Did you know there are two forms of adult male orangutans? This little-understood differential growth in males is called “bimaturism” or “arrested development”.The first type, called flanged male, is the typically depicted male orangutan with the big cheek pads (flanges) and a large throat sack under the chin for making long calls. The flanged male is the dominant male, and there is usually only one flanged male within a set territory. The second phenotype is called unflanged male, these male orangutans lack both the cheek pads and throat sacks, and more than one unflanged male can be found within one flanged male’s territory. Also, unflanged males are usually smaller in body size than their flanged counterparts. These unflanged males are sexually mature, and often resort to different mating tactics than the dominant flanged male. Females show preference for the dominant flanged male, and are less respective towards the unflanged males, which can lead to instances of “forced copulation”. Unflanged male orangutans can turn into flanged males, why and when they undergo this transition isn’t fully understood. Scientist have hypothesized that there is some sort of auditory suppressor emitted from the flanged male when he makes his long calls that keep unflanged males in this underdeveloped state. When a dominant flanged male dies or is ousted from the area, an unflanged male can develop cheek pads and actually grow in size, taking over the territory!
Source: http://www.orangutan.org/orangutan-facts/orangutan-behavior Photo: A. Gibson (1a) and H. Morrogh-Bernard (1b). http://www.sciencedirect.com/science/article/pii/S0047248406001795