At the turn of the 20th century, a famous horse named Clever Hans toured Germany. The horse stunned crowds as his trainer demonstrated the animal’s alleged ability to understand German, tell time and even solve math problems.
But when an independent team investigated Clever Hans, they concluded his act was a fraud. It turned out that the horse actually just responded to subconscious body-language cues from his trainer and couldn’t really comprehend human language or perform arithmetic feats.
Today, researchers are still testing the limits of animals’ ability to understand humans, but examples like Clever Hans show just how difficult that task can be. So do we know if any animals can actually understand human language?
Animal language studies with primates, birds, dolphins, and other animals flourished through the 1960s and 1970s, but skeptics went on to criticize this research, saying the animals were only mimicking their trainers like Clever Hans. Though many animals are adept at picking up on context clues like body language and tone of voice, it’s less clear if they understand the meaning of words or more complex features of language like grammar. But today, research is beginning to suggest that, with training, some animals may understand certain features of human language, such as the sound and meaning of specific words.
“Humans are special in many ways, and certainly language is unique to humans,” Simon W. Townsend, a professor of evolutionary anthropology at the University of Zurich, told Live Science. “But I think more and more, we’re finding that there are really quite some similarities between the communication systems of animals and humans.”
Related: Can animals learn another species’ ‘language?’
Can primates learn human language?
One of the most famous animal language experiments involves Koko, a female western lowland gorilla who learned modified American Sign Language (ASL). Koko, who died in 2018, could use about 1,000 signs and respond to over 2,000 words in spoken English.
However, experts warned that there’s a big difference between learning some modified signs and actually mastering ASL, and Koko was never anywhere near fluent in sign language. Critics also pointed out that Koko sometimes signed nonsensically, and her trainers tended to lean on their own interpretations to make sense of Koko’s signs.
According to comparative psychologist and linguist Michael Tomasello, the title for most advanced language skills in a primate goes to Kanzi, a male bonobo (Pan paniscus) who lived from 1980 to 2025. Kanzi communicated through a lexigram board, a keyboard of about 200 arbitrary symbols that corresponded to things in his environment.
Townsend, who worked with Kanzi, says the bonobo understood the symbols on his lexigram board well and could use them to communicate with his trainers. Kanzi used symbols for people’s names, common objects, actions and locations, and he had vocalizations for “yes” and “no.” He used his lexigram board to ask for objects or request certain actions.
“He’s learned symbolic communication, which is a feature of another species’ communication system,” Townsend said. But, Townsend added, human language is more than just using symbols; it also involves combining those symbols into larger structures to create more complex meaning — a feature of language called syntax. If Kanzi grasped syntax, that would indicate a deeper understanding of human language.
So far, studies have shown that Kanzi could order symbols at a rate better than random chance, but some critics doubt the bonobo’s grammatical abilities. Townsend and other researchers studied Kanzi up until his death to learn the limits of his syntax skills. Their research is currently unpublished.
Kanzi understood some spoken English, too. In a 1993 study, researchers found that when the bonobo listened to brand-new sentences, like “Put on the monster mask and scare Linda,” he could complete the requested task about three out of four times, outperforming a 2.5-year-old child.
Can dogs understand humans?
Most animals have no hardwired evolutionary reason to care about human language. But dogs, who’ve lived alongside humans for at least 14,000 years, have a special motivation to listen to their owners and respond appropriately.
Dogs are highly tuned in to the specific sounds and tones of voice that humans use to communicate. In fact, dogs show interest in human voices and gestures as early as 8 weeks old. Dogs are so used to human voices that they can tell when speech has been scrambled, and neuroscience research even suggests that dogs have their own mental representations of words, suggesting a deeper comprehension of language similar to humans’.
Related: Are dogs smarter than wolves?
Some prodigious dogs may even grasp basic grammar. One of them was Chaser, a highly trained border collie, known as the “smartest dog in the world,” who learned over 1,000 words. According to a 2011 study, she could differentiate commands such as “bring the sock to the ball” and “bring the ball to the sock,” indicating a basic understanding of syntax.
Federico Rossano, an associate professor of cognitive science at the University of California, San Diego, is using button boards — a series of buttons that play a recorded word when pressed — to investigate the linguistic capabilities of our canine companions.
In 2024, Rossano showed that dogs respond to the actual sounds of words and not just to their context, like their owner putting on their shoes while saying it’s time to go outside. In a study of 59 dogs, the canines listened as a researcher pressed buttons with words related to “outside,” “play” and “food,” as well as a nonsensical word to serve as a comparison. The researcher wore headphones so their reactions and body language didn’t affect the dogs’ responses.
Even without these context clues, the team found that when dogs heard the recordings relating “outside” and “play,” they tended to start exhibiting behaviors related to going outside or playing, respectively, such as running to the door or grabbing a toy.
The research is a promising step toward showing that dogs can understand human speech even without the context clues that come along with it. But other studies have found that dogs can’t differentiate between words that differ by a single sound, such as “sit” and “set,” so there may be a limit to how much they can understand.
Rossano emphasized that his studies are not meant to prove that dogs can “talk.” Some Instagram-famous pets have gone viral for seemingly advanced communication through button boards, but these cases aren’t the focus of his studies. For now, Rossano is focused on investigating the basics.
Like other animal language researchers, Rossano still has a lot of questions. He’s currently investigating whether dogs use buttons to refer to people or objects that aren’t currently present, and whether they combine buttons to refer to things they don’t know the names of. Both of these feats would imply more complex cognitive processes.
Federico admits he began the research with some skepticism, but his view of dogs’ language capabilities has “100%” changed since then.
“Maybe there’s more of a mind there than we give them credit for,” he said.
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