A chimpanzee mother cared for her disabled infant in the wild in Tanzania, Japanese researchers reported in a study published this week, research they hope will help in understanding the evolution of social care in humans.

A team of Kyoto University researchers discovered that a "severely disabled" female chimpanzee baby was born in a group in Tanzania's Mahale Mountains National Park in 2011, and recorded behaviour of the group for about two years.

"The observed infant exhibited symptoms resembling Down syndrome, similar to those reported previously for a captive chimpanzee," they said in an abstract of the study published Monday in the online edition of Primates, an international journal of primatology.

"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" when the infant disappeared and was believed to have died, they said.

Allomothering refers to care of infants performed by those other than the biological mother.

The mother and the sister of the chimpanzee supported its body with their arms when the mother was breastfeeding it, Michio Nakamura, associate professor at Kyoto University's Wildlife Research Center, told AFP on Wednesday.

"Usually, a chimpanzee baby can hang onto their care-giver by itself, but this infant's legs were not powerful enough," he said.

"It is the first time it was observed in the wild that a disabled chimpanzee was receiving social care."

"We believe the study offers a fresh clue as to how human society, which socially cares for disabled members, has evolved," he said.

Signs of social care for the disabled have been discovered in research on human ancestors, and "there has been discussion that the ability to give care was probably obtained when our ancestors became humans," he noted.

The mother of the baby did not allow nonrelatives to take care of the disabled infant even though she had been previously relatively tolerant of allomothering by nonrelatives for her other children, the researchers said.

"Other group members did not show any averse or fearful reactions to the disabled infant," the study said.