Lemurs without teeth and one-armed macaques: How disabled primates survive in the wild

Nico was an adult male blue-headed cercopithecus living on a small island in the Quirimbas Archipelgo of Mozambique. He was one of the few monkeys on the island who had been named, for one simple reason. Most of his peers looked too similar for human eyes to tell them apart. But Nico was different in that he had a completely atrophied hand. It looked as though he had caught his wrist in some kind of wire or rope that had subsequently cut off its blood supply.

He was also one of the first monkeys we met. Nico seemed to be very used to human presence. While the large part of his group of Cercopithecus mitis hunted for seeds on the ground, he approached and gazed at us expectedly. A few days later, we saw a man giving him food. Perhaps this propensity to get close to humans had been occasioned by his injury, or perhaps the contrary was true. It was impossible to know, just like it was impossible to know if he would have been able to survive in a place where there were no humans to feed him every once in a while, or in a place where predators were present.

Nonetheless, I couldn’t help but think about how Nico was able to get through life rather well, given that he had one hand. There seemed to be no significant differences between him and the other males. The idea that natural selection acts as an unstoppable force might cause us to think that animals with physical disabilities have no place in nature; that only humans, thanks to our habit of caring for each other, have escaped this harsh reality. But anyone who has passed enough time with primates in the wild knows that this doesn’t capture their reality.

Since the beginning of field-based primatology, numerous anecdotes of primates with disabilities surviving for years have been reported. Yet, few studies have sought for deeper understanding of their lives. That is, until 2023, when a research team from Canada’s Concordia University decided to analyze and collect all cases of primate disability in a literature review. In total, they examined 114 articles and found some general trends. Although many species are represented in the review, the most frequent are chimpanzees and macaques, probably because they have been the most studied. The vast majority of the disabilities that appear are physical, involving limbs.

Humans are an undeniable presence among the accounts collected in the review. Its authors were struck by how frequent it was to find a relationship between the physical disabilities of primates and the environmental impacts caused by humans. This correlation was found in 60% of the articles. A striking example is found in an account from the Beza Mahafaly Special Reserve in Madagascar, where lemurs (Lemur catta) were losing their teeth prematurely. Among these primates, the hard fruit of the tamarind normally represents a small part of their diet. However, human disturbances on the reserve had reduced the availability of other foods and the lemurs had been forced to include more tamarind fruit in their diet, which was wearing down their teeth faster.

Still, these toothless lemurs and other primates with disabilities found ways to survive. How did they manage to do so? The literature review suggests three factors: flexible behavior that allows them to adapt to their disability, social environment and the potential to create new behaviors.

When the lemurs lost their teeth, they spent more time on eating the hard fruit, licking it before they eat, incorporating more leaves into their diet and looking for fruits that have already been partially processed by another lemur. This is a clear example of how primates are capable of adapting to some anthropogenic disturbances thanks to their flexible behavior.

As for social environment, maternal care in the first months of primates’ lives appears to be especially important. On Japan’s Awaji Island, there is a group of macaques with a high rate of members with disabilities. Between 1978 and 1995, 14% of their offspring were born with some kind of limb deformation. However, the vast majority made it through their first year of life because their mothers were able to compensate for their disability by providing them with extra care and adapting to the needs of their kids.

One of these mothers was Yuki, who was also one of the macaques who had one of the most serious disabilities, having been born with two completely atrophied hands. She had learned to walk on two feet and when climbing trees, she always chose those with a trunk-and-branch structure that allowed her to climb without using her hands. Her child was also born with a disability that prevented her from holding onto her mother’s body, so Yuki transported her by holding onto the small one’s body with her two deformed arms.

Special treatment for infants with disabilities is a widespread behavior among primates, who have extremely rarely been witnessed abandoning their children. Mothers of these infants have been observed spending more time with them and staying close to them for longer periods while their offspring is young. When they are slower than normal, these children are waited on, even when this causes the family to lose contact with the primary group, and they are guided towards sources of water and food when they are blind.

Although they are less frequent, cases of other members of the group helping members with disabilities have also been observed. A macaque from the Awaji island named Maki adopted Meg, a six-month-old orphan with a disability. Later, when Maki became one of the group’s dominant males, he continued to allow Meg to spend time and eat with him.

From disability to innovation

Finally, many cases of new behavior occasioned by disabilities have been reported. Late was another macaque from the same group as Yuki, and she hit upon a way to nurse her daughter Ribbon with minimal effort. Ribbon was born with deformations of the hands and feet and couldn’t grab on to Yuki’s body to breastfeed. So, Late would sit near a rock or tree and squeeze Ribbon between her body and its surface to avoid holding Ribbon up with her arms.

When Ribbon grew up and no longer depended on her mother for food, she continued to find innovative solutions for herself. Macaques use their hands to remove leaf litter and catch invertebrates; but Ribbon, who lacked functional hands, devised her own method. She tapped the ground with her hands and feet to remove leaves and caught invertebrates with her mouth.

On some occasions, this kind of new behavior was adapted by the entire group. Tinka was an adult chimpanzee from a community in Sonso, Uganda who had paralysis in both hands. Normally, chimpanzees use their hands to scratch their back, but Tinka invented his own style. He held a vine with his foot and rubbed against it. It must have been a satisfying way to scratch because, before long, other able-bodied members of the group began to imitate him. Using his ingenuity, Tinka managed to solve a problem specific to him, but in so doing, brought value to the rest of the group.

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