Penises come in all shapes and sizes – but there’s a surprising reason why the tip is mushroom-shaped.

Scientists say it is an evolutionary design that amounts to infidelity.

Investigating why the human penis looks like this, a team of scientists in New York conducted a series of experiments.

They first speculated that the head of the penis – which is more of a mushroom shape – is actually a “tool” for removing semen from love rivals.

That’s enough to make you squirm, but it makes evolutionary sense.

The men wanted to make sure they were the ones carrying on their bloodline – survival of the fittest, and all.

To ensure that their sperm is the only one inside a woman, the ridge that separates the head from the shaft, called the coronal ridge, can help.

During penetration, it can help remove semen from inside the vagina.

The researchers, led by Professor Gordon Gallup of the State University of New York, put it bluntly.

Their article said: “If a woman copulated with more than one man in a short period of time, this would allow subsequent men to ‘collect’ the sperm left behind by others before ejaculating.

The team put their theory to the test in 2003, using artificial genitalia and fake semen made from a mixture of starch and water.

Indeed, the latex penis and its pad were able to extract more than 90% of the “sperm” substitute, in a single thrust.

A member without a coronal crest only managed to remove 35% of the sperm.

The thrust depth was also important to clear the vagina.

Professor Gallup told the BBC: “We theorize that due to competition for paternity, human males have evolved uniquely configured penises that function to displace semen from the vagina left behind by other males.

“Thus, the human penis can allow males to substitute their sperm for the sperm of their competitors.”

Cavemen versus modern man

While men’s penises haven’t changed since the dawn of time, sex and relationships have certainly changed.

The harvesting mechanism would have been much more useful hundreds of thousands of years ago, when humans were much busier with procreating.

However, our instinctual sexual desires and behaviors in many ways have remained the same.

The researchers said that in their research they found evidence that men still unconsciously use their penises as a “sperm-moving device”.

Two surveys of around 500 students – men and women – were conducted.

Participants recalled that sex with partners involved “deeper and more vigorous penile thrusting” if they had been separated or if the woman was suspected of cheating.

This casts shabby doubts on the phrase “absence makes the heart grow fonder”, but supports the researchers’ theory.

Professor Gallup said: “Under conditions which increase the threat of female infidelity, female students have reported changes in sexual behavior which would serve to purge the vagina of extraneous semen prior to ejaculation.”

Some experts have said they think the idea is wrong and does not sufficiently explain why the penis has the shape it does.

There is no standard penis shape and although most spikes look like mushrooms, they can have a variety of differences.

Size, girth, angle, or circumcision are some of the factors that can cause men to have different looking penises.