Japanese researchers said they succeeded in observing how a group of chimpanzees interacted with a disabled infant in the wild over a period of nearly two years, in an experience that was likely a global first.

The group said it located a newborn female chimpanzee with symptoms of Down syndrome and damage to its spine in Mahale Mountains National Park in Tanzania in January 2011. It also appeared to have weak legs, said Michio Nakamura, a member of the group and an associate professor at Kyoto University.

There...