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Orangutans are less aggressive than other primate species, mostly due to their social structures. Male orangutans form temporary groups, especially during mating, but don’t participate in raising their offspring. Because female orangutans give birth infrequently -- only once every seven years on average -- competition for mating privileges occur less often than in other species, such as chimpanzees. Orangutan aggression is infrequent, but does occur.
Stunted Male Development Reduces Aggression
When male orangutans reach full sexual maturity, their growth reaches 300 pounds. They also develop flanges, thick cheek pads and laryngeal sacs that they use to make a loud, roaring sound called a long call. This call advertises to females that they’re available for mating and also repels other males. Young males release stress hormones when they hear the long call, which curtails testosterone production and prevents them from developing the cheek pads and growth of fully mature orangutans. As long as fully mature males are in the area, young males won’t develop flanges.
Aggression Between Two Mature Males
Most of the aggressive behavior observed in wild orangutans is between two fully mature males, usually competing for female attention. During encounters, male orangutans wrestle, bite and scratch. These fights cause injuries and even death. Mature males have battle scars that include missing fingers, healed fractures and deep lacerations and missing eyes.
Aggression Between Subadult Males and Females
Subadult males are smaller than fully mature orangutans, but are still capable of copulating. Females don’t invite subadult males to mate, and mostly avoid them. Out of frustration, and to show aggression and control, subadult males will often force females to copulate.
Aggression Between Mature Males and Females
Females are drawn toward males who are aggressive toward other males, but males that are aggressive to females are dangerous. An aggressive orangutan attacks his female mate if she doesn’t do what he wants. He can cause bruises, broken bones and may even knock her out of trees. Researcher Maria A. van Noordwijk observed female orangutans taking food from mature males, then watching their reactions. She hypothesizes that this behavior is a test that allows a female to see a male’s tendency to be aggressive toward her before she chooses to mate with him.
Aggression Between Females
Aggression between two females is quite rare and typically occurs only during food shortages. Females don’t often injure or kill each other, but engage in aggression displays including breaking tree branches, diving and lunging, and exposing teeth in a grimace.
- Orangutan Foundation International: Orangutan Behavior
- Palomar College Behavioral Sciences Department: Primate Behavior: Social Structure
- Low Testosterone Correlates with Delayed Development in Male Orangutans; Amy Zhou and Cheryl D. Knott
- San Diego Zoo: Orangutan, Pongo pygmaeus, Pongo abelii
- Intersexual Food Transfer Among Orangutans: Do Females Test Males for Coercive Tendency?; Maria A. van Noordwijk
- Anup Shah/Photodisc/Getty Images