Abstract
Social primates must recognise developmental stages of other conspecifics in order to behave appropriately. Infant faces have peculiar morphological characteristics—relatively large eyes, a small nose, and small mouth—known as baby schema. In addition, the infant faces of many primate species have unique skin coloration. However, it is unclear which features serve as critical cues for chimpanzees to recognise developmental changes in their faces. The present study aimed to investigate the relative contributions of facial shape and colour to age categorisation in chimpanzees. We used a symbolic matching-to-sample task in which chimpanzees were trained to discriminate between adult and infant faces. Then, we tested how their age category judgments transferred to a series of morphed faces which systematically differed in facial shape and colour. Statistical image quantification analysis revealed significant differences both in shape and colour between adult and infant faces. However, we found that facial coloration contributed to age categorisation in chimpanzees more than facial shape. Our results showed that chimpanzees use unique infantile facial coloration as a salient cue when discriminating between adult and infant faces. The display of their developmental stages through facial colour may help chimpanzees to induce appropriate behaviour from other individuals.
Highlights
Social primates must recognise developmental stages of other conspecifics in order to behave appropriately
To identify the facial features that differed between adult and infant faces, we performed multiple t-tests for facial shape and colour components extracted from the PCA
The results revealed that infant faces had significantly larger PC1 scores than adult faces (p < 0.001), while there were no significant differences for any other components
Summary
Social primates must recognise developmental stages of other conspecifics in order to behave appropriately. Our results showed that chimpanzees use unique infantile facial coloration as a salient cue when discriminating between adult and infant faces The display of their developmental stages through facial colour may help chimpanzees to induce appropriate behaviour from other individuals. Studies have shown that affiliative behaviours are most often observed toward infants displaying infantile coloration, and decrease as infants age and lose their infantile c oloration[20] Taken together, both facial shape and colour properties can provide critical information related to age, at least in some primate species. An empirical study with eye-tracking demonstrated that adult chimpanzees (Pan troglodytes verus) prefer to look at infant faces when presented with the pictures of conspecific mothers and infants[25]. Given that the facial images were greyscale, the capuchin monkeys must have extracted facial information besides coloration to discern the age categories of faces
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