Why gorillas have such small penises: Scientists uncover the genetic mutation behind apes' 'modest' 1.1-inch genitalia - and it could explain why some men are infertile

Standing at up to 6ft tall and weighing a staggering 500lbs, it's safe to say that gorillas are some of the most formidable creatures in the animal kingdom. 

But despite their impressive stature, male gorillas are lacking in one particular area. 

The great apes have the smallest penises of all apes, with the average member measuring just 1.1-inches long. 

Now, scientists from the University of Buffalo have identified the genetic mutation responsible for this adaptation. 

And they say the discovery could help to explain why some men are infertile. 

Standing at up to 6ft tall and weighing a staggering 500lbs, it's safe to say that gorillas are some of the most formidable creatures in the animal kingdom. But despite their impressive stature, male gorillas are lacking in one particular area (stock image)

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Male gorillas have very small penises and testes, and produce a low amount of sperm.  

Previous studies have attributed this to their polygynous mating system. 

Within a troop, the alpha male is physically larger than the other males, giving him near exclusive access to the females. 

This means his sperm doesn't compete with other males' sperm within the female reproductive tract.  

'There are two ways to compete for mates: You can either use your body or your sperm,' Dr Vincent Lynch, who led the study, said. 

'Most mammals use a combination of both. Gorillas use only their bodies.'

Male gorillas have very small penises and testes, and produce a low amount of sperm. Previous studies have attributed this to their polygynous mating system (stock image)

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The researchers suggest that the lack of sperm competition likely led to the evolution of gorillas' small testicles, as well as their low sperm count. 

And in their new study, they set out to understand if the gene mutations behind them could also explain why some men struggle to conceive.  

'We have a set of genes that are involved in sperm biology and have the signatures of harmful mutations when in gorillas, Dr Lynch said.

'We can then look at those same genes in infertile men and see if they have mutations.

'Here the gorilla genome essentially acts as a discovery tool for finding candidate genes for human male fertility that we previously wouldn't have been able to identify.'

The researchers analysed over 13,000 genes across 261 mammals, and found that 578 (4.3 per cent) showed the same signatures as the gorilla lineage. 

To test the impacts of these genes on male fertility, the team used gene editing to delete them in a fruit fly, Drosophila melanogaster.

They found that deleting these genes affected the male flies' reproductive function. 

Next, the team team analaysed the gorilla genes with those of 2,100 infertile men, revaling 109 genes linked to loss of function in men. 

'Just a few years ago, there weren't enough sequenced genomes and computing power to conduct these kind of studies,' Dr Lynch added. 

'As science collects more genetic data, we'll have a better understanding of why infertility happens.'