Rusty has dark-brown fur with reddish tufts. On the day I visited, he was prowling around a fenced-in area with a hammock and a tree stump. When he saw me, he grabbed onto the wires of the fence. His fingers were thick and strong, and he gave me a hard stare. He bared his teeth. He climbed higher on the fence and looked down at me. His face showed pure malevolence.

I'd gone to see Rusty because I'd been working on a story about monkey smuggling. A Cambodian official, Masphal Kry, was charged in November 2022 with assisting in a poaching scheme. Macaques are protected by international trade law. In order for them to be sold in the United States, they require permits that show they were raised in captivity for research -- rather than captured in the wild.

Kry was arrested at JFK International Airport in New York. Prosecutors said he had been helping to sell wild monkeys from Southeast Asia, passing them off as ones raised in captivity. A reporter covering Asia, I had attended his pretrial hearing in Miami in 2023. He was acquitted in March and returned home to Cambodia.

"I hope a monkey will take the stand," I'd told my dad, a retired businessman, on the phone before the hearing in Miami. "And put his hand on the Bible."

"More likely his foot," my dad said.

My dad, John McKelvey, knows about monkeys. He used to be president of a Kansas City company, Midwest Research Institute, where they kept primates in laboratories. As a child, I would visit my dad at work and could hear the animals rustling in their cages. I loved animals, but I understood the reasons for the research done in the labs.

The study of primates helps scientists learn about the aging process and the brain. Research done on rhesus macaques was crucial for the development of the COVID vaccine. Every year, about 70,000 rhesus macaques are used in medical investigations in the United States, according to Science.

I traveled to Locust Grove in the summer of 2023 because I'd heard so much in the courtroom about the macaques and wanted to see one in real life. I also spoke with scientists about them. One of the scientists, Charles Murry, a University of Washington professor who does heart research, told me what it was like to walk into an operating theater with a monkey surgically draped. He spoke about how important it was to "honor" the animal.

"Nobody does this lightly," he said. But in the end, he said, he needed the monkeys for research because their hearts are so similar to ours.

I guess that's why it was hard to see Rusty with his face twisted in rage and now to hear about the monkeys in South Carolina trying to escape. It makes me wonder about artificial intelligence and whether it will someday replace the monkeys in medical research. In the future, scientists might be able to use machine learning models, employing large datasets instead of monkeys, to see which drug compounds work and which ones are toxic. Scientists may also someday be able to use AI to gather large datasets of human cells, enough to help them determine whether a drug compound works. Then they would not need to use monkeys for their research. When you've seen the monkeys up close, it's hard to shake the feeling that they're so much like us, they shouldn't be subjected to these procedures.

A friend of mine, a Polish scientist, once told me about his work with monkeys. He said that he'd gone to his lab one night to pick up something he'd left behind. He turned on the lights and looked around the room in stunned silence. The monkeys were crouching in their cages with their arms sticking out of the bars. Holding hands, they had formed a circle.

After that, he said, he gave up studying monkeys and focused instead on another subject: snail brains.