apart.
It could, he said, be because the male has a subconscious desire to rid his partner of any trace of another man's semen.
Professor Gallup's colleague, Rebecca Burch added: "A longer penis would not only have been an advantage for leaving semen in a less accessible part of the vagina, but by filling and expanding the vagina it also would aid and abet the displacement of semen left by other males as a means of maximising the likelihood of paternity."
Some believe that the penis may simply have it's shape in order to make sex more pleasurable for women.
And the penis may look more desirable to a woman in its current form, rather than a simple lengthy shape.
What other penis shapes are there?
Theories aside, there are many different kinds of penis shapes, from the pencil, to the sausage and cone shape.
There is no standard shape, and no penis should be considered normal or abnormal.
Size, girth, angle or circumcision are all among the factors that can lead men to have different-looking penises – all of which do the same job.
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