Optional-switch cognitive flexibility in primates: Chimpanzees' (Pan troglodytes) intermediate susceptibility to cognitive set

Within human problem-solving, the propensity to use a familiar approach, rather than switch to a more efficient alternative is pervasive. This susceptibility to "cognitive set" prevents optimization by biasing response patterns toward known solutions. In a recent study, which used a nonverbal touch screen task, baboons exhibited a striking ability to deviate from their learned strategy to use a more efficient shortcut. Humans, on the other hand, displayed the opposite response pattern and almost exclusively used a less efficient, but familiar, response. In the current study, we sought to further explore variation in susceptibility to cognitive set within the primate lineage by conducting the Learned Strategy-Direct Strategy task with 10 chimpanzees (Pan troglodytes). Using multilevel multinomial modeling, we found that chimpanzees' shortcut use was intermediate to baboons' and humans'. However, unlike either baboons or humans, there was pronounced inter- and intraindividual variability in chimpanzees' shortcut use. Additionally, a subset of chimpanzees employed a unique solution, wherein they switched strategies midtrial. Further, we found that chimpanzees did not exhibit switch costs when switching between the learned strategy and the shortcut, but humans did. We propose that differences in abstract rule encoding may underlie differences in susceptibility to cognitive set on the Learned Strategy-Direct Strategy task within the primate lineage.