Out of all primates, who do you think has the biggest testicles? A question that has gripped us all, I’m sure, but the answer might surprise you. Researchers suggest – in proportion to its body size – the northern giant mouse lemur (Mirza zaza) comes out on top. 

The researchers found it odd that these species, which weigh 300 grams (0.7 pounds), are called the northern giant mouse lemur, but it seems it isn’t their body size that’s giant. The study, published in the American Journal of Physical Anthropology, found these lemurs had the highest testis volume relative to their body mass among all primates.

BBC’s Matt Walker puts this in context, writing: “If a man had testicles of an equivalent size, they would weigh 4 kilograms [8.8 pounds].” That’s the size of a grapefruit per testicle.

^{Image credit: Johanna Rode-Margono}

There’s more to this study than the hilarious imagery of large lemur testes. Professor Anna Nekaris, lead researcher from Oxford Brookes University, tells IFLScience that testes size can hint at the mating system of the primate. The lemur’s large testes suggest there’s strong sperm competition between males.

“If they’re fighting against other males, it’s very often about how much sperm they can produce instead of how strong they are,” Nekaris explains.

Higher amounts of sperm means the northern giant mouse lemur is able to produce larger ejaculations and mate more frequently. In contrast to the majority of lemurs, the northern giant mouse lemur is not tied to mating seasons and reproduces all year round.

There’s also less food competition, as these lemurs eat secretions of larvae, which are readily available. As Nekaris suggests, the “readily available resource allows them to be more promiscuous.” The huge testes are a byproduct of promiscuity.

There are, however, certain setbacks to having huge testes. The northern giant mouse lemur can end up bumping their testes while walking and climbing, which results in a “bruised testicle” or two.

Though the species was first discovered in 2006, this is the first ecological study on the northern giant mouse lemur. The species has been classed as endangered since 2014, primarily due to habitat loss. Lemurs, found in Madagascar and some tiny neighboring islands, are members of the clade Strepsirrhini, which is one of the two suborders of primates. The other is Haplorrhini, which the more commonly known primates such as apes and humans fall into.

Nekaris tells IFLScience that nocturnal Strepsirrhini are “largely ignored.” She points out that one of the more well-known primate centers – the Jane Goodall Institute – is unaware of the nocturnal strepsirrhines present there.

“Some of these nocturnal strepsirrhines are so ancient and unique, yet they’ve never been studied before,” she explains.

Information like the testes size can inform conservationists of the factors associated with breeding. While some mouse lemurs in captivity breed really well, others hardly breed at all, Nekaris says.

“This information can also change managed practices in forest, such as cutting or replanting, so they have minimal disturbances to that particular species,” Nekaris tells IFLScience.

If there’s more information on the mating system of nocturnal strepsirrhines, in particular whether they’re seasonal breeders, conservationists can try to ensure their breeding seasons aren’t interrupted by forest management or hunting.

Nekaris is “excited” by the attention a relatively unknown species has gotten through a fun fact – their testes size – and hopes a wider group of people will become invested in protecting not only this endangered species, but other strepsirrhines primates that don’t get as much attention and funding as well.