Female orangutans may prefer males with large cheekpads

Dominant male named Kiko at NZP. Photo: author

Orangutans display an interesting form of sexual dimorphism in that males have large cheek pads but females do not. Males are also larger and have large throat sacs that aid them when they make long calls, which advertise to females and tell other orangutans, "I am here!" However, not all males develop this large size, cheek pads, and throat sac. The cheek pads, large size and throat sacs are characteristic of dominant males. Thus, orangutan males display bimaturism, meaning the timing of development differs. Some males develop these dominant characteristics much sooner than others.  Because adult males aren't always large and with the distinguishing cheek pads and throat sacs, it can be quite difficult to tell non-dominant males and females apart.


These non-dominant males aren't juvenile or infertile. They are perfectly capable of reproducing and previously have been shown to produce about half of all wild orangutan offspring (Utami et al., 2002). These two forms of adult males have different strategies when it comes to mating. The dominant males defend their territory. Their homerange is very large and includes multiple females. Non-dominant males are able to gain access to females when the dominant male isn't around.

Bonnie(female) at NZP. Photo: author

A new study resulting from eight years of field research at Tanjung Puting National Park on the island of Borneo found that females prefer dominant males with cheek pads. This is the longest study to have been done at one site in a single population of individual orangutans in terms of orangutan paternity. Banes and colleagues (2015) determined parentage for orangutans inhabiting the homerange used by Kuasasi, a dominant male. They determined he produced far more offspring once he achieved dominance and that he fathered the majority of offspring in the area. This is in contrast to what Utami and colleagues found in 2002 in Sumatra in which unflanged males produced half of the offspring.

This more recent study is arguably limited by the fact that researchers determined the parentage of orangutans in one dominant male's range (Kuasasi). It is possible this is just one very successful male. Or, females are in fact choosing dominant, flanged males over unflanged males. The authors also point out that their study site may affect their results. Camp Leakey is characterized by both wild orangutans and individuals that were once captive but released to the area in the 70s. Orangutans at this site also receive medical treatment, which may mean that dominant males are surviving attacks that they otherwise would not, affecting the population.

Conducting studies on paternity and life history variables on orangutans is very difficult given that, other than humans, orangutan offspring take the longest to reach adulthood of any primate. Females will reproduce every eight years (Gladikas and Wood, 1990), thus answering these types of questions remains difficult.

Links of interest:
Encyclopedia of Life Page on Bornean Orangutans
 IUCN Redlist Page on Bornean Orangutan
Monopoly of the male orangutan
Orangutan long call


Sources:

Galdikas BMF, Wood JW (1990) Birth spacing patterns in humans and apes. Am J Phys Anthropol 83:185–191

Graham L. Banes, Biruté M. F. Galdikas, Linda Vigilant. Male orang-utan bimaturism and reproductive success at Camp Leakey in Tanjung Puting National Park, Indonesia. Behavioral Ecology and Sociobiology, 2015; DOI: 10.1007/s00265-015-1991-0

Utami, S. S., Goossens, B., Bruford, M. W., de Ruiter, J. R., & van Hooff, J. A. (2002). Male bimaturism and reproductive success in Sumatran orang-utans. Behavioral Ecology, 13(5), 643-652.