An evolutionary biologist has compared the sizes of human genitals to gorillas and apes to look at why there is such a discrepancy between the species

Gentlemen, if you've ever been concerned about the size of your penis, take comfort in the fact that no matter how small it seems, it is one of the largest in the primate world.

An evolutionary biologist has compared the sizes of genitals of humans to gorillas and apes to look at why there is such a discrepancy between the species.

And he concludes the human penis may have grown so long to make it more conspicuous to potential partners, or even as a way of cooling the body down.

The relative size of the male reproductive organs across these species was discussed by evolutionary biologist Darren Curnoe in his latest 'How Did We Get Here?' video series for the University of New South Wales, Australia.

Gorillas have small penises and small testicles, while chimps have medium-sized penises and large testicles.

By comparison, human males have the longest penis and medium-sized testicles.

'Amazingly the size of the reproductive organs, especially the testicles, are a reflection of how the apes are organised socially,' explained Professor Curnoe.

'Why the difference? Well for the testicles it's pretty straight forward.

'It all comes down to the amount of competition the chimps, and their sperm, have when they mate with females.'

He said that if a female is 'receptive to multiple males in each reproductive episode', it maximises her chances of getting high-quality DNA.

So the sperm of these males have to battle it out to fertilise her egg, hence chimps have very large testes that produce a very large number of sperm in each ejaculate

The opposite applies to gorillas, Professor Curnoe continued.

A mature male, a silverback, has exclusive reproductive access to the females in his group so his testes are small because there is no 'sperm war'.

Gorillas (left) have small penises and small testicles, while chimps (right) have medium-sized penises and large testicles. For testicles, this difference is explained by how societies are organised. Chimps have more competition so produce larger numbers of sperm, but gorillas have a hierarchy so they don't need as much

But he said there wasn't a discernible reason why there is so much variation in penis size between humans and great apes.

'Why is ours the longest? It may be because we walk upright on two feet, making it more conspicuous to play a role in attracting more females.

'Or perhaps it acts as a radiator, releasing heat when we walk and helping to keep our bodies cool'

He added that the differences don't just stop there though.

The chimpanzee penis has spines on it, designed to temporarily injure females when they mate in order to delay her from mating with other males too soon.

A study in 2011 discovered evidence to suggest humans once had spines on their penises, but evolved to make them smooth.

Researchers at Stanford University in California found a molecular mechanism that could explain why we lost a piece of non-coding DNA that influences the expression of the androgen receptor gene involved in hormone signalling.

Professor Darren Curnoe said there wasn't a discernible reason why penis size varies so much between primates, but speculated it may be because men walk upright, making the penis conspicuous. Or it releases heat when men walk to keep them cool. The penis in this image was pixelated by the researchers

It has long been thought humans evolved smooth penises after developed a more monogamous reproductive strategy than our ancestors.

Our ancestors may have used penile spines in a similar way to how chimps do, but how this change was made is not known.

Geneticists recently studied cloaca (shown) to see how external genitalia forms and why it grow in different regions of the body in different animals

And in November, a team of geneticists at Harvard Medical School's Department of Genetics discovered not only how such external genitalia forms, they identified why the genitals grow in different regions of the body in different animals.

Snake and lizard genitalia is derived from the same tissue that creates hind legs, while mammalian genitalia derives from the tail bud.

The embryonic cloaca - which eventually develops into the urinary and gut tracts - sends signals that tell nearby cells to form genitals.

The cloaca's location determines which tissues receive these signals first.

In snakes and lizards, the cloaca is located closer to the lateral plate mesoderm, the same tissue that makes the paired limbs, receives the signal.

But, in mammals, the cloaca is closer to what’s known as the ‘tail bud’, found at the bottom of the animal’s trunk, or torso.

The cells that turn into either a penis or a clitoris start on opposite sides of the embryo.

As the embryo forms, and it changes from a flat sheet to a 3D embryo, these cells meet.

They then form a bud, which becomes genitalia.

In snakes and other reptiles, the researchers suggest that these buds don’t merge in the same way, so they form dual penises.

During tests, the geneticists were even able to change the location of where the penis grew by tweaking the signals sent by cells.

'There is still much more to the penis that meets the eye,' concluded Professor Curnoe.