Male lemurs exhibit enlarged testicles in the presence of other males

Dominant male lemurs grow more flexible testicles when other males are present, improving their position for successful mating.

In many species, dominant males have larger testes than subordinate males. Increased Testosterone Research has also shown that males of group-living, multi-male species have larger testes than males of group-living, single-male species. Larger testes mean more sperm can be produced, which increases the male’s chances of producing offspring if a female mates with multiple males.

Gabriel Bueno and Rebecca Lewis Researchers at the University of Texas at Austin tested whether this pattern held true within a single population by looking at testicle size in 23-year-old adult men. Verreaux’s sifaka (Propithecus beleaucii) took place outside of the mating season over a 13-year period in Kirindimitea National Park in western Madagascar.

This lemur group is made up of several groups, some with only one male and some with several. As in all lemur societies, the females hold the most social power, but the males also have their own hierarchy. Dominant lemurs have greasy brown spots on their chests from constant scent marking, while subordinate lemurs have clean white bodies.

The researchers found that the testicles of stained males in multi-male groups were on average 103 percent larger than those of clean males from the same groups and 31 percent larger than those of stained males in single-male groups. The stained lemurs in multi-male groups may be producing more testosterone than other lemurs or they may be suppressing the amount of testosterone that clean males can produce, Bueno says.

Stained males living in multi-male groups not only have larger scrotums in absolute terms, but also larger relative to their body size, which Bueno says suggests indicates that dominant males spend more energy competing for sperm when there are other males around.

“The key is that they can make that switch,” Bueno says. Because the dominant male always has the largest testicles, when a dirty male with larger testicles enters the group, the dirty male’s gonads grow. This “highlights how extraordinarily flexible they are and how they adapt to their social environment,” Bueno says.

“It is surprising that males can flexibly adjust their testis size across a range of environments throughout their lives,” he said. Peter Kappeler The researchers, from the University of Göttingen in Germany, added that their findings raise important questions about the potential costs of maintaining large testes: Enlarging them requires energy that cannot be used elsewhere, and likely requires the animal to search for more food.