Bornean Orangutan Blog Post 3
Image credit: © San Diego Zoo Wildlife Alliance. All rights reserved.
Orangutans are a semi-solitary species. Adult males are more solitary than females, spending around 91% of their time alone. (San Diego Zoo Wildlife Alliance Library staff.) Females spend time with their young for seven years post-birth and often have more than one child of varying ages with them. Adolescent orangutans, particularly females, spend time with other immature orangutans and adults. Both male and female adult orangutans have large habitats with little overlap. Females tend to stay within the range of their habitats while some adult males are more nomadic. Nomadic males will sometimes challenge settled males for their territory or access to fertile females in a nearby territory. Females often tend to claim a home range close to their mothers and often have overlap between the mother's and daughter's territory. Yong adult females have been observed to travel back to their mothers for social visits. This behavior has been observed in young adult males but is less common.
Because orangutans live dispersed, they do not have as strict of a social hierarchy as seen in other primates. There is often some hierarchy when it comes to mating but that is based on age and physical size and is not inherited. Adult females rarely have conflict if they come across one another, but adult males have highly physical altercations when crossing paths, especially if there is competition for a fertile female. The most likely scenario for physical competition is when two flanged males encounter each other near a fertile female. The physical fight can last anywhere from a few minutes to an hour or more. Due to the highly aggressive nature of orangutan combat, adult males may have missing fingers, patches of hair, or even missing eyes as the result of a fight.
Orangutans have a high level of sexual dimorphism, with males being considerably larger than females. Males also develop bi-maturely, with the main development being a face disk or “flanges.” Flanged males also have a large throat sac which allows them to produce the “long call”, a distinctive sound that indicates dominance and readiness to mate. Some “long calls” can be heard from over a mile away. Flanges do not represent sexual maturity as once thought by primatologists, both flanged and unflanged males are capable of reproducing. Females have a preference for flanged males, so unflanged males often resort to “forceful copulation.” Forceful copulation rarely results in pregnancy for the female and is often observed as a domination tactic employed by young adult males. Females often are the ones who initiate sexual contact. Because of the female's preference for flanged males and the fact that willing copulation is more likely to result in a viable pregnancy, flanged males see higher rates of reproductive success than unflanged males. Orangutans practice polygyny, where males will mate with more than one female.
Orangutans have one of the slowest life histories of any primate. The gestation period is anywhere from 227-301 days. (San Diego Zoo Wildlife Alliance Library staff.) Orangutans give solo birth but have been observed to have twins. Young orangutans are not fully weaned off of their mothers for anywhere between six to nine years with the average weaning age being seven. Offspring stay with their mothers for 7-10 years post-birth. These mother/offspring pairings are called dyads. Fathers are not present in the rearing of their offspring, so all child care falls to the mothers and any older siblings who are still around. Primatologists believe that the long dyadic period is due to the amount of knowledge that has to be passed from mother to child. Young orangutans learn the fruiting patterns of the forest they inhabit, how to avoid or safely process toxic fruits, and proper nest-building techniques from their mothers before they strike out on their own.
Sources:
Kenyon-Flatt, B. (2021, August 24). Flanged male orangutans experience more severe early life stress. Labroots. https://www.labroots.com/trending/plants-and-animals/21156/flanged-male-orangutans-experience-severe-life-stress
McGrew, W. C., Marchant, L. F., & Nishida, T. (1996). Great ape societies. Cambridge University Press.
Relationships & mating strategies. (n.d.). https://www.orangutanrepublik.org/learn/orangutan-specifics/relationships-mating-strategies/
San Diego Zoo Wildlife Alliance Library staff. (n.d.). LibGuides: Orangutans (Pongo spp.) Fact Sheet: Reproduction & Development. https://ielc.libguides.com/sdzg/factsheets/orangutans/reproduction
Scott, A. M., Banes, G. L., Setiadi, W., Saragih, J. R., Susanto, T. W., Setia, T. M., & Knott, C. D. (2024). Flanged males have higher reproductive success in a completely wild orangutan population. PLoS ONE, 19(2), e0296688. https://doi.org/10.1371/journal.pone.0296688
Vicki. (2016, July 12). Why do male orangutans have huge throat sacks? - Orangutan Foundation International Australia. Orangutan Foundation International Australia. https://orangutanfoundation.org.au/why-do-male-orangutans-have-huge-throat-sacks/#:~:text=A%20flanged%20male%20has%20big,cheek%20pads%20and%20large%20size.
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