The flanged male is the familiar image: broad fleshy cheek pads, a pendulous throat sac used to produce a long call audible over a kilometre, and a body mass around seventy to ninety kilograms. The unflanged male is sexually mature, fully fertile and looks much like a large female.
The two forms use different strategies. Flanged males call, advertise and are approached by females. Unflanged males travel widely, do not call, and pursue matings that females often resist. Paternity studies in wild populations have confirmed that unflanged males sire a meaningful share of infants, so the small form is not a failure to develop.
Development appears to be arrested rather than delayed, and the arrest is thought to be socially mediated, with the presence of a dominant flanged male suppressing the change. The exact trigger and timing remain unresolved, and captive records show transitions occurring over a period of months once they begin.