At the Edinburgh Zoo, there are 5 drills in total, 2 males, and 3 females. Size is the most obvious indicator of the drill’s sex, where males are significantly larger than females.
The adult male is the dominant individual in the group, he is the biggest and has the brightest pink bum!
The pink bum indicates the level of testosterone (hormone = chemical messenger) the male drill has, the pinker the bum the higher the testosterone level. The hormone allows the male drill to be strong, and more likely fight, this signals the others in the group that he is the dominant one in the group.