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One of the big questions we raise in comparative psychology is about the main difference between humans and animals. There is an easy short answer to what sets us apart from the rest of the animals: language.

Of course, there is no doubt that animals also communicate with each other. But their communication systems aren’t comparable to the complex human language. However, when we think about the evolution of language, it is very unlikely that our language appeared spontaneously within the six million years after our ancestors separated from chimpanzees. It is much more likely that language evolved from pre-existing features of our common ancestors. Thus, comparative psychologists look for features of human language that are also present in animals.

One of the features that is typical for language is the combination of individual characters according to certain rules. These rules are called syntax. When we put signs together, we give them a new meaning. This happens both on a small scale (for example, when we combine letters to form words) and on a large scale (when we combine words to form sentences). Through the unlimited abundance of possible sign combinations, we can express anything we want!

Do animals use syntax? Klaus Zuberbühler, a Swiss psychologist working on primate communication, has found evidence of it in monkeys and apes. But as one can imagine, the most challenging task for scientists is to infer the meaning of animal calls. Researchers do this with Playback experiments, in which sound recordings are played back for animals to observe their response (Zuberbühler, 2020).

Zuberbühler and his colleagues have, for example, investigated the alarm call system in Campbell’s monkeys. These monkeys use alarm calls to warn against eagles which sound something like “Hok!” However, if an “oo” is added to this sound, i.e., “Hok-oo,” it probably means something like “Enemy in the trees!” Campbell’s monkeys also modify another sound in a similar way. The leopard warning call goes like this: “Krak!” However, when the monkeys call out “Krak-oo!” they mean something like, “Beware!” (Ouattara et al. 2009).

Another monkey species that Zuberbühler investigated are the Putty-nosed monkeys from West Africa. Males of this species emit two different calls that serve as alarm calls. The scientists refer to these calls as “hacks” and “pyows.” These calls do not stand for a specific predator like the calls of the Campbell’s monkeys. But the scientists have observed that predominantly “hacks” are produced when an eagle is in sight, and “pyows” when a leopard is nearby. The interesting thing is that Putty-nosed monkeys combine both calls into “pyow-hack” sequences. For example, one to three “hacks” can be followed by up to four “pyows.” When the sounds are arranged in this sequence, they probably get a new meaning, something like, “Let’s go!” Thus, when the males make these calls, the whole group slowly starts moving (Arnold & Zuberbühler, 2006).

In a very recent study, Zuberbühler and colleagues found syntax also in our closest living relatives, chimpanzees (Leroux et al., 2023). Chimpanzees produce so-called “alarm-huus” when they are surprised. They also use “waa-barks” when potentially recruiting conspecifics during aggression or hunting. Observations have suggested that chimpanzees combine these calls specifically when encountering snakes, producing “alarm-huu-waa-barks.”

To find out whether this is true, Zuberbühler and colleagues tested wild chimpanzees in the Budongo forest in Uganda. They presented a fake snake to a chimpanzee and recorded the calls. Indeed, in nearly half of the cases, subjects produced the combined “alarm-huu + waa-bark” call. When they did, more individuals joined the caller after hearing the combination.

In further experiments, the scientists found out that the meaning of the combination must be “recruitment to a threat.” Thus, in that case, the meaning of the call combination is derived from the meaning of its parts: “alarm-huu” means a potential threat, “waa-bark” means recruitment, and the combination means “recruitment to a threat.” It is likely that the caller is asking his fellows to come to defend themselves against the snake.

Well, animals may not have a language like humans, but would you have expected syntax in the African Rainforest? What Zuberbühler and colleagues conclude is that combined structures may not have evolved in humans in the last 6 million years but that the cognitive preconditions to use syntax have developed much earlier.