The human penis is ‘conservative’ compared to other members of the animal kingdom – and now a scientist reveals why this is.
Michelle Spear, a professor of anatomy at the University of Bristol, says penises in the animal kingdom can be spiked, corkscrewed and even detachable.
The reason they have such elaborate shapes and features is to help increase the chance of insemination, she explains in a piece for The Conversation.
That’s because becoming a father in the wild is more competitive – and success is more down to chance.
In comparison, humans have a less competitive and more pair-bonded mating ritual, in that a man and a woman tend to be together as they try for a baby.
As a result, we’ve not had to develop these extreme penile features such as bones and backward-facing spines.
‘The human penis is so uniform, it’s an anatomical outlier,’ Professor Spear says.
‘Understanding why penises evolved, and why they differ so widely, also helps explain why humans have one at all.’
Michelle Spear, a professor of anatomy at the University of Bristol, reveals why human phalluses are so ‘conservative’ compared to other animals (file photo)
As Professor Spear explains, non-human penises tend to be more elaborate because they are adapted for ‘sperm competition’.
This is when multiple males mate with the same female, and their sperm compete internally for fertilisation.
Essentially, various weird and wonderful penises in the animal kingdom are used as a ‘competitive tool’.
The domestic cat, for instance, has backward-facing spines on its penis.
These stimulate ovulation in the female, ensuring sperm meets a ready egg, but also discourage mating with other males by making withdrawal painful.
Perhaps even more gruesomely, male bedbugs use a dagger-like penis to stab through the abdominal wall and deposit sperm, a 2001 study revealed.
This traumatic insemination is not typically deadly, although the female’s injuries take time and energy to heal.
More bizarrely, some species of male ducks have corkscrew-shaped penises that can extend in under half a second.
Male ducks are infamous for their bizarre penises (pictured), which are corkscrew-shaped
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)
‘This is a response to female ducks evolving highly convoluted vaginas with dead-end pockets and spirals that twist in the opposite direction,’ Professor Spear said.
‘This is a textbook example of sexual antagonistic co-evolution, where male traits that increase fertilisation rates are countered by female traits that limit male control.’
Certain mammals such as dogs, chimps and walruses have a penis bone, known as the baculum – a form of structural support that allows penetration without having to rely on blood pressure to get an erection.
A bone in the penis allows a male to mate for a long time with a female – another distinct advantage in the race to inseminate and pass on genes.
In contrast, humans don’t have a penis bone, as we tend to be a monogamous species, and men don’t need to compete for a partner quite as much.
Of course, penises first evolved as a solution to one simple problem – how to achieve internal fertilisation.
‘The first animals lived in the sea before our ancestors started living on land half a billion years ago,’ the academic explains.
‘Today, many aquatic animals still simply release sperm and eggs into the water.
The ‘very strange and unusual’ echidna penis remains a mystery to researchers who still don’t understand why it has four heads
‘However, as organisms moved to land, a new mechanism was needed to transfer sperm into the female body – enter the penis.’
The human penis is also pretty big, 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).
Simon Underdown, professor of biological anthropology at Oxford Brookes University, said there’s a reason for humans being ‘king of the swingers’.
And it is ‘almost certainly’ related to humans starting to walk on two legs (becoming ‘bipedal’).
