Age categorization of conspecific and heterospecific faces in capuchin monkeys (Sapajus apella)

Abstract

Across various species, infant faces share various features referred to as “baby schema”(Lorenz, 1942). Assuming that these features are indeed shared among species, it is possible that nonhuman animals may perceive age information in conspecific and heterospecific faces. We tested whether tufted capuchin monkeys (Sapajus apella) would visually categorize age from faces. In Experiment 1, we trained 4 monkeys to discriminate adult and infant faces of conspecifics using a symbolic matching-to-sample procedure. We then tested whether their categorization transferred to faces of other species (i.e., dogs and humans). In Experiment 2, we trained another 2 monkeys on age categorization of heterospecific (human) faces and tested them with conspecific and dog faces, to assess whether conspecific age categorization in Experiment 1 was specific. In Experiment 3, the 4 monkeys from Experiment 1 were trained with human faces, whereas the 2 monkeys from Experiment 2 were trained with conspecific faces; we then tested all 6 monkeys with faces of dogs and other species including New World monkeys, Old World monkeys, apes, and carnivores. During training, the monkeys quickly learned to categorize adult and infant faces of both conspecifics and humans. However, age categorization failed to transfer to different species in the test phase in all 3 experiments.