“Crows Perform Yet Another Skill Once Thought Distinctively Human: Scientists Demonstrate That Crows Are Capable of Recursion—A Key Feature in Grammar. Not Everyone Is Convinced”, 2022-11-02 (; backlinks):
…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. Both humans and monkeys were shown a display with two pairs of bracket symbols that appeared in a random order. The subjects were trained to touch them in the order of a “center-embedded” recursive sequence such as
{ ( ) }or( { } ). After giving the right answer, humans received verbal feedback, and monkeys were given a small amount of food or juice as a reward. Afterward the researchers presented their subjects with a completely new set of brackets and observed how often they arranged them in a recursive manner. Two of the 3 monkeys in the experiment generated recursive sequences more often than non-recursive sequences such as﹛ ( ﹜ ), although they needed an additional training session to do so. One of the animals generated recursive sequences in around half of the trials. 3—to 4-year-old children, by comparison, formed recursive sequences in ~40% of the trials.This paper prompted Liao and her colleagues to investigate whether crows, with their renowned cognitive skills, might possess the capacity for recursion as well. 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 the birds’ ability to spontaneously generate such recursive sequences on a new set of symbols. The crows also performed on par with children. The birds produced the recursive sequences in around 40% 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. These findings raise the question of what non-human animals might use this ability for, he adds. “They do not seem to possess anything similar to human language, thus recursion is possibly relevant to other cognitive functions”, he says. One speculation is that animals might use recursion to represent relationships within their social groups.
…To address this limitation, Liao and her colleagues extended the sequences from two pairs to 3 pairs—such as
{ [ ( ) ] }. With 3 pairs of symbols, the probability of producing the sequences without grasping the underlying concept of recursion becomes much lower, Liao says. Here, too, the researchers found that the birds were most likely to choose center-embedded responses.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. A key reason, he explains, lies in a feature of the study design: the researchers placed a border around the closed brackets in their sets—which the authors note was required to help the animals define the order of the brackets. (The same bordered layout was used in the 2020 study.) For Rey, this is a crucial limitation of the study because the animals could have grasped that bordered symbols—which would always end up toward the end of a recursive sequence—were the ones rewarded, thus aiding them in simply learning the order in which open and closed brackets were displayed.