northern giant mouse lemur
Northern giant mouse lemur has biggest testicles proportionally than any other primateRussell Mittermeier/creative commons

The giant mouse lemur has the biggest testicles of all primates and if humans were the same size proportionally, they would be the size of grapefruits.

Scientists from the Bristol Zoological Society said the Mirza zaza – which weighs around 300g – published their findings in the American Journal of Physical Anthropology.

The northern giant mouse lemur was first discovered in 2005 and since then it has been listed as endangered on the IUCN Red List. It is at risk from forest clearance and "slash-and-burn" agricultural activities. Bristol Zoological Society is one of the founding partners of a conservation project set up to protect them.

The authors said testis size is an indicator of a species' mating system. It had been thought the northern mouse lemur had a polygynandrous mating system and seasonal breeding. Polygynadrous system involves having more than one mate.

In the study, researchers caught 12 giant mouse lemurs before and during their suggested mating period and took measurements of them – including testis size.

"We show M. zaza has the highest relative testis volume among primates, indicating strong sperm competition and polygynandrous mating," they wrote, adding that based on their assessment, they believe the species is one of the few lemurs that breeds all year round.

Christoph Schwitzer, director of Conservation at Bristol Zoological Society, told the Western Daily Press: "These findings are part of the first ever longer-term study on the northern giant mouse lemur, a species only known to science since 2005.

"Most lemurs are highly threatened with extinction, and our research aims to acquire the knowledge necessary to mitigate for the threats they are facing. Finding this species' enormous testis size was pretty much a side-product of our research and surprised us as much as anybody else".

Study author Johanna Rode-Margono added: "My first thought was my God, how can they walk and climb without bumping their testes everywhere. We always found it odd that they are called 'giant' mouse lemurs when they belong to the family of the dwarf and mouse lemurs. But now we can say that we now know why - giant was not referring to their body size."