On Cayo Santiago island, scientists track the alliances and power struggles of a colony of feral monkeys — collecting data to generate new insights into the social challenges that people with autism face.
41N sits alone under the waxy leaves of a bay rum tree. When he hears us approaching, he looks up from the ground and smacks his lips as though he has peanut butter on the roof of his mouth. The gray hair on his chest hides his identifying tattoo, but this 16-year-old rhesus macaque is hard to mistake: He has freckles on his face and smears of red extending horizontally from his eyes like war paint. Because March is the breeding season here on Cayo Santiago — Puerto Rico’s so-called ‘Monkey Island’ — his face and genitals are tinted a garish coral hue.
For Michael Platt, a neuroscientist who is normally stuck in his lab at the University of Pennsylvania in Philadelphia, this moment was the first time he was able to match a face to a gene. 41N is one of 40 macaques on Cayo Santiago that Platt’s group identified last year as carrying a naturally occurring variant of a gene called SHANK3. This gene codes for a protein that strengthens the connections between neurons. About 1 percent of people with autism have a mutation in SHANK3; on Cayo Santiago, potentially one out of every eight monkeys possesses this SHANK3 variant. As in people, a disruption of this gene affects the monkeys’ social lives. “Are those autistic monkeys?” Platt says, immediately answering his own question with a scientist’s allergy to hype: “I don’t know.”
What makes Cayo Santiago special is that it is a haven for social diversity. Most traditional research colonies would have weeded out uncooperative monkeys or ones with behavioral issues, but the monkeys here are left to themselves. The island was first stocked with macaques from India for medical research in the late 1930s, and now has more than 1,500 crammed into an area the size of around eight city blocks. The primate population density here rivals that of the New York City metropolitan area. The monkeys, slightly larger than house cats, make friends, raise families and mourn the loss of their loved ones. They have formed six tightly knit groups, and within those groups there are defined pecking orders. The social butterflies, for instance, spend time with other social butterflies — forming a popular in-group.
41N and his ilk are neither more nor less gregarious than run-of-the-mill macaques. Rather, they are more adventurous when it comes to picking their friends. Instead of joining a pre-existing social clique, they form relationships with monkeys that don’t necessarily spend time with one another, creating a bridge between cliques. That may be because they’re not skilled at reading social cues, or because they simply choose to find friends outside the norm. It’s too early to tell.
In 2007, Platt launched a wide-ranging research effort here to elucidate the role that genes and the environment play in shaping these animals’ social lives. Over the next five years, he hopes to bring monkeys with natural or engineered genetic variants into the laboratory, probing their atypical brains and testing drugs in experiments that would be impossible to conduct with people.
A decade ago, few in the research community would have anticipated that some of the most intriguing advances in understanding autism might come from feral monkeys on a Caribbean island. Monkeys are not the most convenient research animals: They are expensive to raise and take a long time to study. Many consider it unethical to conduct research — invasive or not — on primates. The result has been that most autism research is conducted using rodents as models.
But Platt and other researchers have begun to draw attention to the limitations that come with that mouse-sized package, namely that rodent brains are different from our own. Mice don’t form societies, and the critical bond between mother and child concludes at weaning. These differences may explain why the majority of clinical trials for neurological drugs based on mouse studies have failed.
Platt’s work has focused on the descendants of wild macaques on Cayo Santiago and in his Philadelphia lab, but he also has plans to study transgenic macaques in China whose genomes have been modified with a man-made mutation in SHANK3 along with the top autism gene, CHD8. A handful of other such transgenic monkeys have recently been engineered for autism research, promising insight into genetic influences on brain development and, potentially, a new platform for testing drugs that influence social behavior. “Mice are great for a lot of things,” Platt says, “but to study social behavior, you really have to study primates.”
In 2013, as Platt’s team was busy collecting field data in Puerto Rico, a monkey unlike any other was born in a laboratory in the southern Chinese city of Kunming. An independent team of researchers there had inserted a mutated gene, MeCP2, into the one-celled zygotes of rhesus macaques and crab-eating macaques, another southeast Asian primate. Mutations in this gene cause most cases of Rett syndrome, which has similarities to autism and can be difficult to study in rodents. Mice with mutations in MeCP2 are anxious and eventually develop seizures, but they don’t look much like people with the syndrome.
One female macaque was born 162 days later, becoming the first-ever transgenic monkey model for autism, but the researchers haven’t described its behavior. In February, a second group of Chinese researchers used a different technique to create macaques with multiple copies of the human MeCP2 gene. MeCP2 duplication syndrome, like Rett syndrome, shares fundamental features with autism. The researchers have reported that the mutant monkeys pace in circles and let out anxious grunts. They also seem to be less social than controls: In a rudimentary behavioral test, the researchers found that the transgenic animals spend less time with other monkeys than controls do.
These tools are part of a resurgence in interest in monkey models, particularly as the gene-editing technique known as CRISPR has allowed quick and precise engineering. Guoping Feng, a neuroscientist at the Massachusetts Institute of Technology, has created a mouse in which SHANK3 has been rendered inoperative with a point mutation. He plans to insert the same mutation into the common marmoset, a palm-sized New World monkey with a white shock of fur around each ear. Marmosets weigh less than a pound and converse with each other using distinctive squeaks. “We say they are talking,” Feng says.
They are also fast breeders: Each pair produces twins or triplets twice a year. Over the past two years, Feng has bred a colony of 120 animals in Cambridge, Massachusetts. “We cannot say that the marmoset will be a better model than the mouse,” he says. “We don’t have proof of that yet, but based on the structure of their brains and their evolution, we think so.”
Platt’s group has also begun working with transgenic monkeys. In May, one of his graduate students visited the Chinese Academy of Sciences, where collaborators have engineered crab-eating macaques with a SHANK3 mutation and are making one with a mutation in CHD8, the top autism candidate gene.
Laboratory monkeys are also being recruited in basic research studies looking at how drugs may affect social behavior. Katalin Gothard, a researcher at the University of Tucson in Arizona, has identified neurons in the monkey amygdala — an emotional center of the brain —that respond specifically to eye contact. Gothard has shown that the strength of the gaze-following reflex in one monkey depends on the facial expressions of the monkey it is watching in a video. In a study submitted for publication, she found that giving male macaques oxytocin increases their likelihood of gaze-following. Gothard says oxytocin could ultimately help people with autism connect to their families and communities.
Platt has seen this struggle to connect firsthand as he has become more involved in the autism community. In November 2014, he visited the Marcus Autism Center in Atlanta, where he met children who had been placed in padded rooms and restrained in order to prevent them from punching themselves in the face. Platt says he hopes his work will directly improve the lives of people with autism, but Cayo Santiago has also given him a lens to think about the spectrum in the context of evolution.
The close monitoring of monkey social life — Platt’s boyhood passion — continues to elicit deep questions. One of the most remarkable monkeys on the island is a male named Pinocchio, who sits near the bottom of the dominance hierarchy, ranked 39 out of 48 males in the powerful Group R. He was originally born into group F in 1999, but has bounced in and out of it twice over the past six years. Pinocchio is one of the floaters that have been observed since the island’s early days but never formally studied. Although Pinocchio has fathered a few children, he is no longer able to reproduce due to an injury, possibly from a fight. He spends a lot of his time alone, which is where we find him.
“Is living outside of groups adaptive?” Platt wonders. His research has been built on the idea that the failure to develop social behavior could be a pathology. But he says he can also see how living outside of the group has some advantages, particularly for a low-ranking male such as Pinocchio.
All that time alone over the years has given Pinocchio time to innovate: He is the only monkey on the island who knows how to open a coconut. When he finds one, he goes down to the concrete dock and tosses it up in the air again and again for hours until it finally breaks open. For this particular monkey, solitude holds sweet rewards.