Toothless Lemurs or One-Armed Macaques: How Disabled Primates Survive in the Wild | Science

Nico was an adult male blue-banded cercopithecus who lived on a small island in the Quirimbas Archipelago in Mozambique. He was one of the few apes on the island that had a name, for a simple reason: all the individuals were too similar to identify them individually. But Nico was different because he had a completely atrophied arm. It looked as if his wrist had gotten caught on some kind of wire or rope that had cut off his blood supply.

He was also one of the first monkeys we met, as he was very used to people. Although most of his group Cercopithecus mitis I was looking for seeds on the ground, he came up to us and looked at us expectantly. A few days later we saw a man feeding him. Perhaps it was this tendency to approach people that led to his injury, or perhaps it was the other way around. We couldn’t know. And if he couldn’t survive in another place where there were no people to feed him from time to time, or where there were predators.

But he couldn’t help but think that Nico was perfectly capable of using one hand, and that there weren’t many differences between him and other men. The idea that natural selection acts as an irresistible force might lead us to believe that animals with physical disabilities have no place in nature; and that only humans, because we care about each other, have escaped this reality. But anyone who has spent enough time with primates in the wild knows that this isn’t necessarily true.

Since the advent of field primatology, numerous anecdotes have been reported of primates with disabilities surviving for many years. Despite this, there are few studies devoted to the subject. For this reason, in 2023, a research team from Concordia University (Canada) decided to analyze and collect all cases of disabilities in primates in a bibliographic review. In total, they examined 114 articles and identified some common trends. Although many species are represented, chimpanzees and macaques are the most common, likely because they have been studied the most. The vast majority of disabilities were physical and affected the limbs.

Once again, humans enter the picture. The review authors were struck by how often they found links between primate disabilities and human-induced environmental changes. In fact, this was the case in 60% of the articles. A striking example is what happened in the Beza Mahafaly Special Reserve in Madagascar, where lemurs (Lemur catta) lost their teeth prematurely. These primates normally rely on hard tamarind fruit for their diet. However, due to human intervention in the reserve, the availability of other food has decreased, forcing the lemurs to include more tamarind fruit in their diet, causing their teeth to wear out faster.

However, toothless lemurs and other primates with various disabilities survive. How do they do this? A review of the literature suggests three reasons: flexible behavior that allows them to adapt to disabilities, their social environment, and the potential to create new behaviors.

When the lemurs lost their teeth, they spent more time eating hard fruits, licking them before eating, included more leaves in their diet, and sought out fruits that had been partially processed by another lemur. This is a clear example of how primates can adapt to some anthropogenic impacts through flexible behavior.

In terms of the social environment, care for mothers in the first months of life is especially important. Awaji Island, Japan, is home to a population of macaques with a high percentage of disabled people. Between 1978 and 1995, 14% of babies born had some form of limb malformation. However, the vast majority survived to age one because their mothers were able to compensate for their disabilities by providing more care and adapting to the specific needs of their offspring.

One such mother was Yuki, one of the macaques with the greatest disability, as both her arms were completely atrophied. She learned to walk on two legs and climb trees. She always chose those that had a structure of the trunk and branches that allowed her to climb without the help of her hands. Her child was also born disabled and could not cling to the mother’s body, so Yuki carried her, pressing her to her body with two deformed arms.

Special treatment of disabled infants is common among primates, with abandonment being extremely rare. The mothers of these infants have been observed to spend more time with them and remain with them for a longer period of their infancy. They also wait for them when they are slower than usual, even if this means they lose contact with the main group, and guide them to water and food sources when they are blind.

Although rare, there are cases of other group members helping disabled individuals. A macaque on Awaji Island named Maki adopted Meg, a seven-month-old disabled orphan. When he later became the dominant male in the group, he continued to allow Meg to accompany him and feed with him.

From Disability to Innovation

Finally, many cases of new behaviors caused by disabilities have been reported. Late was another macaque from the same group as Yuki, who developed a way to breastfeed her daughter Ribbon without much effort. Ribbon was born with malformations of her arms and legs and was unable to latch onto her mother’s nipple. So Late would sit near a rock or tree and squeeze Ribbon between his body and the substrate so that he didn’t have to support her with his hands.

When Ribbon grew up and no longer depended on her mother for food, she managed too. Macaques use their hands to clear away fallen leaves and catch invertebrates; but Ribbon, lacking working hands, developed his own method. He slammed his hands and feet into the ground to pluck leaves and used his mouth to catch invertebrates.

Sometimes new behaviors spread throughout the group. Tinka was an adult chimpanzee from the Sonso community in Uganda who was paralyzed in both arms. Chimpanzees usually scratch their backs with their hands, but Tinka invented a different way. He held a vine with his foot facing outward and rubbed it. It must have been a good way to scratch himself, because soon other able-bodied members of the group began to imitate him. By using her ingenuity, Tinka had managed to solve a personal problem and add value to the group.

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