Dr. Alan Robert Rabinowitz

Growing up with a severe stutter, Alan Rabinowitz felt damaged and alone. Talking to adults had become so traumatic, he often said, he found solace by retreating to his bedroom closet, where he kept a chameleon, snakes, a turtle and a gerbil. Speaking to his pets came more easily. They did not make him feel worthless, and were as voiceless as he was in the human world.

On visits to the Bronx Zoo with his father, Alan gravitated to the old Lion House, with its rows of big cats roaring inside their cages. He found joy in talking to them as well, especially an old jaguar, who was wary and watchful.

“I would sit and whisper to this jaguar, outpouring all my emotions,” Dr. Rabinowitz said in an interview with Natural World Safaris, a tour operator, “and I promised that if one day I found my voice I would become their voice.”

He kept that promise.

With therapy he found his voice — a particularly forceful and articulate one — as a leading big cat conservationist for the Wildlife Conservation Society, which runs the Bronx Zoo, and, since 2006, for Panthera, the wild cat conservation organization that he co-founded.

Mr. Rabinowitz established the world’s first jaguar preserve, in Belize, and a vast tiger preserve in Myanmar. His radio telemetry research on the Asiatic leopard, Asian leopard cats and Asian civets at a wildlife sanctuary in Thailand helped determine how much space each species needed to live and reproduce and led to its designation as a Unesco World Heritage site.

When Mr. Rabinowitz died on Sunday at 64 in a hospital in Manhattan, he had been working for more than a decade on a complex initiative to protect and connect jaguars as they move through human populations and landscapes from Mexico to Argentina.

Howard Quigley, a friend and executive director of Panthera’s jaguar program, said the cause was lymphatic cancer, which had spread to his lungs. Dr. Rabinowitz had been given a diagnosis of chronic lymphocytic leukemia in 2001. He lived in Mahopac, N.Y., in Putnam County.

Dr. Rabinowitz, who had the reputation of a swashbuckler, traveled to jungles, rain forests and mountaintops. He mapped habitats diminished by development; negotiated with governments, some of them dictatorships, to provide safe swaths of land to preserve the wild cats; and argued that their preservation meant saving whole ecosystems.

“He raised a lot of consciousness on behalf of wildlife, not just big cats,” George Schaller, a prominent wildlife conservationist and member of Panthera’s science council, said in a telephone interview. “He made people realize that these are beautiful animals and that they, and their habitats, are threatened and you have to fight for them.”

He played down Mr. Rabinowitz’s swashbuckler reputation as a media invention, calling him a “hard worker who liked to be out in remote areas and studied what people thought were dangerous animals.”

He also became a prolific storyteller, describing his work in many scientific articles and books, including “Beyond the Last Village: A Journey of Discovery in Asia’s Forbidden Wilderness” (2001), “Life in the Valley of Death: The Fight to Save Tigers in a Land of Guns, Gold, and Greed” (2008) and “An Indomitable Beast: The Remarkable Journey of the Jaguar” (2014).

It was clear that Dr. Rabinowitz preferred being in the field, tracking a jaguar or a tiger, to being in a city. Despite the dangers of his work — from disease, attacks by predators and iffy airplanes — he reveled at being in proximity to a big cat.

In 2011, he took a crew from CBS’s “60 Minutes” to search for jaguars in the Brazilian wetlands near Bolivia.

“Look at her,” he said excitedly as a camera spotted a jaguar during a nighttime hunt. “God, she’s beautiful.”

Alan Robert Rabinowitz was born in Brooklyn on Dec. 31, 1953. His father, Frank, was a high school physical education teacher, and his mother, Shirley (Felman) Rabinowitz, was a homemaker. His stutter led public schools in Far Rockaway, Queens, to put him into classes with children with Asperger’s syndrome, dyslexia, attention-deficit/hyperactivity disorder and other conditions that other children around him often mocked. He underwent hypnosis and shock therapy and was given drugs.

“It made me realize that adults thought I was broken, so I gave up trying to communicate with them,” he told Publishers Weekly in 2014. He added, “I have no memories of being able to speak without severe disfluency, and I remember a childhood filled with fear and pain.”

He found relief when he was 18, at a clinic in upstate New York, where he learned to speak fluently. He graduated from Western Maryland College (now McDaniel College) in Westminster, Md., with a bachelor’s degree in biology and chemistry.

At the University of Tennessee, where he studied black bears, raccoons and bats, he earned a master’s and a Ph.D. He wrote his dissertation about the ecology of the raccoon in Great Smoky Mountains National Park.

Dr. Rabinowitz was a research fellow at the Wildlife Conservation Society when Dr. Schaller, who was a top executive there, suggested that he go to Belize to study jaguars.

“He had a vision for himself that he hadn’t realized,” Dr. Schaller said. “When you meet someone like that you have to give him a try.”

That set Dr. Rabinowitz on a path of exploration and adventure, one that dealt with not only jaguars, lions and tigers. In northern Myanmar, for example, he discovered a previously unknown species of deer, the leaf muntjac, and in the Himalayas he met the last known Mongoloid pygmies in the world, called the Taron.

Recalling his meeting with one pygmy, Dr. Rabinowitz said he had communicated nonverbally with him.

“He started making gestures about young children, which I didn’t quite understand at first,” he said in a 2013 interview with the On Being Project, which focuses on subjects involving moral imagination and social courage. When he realized that the man had asked him why he had no children, Dr. Rabinowitz answered through a translator, “Why do you assume I have no children?”

The man replied, "Because you act like a man who still has this deep, deep hole inside of him.”

The conversation led him to think differently about his family and to decide with his wife, Salisa Rabinowitz, to have children. (He and Salisa Sathapanawath had met in Thailand, where Dr. Rabinowitz was giving a lecture at the university she was attending. They married in Thailand in 1992.) They had a daughter, Alana, and a son, Alexander.

His children and his wife survive him, along with his sisters, Susan Klein and Sharon Zivi

Some years after persuading the government of Belize to set up the jaguar preserve, the Cockscomb Basin Wildlife Sanctuary, Dr. Rabinowitz was walking through it when he saw the tracks of a large male. He began to follow it until it was getting dark — and realized that it was lurking behind him.

Terrified, he thought to make himself small and unthreatening. “So I squatted down and I was expecting the jaguar — hoping the jaguar — would just walk off,” he told On Being. “Although I loved watching it, I was also scared. And the jaguar just sat down. And he just sits there on the trail, the trail I have to go back on. Sitting there, looking at me.”

Still uncertain how to proceed, Dr. Rabinowitz stood and fell on his back, thinking he was now easy prey. “The jaguar let out kind of a guttural growl and stood up and walked toward the forest,” he said. “And right before it went into the forest, it turned and it looked back at me for a few seconds and our eyes met. And I remember that look so clearly from the cages in the cat house at the Bronx Zoo.”

In “A Boy and a Jaguar” (2014), an autobiographical children’s book, Dr. Rabinowitz wrote a coda to the jaguar encounter.

With an illustration of him and the jaguar looking peacefully at each other, he wrote: “We are both whole. We are both at home.”

Then, he added, he leaned toward the jaguar and whispered to it, “Thank you.”