Crows can be among the most intelligent animals in the animal kingdom. They are capable of making rule-guided decisions and of creating and using tools. They also appear to show an innate sense of what numbers are. Researchers now report that these intelligent birds can understand recursion, which is the process of embedding structures into other, similar structures. This ability was long believed to be unique to humans.
Recursion are key features of language. It allows us to create complex sentences from simple ones. Consider the sentence “The cat chased the mouse.” Psychologists believed that recursion was a human trait. Some believed it was a key feature that distinguished human language form other forms of communication between animals. However, questions remained about this assumption. “There’s always been interest in whether or not nonhuman animals can also grasp recursive sequences,” says Diana Liao, a postdoctoral researcher at the lab of Andreas Nieder, a professor of animal physiology at the University of Tubingen in Germany.
In a study of monkeys and human adults and children published in 2020, a group of researchers reported that the ability to produce recursive sequences may not actually be unique to our species after all. Humans and monkeys were presented with a display that featured two pairs of bracket symbols in random order. The subjects were instructed to touch the bracket symbols in the order of a “center embedded” recursive sequence, such as () or (). After giving the correct answer, the subjects received verbal feedback and the monkeys were given small amounts of food or juice. The researchers then presented the subjects with a new set of brackets. They observed how often they arranged them in an recursive fashion. The experiment resulted in two monkeys that generated recursive sequences much more often than the nonrecursive sequences ( ), but they had to be trained again. One of the monkeys generated recursive sequences in approximately half of the trials. Three- to four-year-old children, by comparison, formed recursive sequences in approximately 40 percent of the trials.
This paper inspired Liao and her collaborators to investigate whether crows might also have the ability for recursion, given their cognitive skills. Adapting the protocol used in the 2020 paper, the team trained two crows to peck pairs of brackets in a center-embedded recursive sequence. The researchers then tested whether the birds could spontaneously create such recursive sequences using a new set symbols. The crows performed as well as children. The birds produced the recursive sequences in around 40 percent of trials–but without the extra training that the monkeys required. The results were published today in Science Advances.
The discovery that crows can grasp center-embedded structures and that they are better at doing so than monkeys “is fascinating,” says Giorgio Vallortigara, a professor of neuroscience at the University of Trento in Italy, who was not involved in the work. He also suggests that these findings raise questions about the possible use of this ability by non-human animals. He says that they don’t seem to have any language similar to humans, so recursion could be relevant to other cognitive functions. One theory is that animals might use the recursion to show relationships within their social networks.
When the 2020 study on recursive capacities in humans and monkeys was published, some experts remained unconvinced that the monkeys understood recursion. Instead, some argued, the animals chose the recursive sequences by learning the order in which the brackets were displayed. If the training sequence was [( ) ], then the monkeys would choose a bracket pair they knew from training and then choose the new bracket pair that they hadn’t seen before. They would then choose the matching bracket from their training session at the beginning of the sequence. This was because they had learned that it is at the end.
To overcome this limitation, Liao with her colleagues increased the number of sequences from two to three pairs. Liao states that three pairs of symbols reduces the likelihood of creating the sequences without understanding the underlying concept. The researchers also found that birds were more likely to choose center-embedded answers.
Some scientists remain skeptical. Arnaud Rey, a senior researcher in psychology at the French National Center for Scientific Research, says the findings can still be interpreted from a simple associative learning standpoint–in which an animal learns to link one symbol to the next, such as connecting an open bracket with a closed one. He explains that the key reason lies in the study design. The researchers placed a border around closed brackets in their sets. This was necessary to help animals determine the order of the brackets. (The same bordered layout was used in the 2020 study.) Rey considers this a critical limitation in the study. Animals could have grasped the fact that the bordered symbols would always point to the end of a recursive sequence. The ones who were rewarded for their understanding were able to simply learn the order in which the open and closed brackets were presented.
Rey believes that the idea of “recursive computing” as a distinct form of cognition is flawed. Even in humans, he says, this capacity can most likely be explained simply through associative learning mechanisms–which is something he and his colleagues proposed in a 2012 study of baboons–and to date, there have been no satisfactory explanations of how the ability to recognize and manipulate such sequences would be coded in the human brain. Rey says that researchers currently fall into two camps. One believes that human language is built upon unique abilities such as the ability understand recursion, and another believes that it evolved from simpler processes such as association learning.
But Liao points out that even with the borders, the birds still had to work out the center-embedded sequence where open and close brackets were paired from outside in. This means that if the birds learned that the open brackets were at one end of the sequence, and the closed brackets at the other, they would have the same proportion of correct and mismatched ( ) responses. She and her colleagues discovered that the birds chose the latter over the former, even when there were three bracket pairs.
Liao’s observation that birds, whose ancestors were long ago different from primates on the branching tree evolutionary tree of life, are able to generate recursive sequences suggests that this ability is either “evolutionary old” or that it evolved independently through what is called convergent evolution. This observation, Liao says, suggests that birds’ brain may not have the layered brain neocortex found in primates.
For Mathias Osvath, an associate professor of cognitive science at Lund University in Sweden, who was not involved in the new paper, its findings fit into a long line of studies indicating that birds possess many of the same cognitive skills as primates. Osvath states that this data just adds to the amazing list of data showing that birds are completely misunderstood. “Saying that mammals took over the world cognitively is just simply wrong.”