Scientists have established which primate has the largest testes, relative to its body size.

The biggest swinger, so to speak, is not the out-sized gorilla or orang-utan.

The northern giant mouse lemur has testicles that are two orders of magnitude bigger than a human's

It is not humans, despite our often-grandiose sense of self.

Nor is it the bonobo, the smaller of the chimpanzee species that is renowned for freely using sex as a way to socially bond.

The primate with the largest testicles is the exceedingly small northern giant mouse lemur (Mirza zaza) of Madagascar, an animal only discovered to be a unique species in 2005.

The discovery was made during a study by Dr Johanna Rode-Margono and Professor Anna Nekaris of the Nocturnal Primate Research Group at Oxford Brookes University, UK.

That means 5.5% of the male lemurs’ bodies were testes

Together with colleagues, including Dr Christoph Schwitzer of the Bristol Zoological Society at Bristol Zoo Gardens, UK and Professor Peter Kappeler at the German Primate Center, in Göttingen, Germany, they captured seven male northern giant mouse lemurs, and five females, living in the Ankarafa Forest, northwestern Madagascar.

This was the first ecological study ever conducted on the lemur, which is endangered.

They weighed the primates and measured the volume of the males’ testes before releasing the animals.

The volume of the lemurs’ testes was “especially surprising”, the researchers write in the American Journal of Physical Anthropology.

Males had an average testes volume of 15.48 cubic centimetres.

That’s eight times the volume normally expected for strepsirrhines, the primate group that includes lemurs, galagos and pottos.

If a man had testicles of an equivalent size, they would weigh 4 kilogrammes

Each male only weighed around an average of 280 grams.

That means 5.5% of the male lemurs’ bodies were testes, if they are considered to proportionately weigh the same.

In humans, relative testicle size varies around the world, but generally speaking they lie between the size of chimpanzee testes, which are relatively larger, and gorillas, that are relatively smaller.

As a comparison, an adult male human has an average testes volume that is about 0.05% of his body, though the precise proportion differs depending on the men sampled.

Relatively speaking, the northern giant mouse lemur has testicles that are two orders of magnitude bigger than a human's.

Or put another way, if a man weighing 80 kilogrammes had testicles of an equivalent size to the lemur, they would be as large as a decent size grapefruit and weigh 4 kilogrammes.

The discovery is more important than it sounds.

Not because of the mirth it may generate, but because varying testes size may indicate how different animals mate; whether they are monogamous, polygamous or otherwise.

"There are still debates whether large testicles are because of a higher amount of sperm, or because of a higher mating frequency," Dr Rode-Margono told BBC Earth.

But male primates generally have larger testes the more promiscuous they are, and testes size can sometimes increase before any mating season.

It may have evolved relatively huge testes to allow males to maintain their ability to compete

The grey mouse lemur, for example, has a short mating season and a polygynandrous mating system, where two or more males mate with two or more females.

That means males compete with each other for females and their sperm may compete too.

So in this species, the size of the males’ testes grows before the mating season begins.

However, the nocturnal northern giant mouse lemur does things differently.

"We were very surprised because lemurs tend to mate only highly seasonally, so investing in giant testes would not make sense," Professor Nekaris told BBC Earth.

But the northern giant mouse lemur appears to mate all year round, one of the few lemur species to do so.

It is also polygynandrous.

So it may have evolved relatively huge testes to allow males to maintain their ability to compete to fertilise females all year.

And though it's not yet confirmed, that suggests the lemur also produces litters of offspring throughout the year.

During the study, the researchers also discovered up to three males sharing a nest with a single female.

Sometimes the males were brothers, but often they were not, reinforcing the idea that males compete for access to females and may require outsized testicles to give themselves the best chance to fertilise their partner.

So why don‘t other primates have such large testicles?

Other species might have other mating strategies, says Dr Rode-Margono.

"Some such as gibbons live in pairs and do not need an increased amount of sperm as they mate with only one partner."

"Others such as the gorilla stay in harems, that means one male mates with several females. But because they mostly defend the females physically against other males, they also do not need a large amount of sperm."

“Lemurs are renowned for their highly seasonal mating patterns, mating only a few days a year," adds Professor Nekaris.

But the behaviour of the giant mouse lemur "just goes to show that there is so much to learn about these rare nocturnal primates."

"Our study also shows why the diminutive ‘giant’ mouse lemur is truly a giant among primates.”

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