Colour matters more than shape for chimpanzees' recognition of developmental face changes

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.

Figure 1

The morphed face stimuli used in test phase: (a) faces with varying facial colours between adult and infant faces with infant facial shapes, (b) faces with varying facial colours between adult and infant faces with adult facial shapes, (c) faces with varying facial shapes between adult and infant faces with adult facial colours, (d) faces with varying facial shapes between adult and infant faces with infant facial colours, and (e) faces with co-varying facial shapes and colours. The facial images were generated with Psychomorph software (version 6; https://users.aber.ac.uk/bpt/jpsychomorph/).

Figure 2

The procedure of experiment (symbolic matching to sample).

Figure 3

Facial shape variations along the axis of the first principal component: The theoretical values of − 3, − 1.5 SD, average, + 1.5, and + 3 SD. The facial images were generated with TpsRelw software (version 1.70; https://tpsrelw.software.informer.com/).

Figure 4

Facial colour variations along the axis of the first principal component. The theoretical values of − 3, − 1.5 SD, average, + 1.5, and + 3 SD. The facial images were generated with MATLAB (version 2018a; https://uk.mathworks.com/products/matlab.html).

Figure 5

The number of infant responses for each stimulus in 16 trials. (a) Colour effect on infant shape, (b) Colour effect on adult shape, (c) Shape effect on adult colour, (d) Shape effect on infant colour, (e) Colour and shape effect.