Researchers have for the first time witnessed how aggressive Bornean orangutans can be toward one another. A recent study from the University of Zurich in Switzerland documents the extraordinary incident of a female orangutan recruiting a male as a hired gun to help corner and attack another female. While similar behaviors have been seen in other primates, lethal fights between females had never before been observed in orangutans.

"This is quite unexpected, as in wild orangutans males and females have never been reported to form coalitions before," Anna Marzec, lead author of the study, said in a news release. "It is also the first report of males supporting females in their conflicts, with lethal outcome."

Aggression, which is often used to gain access to limited resources, is fairly common among primates; lethal attacks, on the other hand, are very rare - especially among females.

Female Bornean orangutans (Pongo pygmaeus wurmbill) live alone and generally settle in or near the area where they were born, whereas males have a much broader range. Therefore, the two sexes really only associate with one another during the few months before a female orangutan is ready to conceive, which happens approximately every seven years, researchers explained.

Marzec and her team had been monitoring a population of lone female Bornean orangutans in the swamp forests of Indonesia's Mawas Reserve since 2003, during which time collected over 26,000 hours of data.

Over the course of their study, researchers had witnessed a total of six female-female attacks, none of which resulted in any visible injuries. That is, until the July 2014 attack, which involved Kondor, a young female who had lost her infant just weeks before, and Sidony, a much older resident female who did not interact much with neighboring apes.

While the two females had a history of aggressive interaction, it appears that a bit of sexual tension ultimately inspired the lethal attack.

In the week before, researchers saw Kondor with a male named Ekko. During this time the two of them encountered Sidony and her dependent son, which prompted Ekko to sexually inspect Sidony, before returning to Kondor to mate. Kondor interrupted these sexual activities, however, when Sidony started to move away and attacked her.

Ekko joined the fight, which lasted 33 minutes. Ekko and Kondor continuously worked as a coordinated team, where one attacked and the other blocked Sidony's escape route. As Kondor instigated two further shorter attacks, Ekko inflicted serious injury used is sharp canines. While another male, Guapo, was able to chase Ekko away and separate the two females, Sidony sustained major injuries in the first part of the attack and died two weeks later.

Researchers say this act of aggression is unlike any witnessed of primates, thus shedding light on the complex social behavior of orangutans. Their study was recently published in the journal Behavioral Ecology and Sociobiology.

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