Male lemurs grow bigger testicles when there are other males around

Dominant male lemurs flexibly grow their testicles when other males are around, putting them in a better position to successfully mate.

In many species, dominant males have larger testes than subordinates, often because they have more testosterone. Studies have also shown that males in species that have lots of males living together have bigger testicles than in species where only one male lives in a group. Because larger testicles can make more sperm, this improves a male’s chances of fathering offspring if females mate with several males.

Gabrielle Bueno and Rebecca Lewis from the University of Texas at Austin tested if this pattern holds true within a single population by looking at the testes size of 23 adult male Verreaux’s sifakas (Propithecus verreauxi) in Kirindy Mitea National Park in western Madagascar. This was done outside the mating season over a 13-year period.

This population of lemurs is made up of several groups, with some having only one male and others having multiple males. Although females hold the most social power, as in all lemur societies, males also have their own hierarchy. Dominant ones have a brown, greasy stain on their chest due to constant scent marking, whereas subordinates have clean white torsos.

The researchers found that stained males in multi-male groups have testicles that are, on average, 103 per cent larger than those of clean males within their group, and 31 per cent larger than those of stained males in single-male units. The stained lemurs in multi-male groups may be producing more testosterone than the other lemurs, or suppressing the amount of testosterone the clean males can produce, says Bueno.

Stained males in multi-male groups don’t just have more sizeable scrotums in absolute terms – they are also larger relative to their body size. Bueno says that this shows dominant males invest more energy in sperm competition when there are other males around.

“Importantly, they are able to switch that,” says Bueno. The dominant male always has the biggest testicles, so if a clean male with larger testes enters the group, the stained male’s gonads grow. This “highlights how insanely flexible they are and how in tune they are with their social environment”, says Bueno.

“The flexible adjustment of testes size of males living under different conditions across their lifespan is remarkable,” says Peter Kappeler at the University of Göttingen in Germany. This raises important questions about the potential costs of maintaining large testes, he adds. Growing larger testicles uses up energy that then can’t be spent elsewhere, and is likely to mean they have to find more food.