Chimps are a lot like humans in many ways. We share most of our genome, our skeleton, and our fondness of socialising. Congenital disorders - like Down syndrome - are, sadly, another thing we both share.
1 in ~175 chimp births results in a congenital condition. This can include things from spinal issues that prevent them using their legs. Or a suite of biological and behavioral issues similar to those observed in human Down syndrome.
In most cases these sorts of conditions result in a degree of social stigma amongst other chimps. In captivity they're often abandoned and their mothers' have to be persuaded to look after them. Even then, few live past infancy.
However, in the wild things may be different. Primatologists have documented how wild groups treat an infant with Down syndrome-like conditions for the first time. They found there was very little of that "social stigma" seen when this happens in captivity.
The infant
The infant in question has the catchy name of XT11 (wild chimp babies have a high mortality, so aren't given a full name until they grow up). XT11 was born in 2011 to Christina, who had already had five children before.
However, within a couple of months it became apparent that XT11 wasn't like Christina's other children. She was noticably smaller and weaker; and kept letting go of Christina whilst she was trying to hold on. As she grew up, her behaviour developed a lot more slowly than other chimps. She had trouble sitting up until she was 20 months old (most chimps can do it by 6 months). In general, XT11 was a lot less active than other chimps as well.
And that isn't the end of her list of issues. She also had a thinner coat (with a rather noticable bold spot), an extra finger and a mass on her abdomen. This was likely a hernia.
These issues are all similar to chimps in captivity with Down syndrome-like condition. However, the authors couldn't confirm that XT11 had this (since they couldn't interfere with the wild population to carry out the tests).
The parent
In captive chimps, these sorts of congential disorders are often a major problem. Another case of a Down syndrome-like condition resulted in the child being neglected by their mother. In the end, they had to be raised by humans.
Yet Christina didn't display any sign of neglect or mistreatment towards XT11. She cared for her just like she did any of her other children. She had to adapt her behaviour to deal with the fact that XT11 needed more help to survive and wasn't comfortable in the normal positions. For instance, XT11 couldn't hold onto Christian properly, so she held her underneath her body instead. Sometimes this did cause issues for Christina, making it harder to climb trees and so forth. Yet she still treated XT11 right.
Except in one way. Whilst most chimps are happy to let others aid their kids; Christina only let family members handle XT11. But this isn't because other members of the group mistreated her. They showed no signs of fear or neglect towards XT11. Perhaps it was Christina recognising the extra level of care XT11 needed.
The moral?
XT11 was treated normally by Christina and her group until she died at 23 months old.
Since this is essentially an anecdotal report, we can't draw an grand inferences from this case. It doesn't hold the key to understanding chimps, human evolution, or anything like that. However, the fact that this case is so different from the cases in captivity reinforces the idea that maybe captive apes aren't the best representation of their species.
Reference
Matsumoto, T., Itoh, N., Inoue, S., & Nakamura, M. (2015). An observation of a severely disabled infant chimpanzee in the wild and her interactions with her mother. Primates, 1-5.
