EXCLUSIVEScientist reveals the surprising reason why human penises are so big

If you’re ever unsatisfied with the size of your penis, it may be comforting to know that it’s pretty big – that is, relatively speaking. 

According to biologists, the average human male actually has a large phallus, at least in comparison with some of our closest living relatives. 

While the average human manhood measures 5.1 inch (13cm) when erect, the chimp and the bonobo only boast a 3.1 inch (8cm) shaft. 

Meanwhile, the orangutan’s member measures 3.3 inch (8.5cm) and the gorilla only has a tiddler – just 1.25 inch (3cm). 

Now, a scientist reveals the surprising reason why we really are ‘king of the swingers’.

Simon Underdown, professor of biological anthropology at Oxford Brookes University, said it’s ‘almost certainly’ related to humans starting to walk on two legs.

Becoming ‘bipedal’ has made it more difficult for humans to get semen to the right spot for conception – i.e. further into the vagina and nearer the cervix – and as a result, we have evolved larger penises. 

‘Chimps and other primates don’t need to be big because they’re not bipedal and hence getting things to the right location is not an issue,’ he told MailOnline.

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To fully understand why humans now have relatively large penises, we have to look back millions of years in the story of human evolution and the other great apes. 

The last common ancestor shared by the modern humans (homo sapiens) and apes in the Pan genus (chimps and bonobos) lived an estimated 13 to 5 million years ago. 

This common ancestor was quadrupedal – meaning it moved around on four legs. 

But while chimps and bonobos stayed on four legs to get around – doing a sort of ‘knucklewalk’ – humans gradually evolved to walk on two legs. 

Humans likely became bipedal to adapt to Africa’s savannah grasslands, free up the hands to use and carry tools, and to intimidate predators.

‘The selective pressure to walk on two legs was massive and sustained for millions of years,’ Professor Underdown told MailOnline. 

But there’s been many ‘knock-on effects’ of becoming bipedal – one of which is that we gradually grew larger penises to improve sperm delivery, the theory goes. 

Chimps and bonobos are used to sitting with bent legs, which on an anatomical level helps the successful delivery of sperm during copulation. 

The average human male has a large phallus, in comparison with some of our closest living relatives. While the average manhood measures 5.1 inch (13cm) when erect, the chimp and the bonobo only boast a 3.1 inch (8cm) shaft. Meanwhile, the orangutan’s member measures 3.3 inch (8.5cm) and the gorilla only has a tiddler – just 1.25 inch (3cm). Pictured, a male chimp

Largest mammal penis relative to body size 

The mammal with the largest penis relative to body size is the fossa Cryptoprocta ferox – officially a Guinness World Record holder. 

This puma-resembling carnivore, endemic to the island of Madagascar, has a penis length of 6.6 inch to 7.8 inch (17–20cm), reaching to between its forelegs when full-sized. 

The mammal measures a total head-to-tail length of 47 inch to 69 inch (120–150cm) – so its penis is nearly one-sixth as long as its entire length.

The blue whale has the largest penis in nature – up to 10 feet long. 

‘Just think about how quick most animal mating is when you see it on David Attenborough documentaries,’ Professor Underdown said. 

In comparison, the human build is naturally evolved for standing but makes the angles required during sex somewhat more physically awkward.

This can impair sperm delivery, so humans evolved bigger penises to compensate. 

‘These changes are assumed to take place around 2 million years ago with the appearance of homo erectus, so we can assume Neanderthals probably had the same set up,’ Professor Underdown added. 

The academic said the theory is based on a ‘widely accepted understanding of differences between us and chimps as a result of bipedalism’. 

Mark Maslin, professor of palaeoclimatology at University College London, says the human penis is comparatively ‘extremely dull’. 

‘It does not have lumps, ridges, flanges, kinks or any other exciting feature that other primates have,’ he wrote in an article for The Conversation. 

Interestingly, human testicles are also ‘rather small’ when compared to other living primates, such as the chimp and the bonobo. 

Human testicles are ‘rather small’ when compared to other living primates such as the bonobo and the chimp. This image shows male and female great ape sexual organs, compared for size (bonobos are flat chested until they get pregnant)

Mark Maslin, professor of palaeoclimatology at University College London, says the human penis is ‘extremely dull’ relatively speaking. This image shows more complex primate penises – brown lemur (a), white-bellied spider monkey (b), black-capped squirrel monkey (c), macaque (d, e, f), baboon (g) and chimp (h)

‘A chimpanzee’s testes weigh more than a third of its brain while ours weigh in at less than 3 per cent,’ Professor Maslin said. 

‘Essentially, male chimps have sex all the time with any female and with any excuse. 

‘A female therefore may contain sperm from multiple partners at any one time, which puts the sperm itself – and not just the animals that produce it – into direct competition. 

‘For this reason, chimpanzees have evolved huge testicles in order to produce massive amounts of sperm, multiple times a day.’ 

Meanwhile, modern human testes are of ‘very modest size’ and produce a relatively small amount of sperm. 

In fact, human sperm count reduces by more than 80 per cent if men ejaculate more than about two times a day.