Two female orangutans have been seen cannibalising the bodies of their recently deceased babies.
Such behaviour has never before been recorded in any great ape species.
The two incidences occurred just one month apart in the same region of forest in Indonesia.
The conservationist who witnessed both incidences suspects they were examples of aberrant behaviour, triggered by stressful living conditions suffered by both mothers.
Humans aside, chimpanzees were the only great apes known to engage in cannabilism, the eating of members of the same species. The behaviour had also been inferred but not seen in gorillas, after the remains of infants were found in the faeces of two adults.
But until now, no ape has been recorded eating its own offspring.
"Cannibalism has been documented in chimpanzees and reported in gorillas. Never before has any ape species been seen treating its own offspring as a consumable resource," says David Dellatore of Oxford Brookes University, in Oxford, UK.
That was until Dellatore begun tracking orangutans living in Bukit Lawang, an area of forest within the Gunung Leuser National Park in Sumatra, Indonesia.
Dellatore, who now works with the Sumatran Orangutan Society based in Medan, Sumatra, initially monitored the physical health of once captive orangutans that have been rehabilitated and released back into the wild.
But soon he noticed that tourists in the area were interacting closely with the apes. Despite a ban on doing so, some tourists would feed or touch the semi-wild apes. So Dellatore switched his research to monitoring the behavioural health of the orangutans, following them from dawn till dusk.
During this research he twice witnessed female apes he recognised eating the corpses of their recently deceased babies.
"While following Edita, whose infant had just died in the forest, on the eighth day myself and my assistant Tumino saw her begin to consume the corpse," Dellatore says.
"At first we did not believe it, but there was no mistaking it. Edita was engaging in filial, or mother-infant cannibalism."
"Then a month later I was following Ratna by myself, whose infant had also just died, and observed her also cannibalising her dead infant."
Seeing the first instance surprised Dellatore, while he found the second even more shocking.
"Such behaviour had never been seen before in more than four decades of orangutan research. Surely it's not happening here twice in a one month period?" Dellatore recalls asking himself.
But Dellatore managed to collect further evidence of the second event. "I recovered a fallen piece of the infant's skeleton that Ratna spat out, as well as rather clear video footage of the event."
Dellatore is unsure why the orangutans behaved so. "It makes little evolutionary sense for orangutan females to kill their infants, nor is there any evidence that this happened here," he reports in the journal Primates.
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