Researchers have for the first time witnessed the death of a female orangutan at the hands of another female. Even more extraordinary is that the perpetrator recruited a male orangutan as a hired gun to help her corner and attack the victim. Before this observation, lethal fights between females had never been observed in orangutans; in other primates such fights occur mainly between males, according to Anna Marzec of the University of Zurich in Switzerland. She is the lead author of a report on the fatal incident, which appears in Springer's journal Behavioral Ecology and Sociobiology.

Aggression serves ultimately to gain access to limited resources. Although aggression among primates is frequent, lethal attacks are very rare, especially among female individuals. Female Bornean orangutans live alone and typically settle in or near the area where they were born, whereas males generally disperse. The two sexes regularly associate only during the few months before a female orangutan is ready to conceive, which happens approximately every seven years.

The research team around Marzec had been following a population of Bornean orangutans (Pongo pygmaeus wurmbill) in the swamp forests of Indonesia's Mawas Reserve since 2003 and already collected over 26,000 hours of information on the adult females alone when they observed the fatal attack in July 2014. During this period, only six female-female attacks had been observed, none of which had caused visible injuries. Other long-term studies of orangutans similarly have never reported such violent female attacks.

The case 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 neighbouring apes. The two females had a history of aggressive interaction: a few years earlier the researchers had witnessed an encounter between them during which Sidony hit and bit Kondor, who had apparently approached Sidony's seven-year-old daughter.

In the week before the lethal attack, Kondor was seen with a male called Ekko. The two of them encountered Sidony and her dependent son. After Ekko sexually inspected Sidony, he returned to Kondor to mate with her. Kondor interrupted these sexual activities when Sidony started to move away, and attacked her.

Ekko joined the fight, which lasted 33 minutes. They continuously attacked as a coordinated team. While one attacked, the other blocked the victim's escape route. Kondor instigated two further shorter attacks. Ekko, who had long canines typical of a male, inflicted the most serious injuries and effectively prevented Sidony's escape.

The dynamics changed when another male, Guapo, arrived and chased Ekko away. Guapo then mated with Sidony. Kondor continued to harass and bite her. Whenever Sidony screamed, Guapo positioned himself between the females or escorted the older ape away. Sidony sustained major injuries in the first part of the attack. Although Guapo successfully protected her from further damaging attacks, Sidony died two weeks later.

The case does not comfortably fit into the pattern of joint coalitional killings normally seen in primates. The attack involved between-sex coalitions, with the males being either the back-up or bodyguard of a female.

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