Chimpanzees in the wild have been observed caring for a “severely disabled” infant, according to Japanese researchers who say the behaviour may shed light on the development of human social behaviours.
The infant “exhibited symptoms resembling Down syndrome” yet was able to survive for almost two years, the researchers said in a paper published in the journal Primates.
Disabilities included apparent damage to the female chimpanzee’s spine, a sixth finger on her left hand and an inability to sit up on her own or hold on to her mother.
Associate Professor Michio Nakamura of Kyoto University told The Japan Times the infant also exhibited facial expressions consistent with a mental health issue.
The researchers said while congenital disabilities occured in many primate species, there have been no reports examining how a chimpanzee mother copes with such a disabled infant in the wild.
The mother did not allow non-relatives to take care of the infant, despite accepting this help for other offspring, but was assisted by another of her female offspring.
Care provided by an individual other than the biological mother is known as allomothering, and is relatively uncommon among chimpanzees.
"The mother's compensatory care for her infant's disabilities and allomothering of the infant by its sister might have helped it to survive for 23 months in the wild," the researchers wrote.
"The mother scooped the infant up and carried her when moving since she would drop without help," Professor Nakamura told the Japan Times.
"When breastfeeding, the mother raised the infant to her nipple to feed her."
Infant not seen since December 2012
Other members of the chimpanzee group did not show any form of hate or fear towards the infant.
Professor Nakamura suggested that this study could help explain how humans evolved into social animals.
"One characteristic of human society is that people reasonably take care of the disabled and those in vulnerable positions," he said.
"It's interesting to observe a chimpanzee looking after a disabled infant in terms of finding out when such sociality occurs, as they are the closest modern species to humans."
Researchers from Kyoto University have been studying a group of chimpanzees in Tanzania since 1965 as part of the Mahale Mountains Chimpanzee Research Project, and the group in this study was described as "well habituated" to human presence.
Professor Nakamura said the infant had not been observed since December 2012 and was probably dead.
He said this could have been due to malnutrition, since it was not observed eating solid food, or the pregnancy of her elder sister, which would have made it difficult for her to continue providing care.