Frans de Waal was just starting his career in primate research in the mid-1970s when he watched a male chimpanzee aggressively confront another at a zoo enclosure in the Netherlands. Awhile later, the chimp calmed down and held his palm up, a gesture of reconciliation. The two chimps hugged.

The moment sparked a lifetime exploration, probing the inner lives of chimps, apes and other species for evidence of empathy, morality and sentient awareness — traits long assumed to exist at high levels only in humans. In other words, Dr. de Waal often said, we are not so special.

“I consider human cognition as a variety of animal cognition,” said Dr. de Waal, who died March 14 at 75 at his home in Stone Mountain, Ga. “We are exceptionally smart, but we are not fundamentally different.”

For more than five decades, Dr. de Waal was distinguished for his wide-ranging curiosity — from studying acts of altruism in chimps to questions of fluid gender roles in primates — as well as his storytelling flair.

In more than a dozen books and frequent talks around the world, Dr. de Waal shared anecdotes and his deadpan humor (often mocking ideas of human exceptionalism) while bringing his work to a wide audience.

“I hate the so-called ‘ivory tower’ of science and feel that I have an obligation to communicate with the general public,” said Dr. de Waal, a longtime professor of psychobiology at Emory University in Atlanta and a research scientist at the school’s Yerkes National Primate Research Center (now the Emory National Primate Research Center.)

One of Dr. de Waal’s favorite tales was about Kuni, a bonobo, a primate found in central Africa. A bird slammed into the walls of her glass enclosure at a British zoo. Kuni gently picked up the stunned bird and took it to the top of the tallest tree in her habitat. She unfolded the bird’s wings and set it loose, like a toy airplane. The bird was still too disoriented, and Kuni watched over it for hours until it could fly away.

“[Kuni] tailored her assistance to the specific situation of an animal totally different from herself,” Dr. de Waal wrote in his 2005 book, “Our Inner Ape.”

In other books and lectures, he described problem-solving such as two chimpanzees joining forces to lug a heavy box or signs of apparent compassion with male chimps taking over care for the young when females were absent. He helped popularize the term “alpha male” in primate terms — not as a swaggering victor but as a leader who shows care and wise judgment for the entire group.

An example was how a group of bonobos helped an ailing newcomer named Kidogo adjust to a new zoo setting. “[They] took him by the hand and led him to where the keepers wanted him, thus showing they understood both the keepers’ intentions and Kidogo’s problem,” he wrote.

Dr. de Waal stressed that such behaviors should not be viewed as just simple versions of human interactions. Instead, he said, they should be regarded as a different, but equally rich, array of emotions and social learning that includes passing on knowledge and sharing a sense of community and generational continuity.

“The possibility that empathy is part of our primate heritage ought to make us happy, but we are not in the habit of taking pride in our nature,” he wrote in a 2005 essay in New Scientist. “When people commit atrocities, we call them ‘animals,’ but when they give to the poor, we praise them for being ‘humane.’ We like to claim the latter behavior for ourselves.”

Among some detractors, Dr. de Waal was accused of trying too hard to tease out deeper meanings from his observations of primates. In response, he described his critics as too influenced by concepts — passed down by cultures and religions — that only humans are capable of such higher values.

“One can only shudder at the thought that the humaneness of our societies would depend on the whims of politics, culture or religion,” he wrote in the book “The Age of Empathy: Nature’s Lessons for a Kinder Society” (2009).

His work was more widely seen as advancing research by leading primatologists such as Jane Goodall and evolutionary psychologist Desmond Morris, whose 1967 book “The Naked Ape” outlined compelling comparisons between apes and humans.

More recently, Dr. de Waal found allies among groups seeking “personhood,” legal status for primates and other animals such as dolphins, whales and elephants. “Uniquely human emotions don’t exist,” Dr. de Waal wrote in a 2019 essay in the New York Times. “Like organs, the emotions evolved over millions of years to serve essential functions.”

In 1995, the Republican House Speaker Newt Gingrich (R-Ga.) recommended Dr. de Waal’s book “Chimpanzee Politics: Power and Sex Among Apes” (1982) as reading for freshman GOP congressmen as a guide on how to get ahead. Dr. de Waal lamented that his concepts of “alpha male” had been hijacked by politicians. He became so dismayed during the 2016 election campaigns that he added a clarifying message to one of his books.

“Merciless tyrants do sometimes rise to the top in a chimpanzee society,” he wrote, but added that the most successful alphas “are typically not necessarily the biggest, strongest, meanest ones around since they often reach the top with the assistance of supporters. Most alphas protect the underdog, keep the peace and reassure those who are distressed.”

‘So humanlike’

Franciscus Bernardus Maria de Waal was born on Oct. 29, 1948, in the southern Netherlands town of ’s-Hertogenbosch, also known as Den Bosch, and was raised in nearby Waalwijk. His father was a banker, and his mother was a homemaker.

He began his field studies with macaques, a small primate species found in pockets across Africa and Asia. He received the equivalent of a bachelor’s degree at Radboud University in Nijmegen in 1970 and a postgraduate degree at the University of Groningen in 1973. Two years later, he began researching chimpanzees at a zoo complex in Arnhem, Netherlands, and received a doctorate in biology from Utrecht University in 1977.

He moved to the United States in 1981 to take a take a position at the Wisconsin National Primate Research Center in Madison and subsequently lectured at the University of Wisconsin. In 1991, he became a research professor at Emory and later was named director of its primate center.

In a 2016 interview on NPR’s “The Diane Rehm Show,” Dr. de Waal told guest host Susan Page he believed that zoos, circuses and other captive settings for animals should not be banned but monitored rigorously to ensure adequate care.

“I’m more of what they usually call an animal welfare-ist,” he said. “It doesn’t mean that we need to stop all these things, but we need to really closely pay attention on how we treat animals.”

Dr. de Waal’s wife of 44 years, Catherine Marin, confirmed the death and said the cause was stomach cancer. Survivors also include five brothers.

At the zoo in Arnhem in 2016, Dr. de Waal’s mentor, biologist Jan van Hooff, took the rare step of entering the chimpanzee habitat to comfort Mama, a 59-year-old chimp and the alpha female for four decades in the chimp colony. She was close to death. Mama appeared to recognize van Hooff. She reached out to hug him, draping one hand over the back of his hand and gently tapping his hair.

“So humanlike,” said Dr. de Waal of his friend’s encounter, which became a viral video and inspired title of Dr. de Waal’s 2019 book “Mama’s Last Hug: Animal Emotions and What They Tell Us about Ourselves.”

“People were surprised how humanlike the expression of Mama was and how humanlike her gestures were,” he said on NPR’s “Fresh Air” show. “And that is something that struck me is … everyone knows that chimps are our closest relative, so why wouldn’t the way they express their emotions be extremely similar to ours? But people were surprised by that.”