Physical disabilities among apes are unfortunately not rare, with many losing hands and feet to snares left by hunters in the forests to catch other animals. But congenital disabilities, or those present from birth, are far less well documented, especially for those in the wild. Now, a new study published in Primates details the social and behavioral development of a wild chimpanzee thought to be born with "symptoms resembling Down syndrome" over her two years of survival, and how her mother coped.
The researchers, who were studying a group of habituated chimpanzees living in Tanzania’s Mahale Mountains National Park, didn’t at first notice anything wrong with the infant, named XT11. But at around six months old, they observed that while her brother became much more active at this age, she showed little change. She remained behaviorally and socially limited, unable to support herself and still totally reliant on her mother for both transport and food when other similar-aged infants were much more independent.
Along with the presumed mental disability, XT11 also had some physical abnormalities. She had what appeared to be a hernia on her belly, which grew and shrunk, as well as apparent damage to her spine, an additional finger on her left hand, and she often kept her mouth half-open. In spite of this, her mother quite incredibly managed to keep her alive for almost two years.
A series of images showing the disabled infant XT11 with her half-open mouth (a), her hernia (b), lack of activity (c), extra digit (d and e), and bald patch down the length of her spine (f). Matsumoto et al. 2015
And it was this behavior of XT11’s mother, a 37-year-old chimpanzee called Christina, that was of most interest to the researchers. No studies had previously examined how a mother chimpanzee would cope with a disabled child in the wild. They found that not only did she adapt her own behaviors to help support her infant, such as helping it suckle, but she even gave up others, such fishing for ants. This, in addition to the fact that Christina’s other daughter also helped care for the disabled sibling, meant that XT11 survived for 23 months, way beyond what might have been expected.
Because the poor infant couldn’t move independently, and continually clung to her mother, Christina had to adapt her locomotion to become tripedal when climbing, constantly keeping one hand on XT11 for support. This meant that Christina had to give up trying to fish for ants – a tasty treat for any ape – when in the trees because she couldn’t hold XT11 and fish at the same time. It could have been this limitation that meant that the infant's sibling took over the care occasionally, allowing Christina the chance to forage unhindered.
The research gives some hint into social caring among great apes. Interestingly, Christina would only let relatives care for XT11, possibly, as the researchers suggest, because she understood that the infant required extra care. It could have been this reluctance to let others look after the disabled infant that was the unfortunate end of her, because as soon as her sister had her own baby, she would no longer care for XT11, and perhaps Christina could not care alone.