This footage shows a 20-year-old female orangutan called Ratna consuming the corpse of her dead seven month old infant
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.
Such behaviour has never been seen before in more than four decades of orangutan research
Conservationist David Dellatore, who witnessed the incidents
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.
A grainy image of Edita with the corpse of her infant (D. Dellatore)
"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.
But he points out that it is not uncommon for orangutans and other nonhuman primate mothers to carry their deceased infants. "It may be part of a grieving process," he says.
Indeed, Edita, a 23 year old female, carried and protected the body of her one year old infant for seven days, occasionally inspecting it while vocalising a whimper. Only on the eighth day did she start to consume it, when it was already heavily decomposed. Twenty year old Ratna's seven month old infant appeared unwell a few days before death.
Dellatore is reluctant to make any definitive claims as to why the behaviour occurred. But he suspects that the mothers' stressed upbringing may have triggered their later actions.
"Semi-wild orangutans are all exposed to considerable traumas, such as witnessing the deaths of their own mothers," he says. To feed the pet trade, an orangutan is often captured from the wild as an infant, with its mother being killed as she would not otherwise let her baby go. Captive orangutans also suffer long periods of social isolation.
"Studies have shown that early social deprivation can have deleterious effects on later levels of cognitive ability. It is possible that the cannibalism events are an extension of these effects," he says.
Although rare, mothers have been recorded cannibalising their infants in a few species of monkey. In galagoes, another primate species also known as bushbabies, the behaviour has been linked to stressful living conditions.
The presence of tourists may also be stressing the apes.
Dellatore supports proper ecotourism in the area, which can bring in important funds that can help conserve the great apes. But he says too many tourists visit and interact with the apes without a sense of environmental or social responsibility.
His organisation is running an ecotourism development programme in Bukit Lawang to try and mitigate these problems.