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While laughter is often considered uniquely human, tied to language and sense of humor, all great apes produce remarkably similar vocalizations during play that share evolutionary origins with human laughter.

In a new study, an international team led by two Indiana University researchers has discovered that bonobos, our closest living relatives along with chimpanzees, tend to be more optimistic after hearing the laughter of their fellow apes—and these findings have implications for understanding the evolution of positive emotions in primates, including humans.

The study, titled "Bonobos tend to behave optimistically after hearing laughter" and recently in Scientific Reports, reveals that laughter seems to enhance positive emotions in bonobos, just like in humans, and influence their decision-making to expect more positive outcomes.

This research is the first to demonstrate that great ape laughter may have cognitive and emotional effects similar to those seen in humans. Sasha Winkler, a Visiting Research Scholar in the Cognitive Science Program in the College of Arts and Sciences at Indiana University Bloomington, and Erica Cartmill, Professor of Anthropology and Cognitive Science in the College, authored the study with collaborators Isabelle Laumer from the Max Planck Institute of Animal Behavior in Germany and Heidi Lyn from the University of South Alabama.

In the study, the researchers conducted a cognitive bias test—a method in animal psychology that assesses mood by seeing whether subjects interpret uncertain situations more positively or negatively—to determine if hearing laughter would make bonobos more likely to approach ambiguous stimuli, a behavioral indicator of optimism.

"We know that other apes, like chimpanzees, have contagious laughter during play," said Dr. Winkler, the lead author of the study. "We were wondering if that behavior could be explained by positive emotions produced from the sound itself."

The study authors first trained bonobos to approach black boxes that always contained food rewards and to avoid white boxes that were always empty. After hearing either bonobo laughter or a control sound, the bonobos were presented with gray boxes they had never seen before. When humans and animals are in positive moods, they are more optimistic, meaning they should anticipate greater rewards when faced with ambiguous cues like the gray boxes.

"Think of it like the rose-colored glasses effect," explained Winkler. "The bonobos were much more likely to approach the gray boxes after hearing laughter, treating them like the rewarded boxes, and indicating a more optimistic expectation of finding a treat."

This study provides the first experimental evidence that great apes not only produce laughter, but also experience emotional changes after hearing it, revealing deep evolutionary roots of the cognitive effects of positive vocalizations.

"The tendency to behave more optimistically after hearing laughter suggests that the sound alone induced a positive emotional state in bonobos," noted Professor Cartmill. "This is the first study of which we're aware to measure a positive affect shift in non-human primates from a brief experimental intervention."

Great apes, including bonobos, chimpanzees, gorillas, and orangutans, all produce vocalizations resembling human laughter during play. Previous research using acoustic analysis suggests that these vocalizations all share an evolutionary origin with human laughter. In other words, human laughter likely evolved from a sound made during play in a common ancestor of all the great apes.

These new findings go beyond behavioral and acoustic similarities to reveal shared cognitive and emotional effects of laughter.

"Our results suggest that laughter in other apes shares not only phylogenetic and behavioral similarities with human laughter but also perhaps some of the same cognitive-emotional underpinnings," said Winkler. "This emotional contagion appears to have been present in the primate lineage long before the evolution of language."

The findings are significant because emotional contagion—the tendency to "catch" emotions from others—is thought to be an important aspect of empathy. According to Winkler, "Studies like ours can help to untangle the evolutionary building blocks of empathy, communication, and cooperation in humans."

This study is part of growing research into positive emotional states in animals, a relatively new area of research.

"Our emotions influence many aspects of cognition, including memory, attention, and decision-making," said Cartmill. "But research has historically focused on negative emotions with clear behavioral correlates, like fear and aggression. We wanted to better understand the relationship between positive affect and cognition in our closest living relatives."

The researchers hope their findings will encourage more comparative studies on positive affect.

The research was done with bonobos living at the Ape Initiative in Des Moines, Iowa, and run by Winkler during her doctoral studies at UCLA. Among the four subjects was the bonobo Kanzi, who recently passed away and was famous for his ability to communicate using keyboard language symbols.

"I feel incredibly grateful to have had the opportunity to work with Kanzi while he was still alive," said Winkler. "We hope this brings greater public awareness of the remarkable similarities between us and bonobos, who are an endangered species. We have so much to learn from these incredible animals."