sidonay murdered orangutan
Sidony - the orangutan killed by another femaleAnna Marzec

A female orangutan has been seen killing another female with the help of a male "hired gun" that helped corner and attack the victim. This marks the first time lethal fights between female orangutans have been observed by humans.

Scientists from the University of Zurich in Switzerland witnessed the attack in Indonesia's Mawas Reserve. Researchers have been following a population of Bornean orangutans in the forests there since 2003 and have over 26,000 hours of data on adult females and their behaviour.

The fatal attack took place in July 2014. Before this, just six female-female attacks had been observed and none of them had resulted in visible injuries. Other long-term reports have not shown any evidence of females killing other females.

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

Published in the journal Behavioural Ecology and Sociobiology, researchers wrote: "Here, we report the first case of lethal female-female aggression ... A young female, who had recently lost her infant, attacked an old resident female." The two females had a history of aggressive interactions – years earlier the older orangutan (called Sidony) had hit and bitten the younger female (Kondor) after she approached her daughter.

A week before the attack Kondor was seen with a male called Ekko. They came across Sidony and while the male inspected her, he returned to Kondor to mate. However, the younger orangutan stopped the sex and attacked Sidony, shortly after which Ekko joined in.

In total, the attack lasted 33 minutes, with both Ekko and Sidony acting as a coordinated team – when one attacked, the other blocked Sidony's escape. Most of the serious injuries were from Ekko's attacks.

Another male arrived and chased the Ekko away, but Kondor continued to attack and bite her. The other male warded off the attacks by positioning himself between the two and eventually Sidony got away. However, the injuries she had sustained were serious and she died two weeks later.

"The interaction's unique feature was that the attacking female was supported by an unflanged male, who had been in consort with her during the week preceding the attack and was responsible for the lethal injuries to the victim," authors said.

The team said the case does not fit into patterns of joint killings normally seen among primates. "This is quite unexpected, as in wild orangutans males and females have never been reported to form coalitions before," said lead author Anna Marzec. "It is also the first report of males supporting females in their conflicts, with lethal outcome."

Researchers believe the killing suggests females must hold "great leverage" over males when fertile and coerce them into providing services on their behalf. "We conclude that even in a species in which coercion is frequently observed in male-female interactions, female leverage over males can coax males into providing services, such as coalitionary support."