|Dominant male named Kiko at NZP. Photo: author|
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|
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
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.