Abstract
Despite not knowing the exact age of individuals, humans can estimate their rough age using age-related physical features. Nonhuman primates show some age-related physical features; however, the cognitive traits underlying their recognition of age class have not been revealed. Here, we tested the ability of two species of Old World monkey, Japanese macaques (JM) and Campbell's monkeys (CM), to spontaneously discriminate age classes using visual paired comparison (VPC) tasks based on the two distinct categories of infant and adult images. First, VPCs were conducted in JM subjects using conspecific JM stimuli. When analyzing the side of the first look, JM subjects significantly looked more often at novel images. Based on analyses of total looking durations, JM subjects looked at a novel infant image longer than they looked at a familiar adult image, suggesting the ability to spontaneously discriminate between the two age classes and a preference for infant over adult images. Next, VPCs were tested in CM subjects using heterospecific JM stimuli. CM subjects showed no difference in the side of their first look, but looked at infant JM images longer than they looked at adult images; the fact that CMs were totally naïve to JMs suggested that the attractiveness of infant images transcends species differences. This is the first report of visual age class recognition and a preference for infant over adult images in nonhuman primates. Our results suggest not only species-specific processing for age class recognition but also the evolutionary origins of the instinctive human perception of baby cuteness schema, proposed by the ethologist Konrad Lorenz.
Highlights
In the AI condition, an adult stimulus was presented in the familiar phase and an infant image was used as the novel stimulus in the test phase; the stimuli were reversed in the IA condition
These results demonstrate that Japanese macaques (JM) subjects possess a spontaneous ability to discriminate infant images from adult images
When adult images were used in the familiar phase (AI condition), the monkeys looked at the novel infant image longer than the adult image
Summary
Human physical features (e.g., body size, facial appearance, sexual organs, and hair color) change with age. When interacting with an unfamiliar person, those physical features facilitate the estimation of his/her rough age, or at least age class (e.g., infant, juvenile, young adult, adult, or elder). ‘‘Own-age bias’’ is an example of a cognitive trait used by humans for age recognition This bias suggests that one’s age estimations of an unfamiliar person are more sensitive and precise when they are about one’s own age than when they are of a very different age [2]. We adapt our way of speaking to very old or very young people [5] These age-dependent differences in social interactions are a part of high-order social cognitions that have evolved through complex interactions in social environments, in the primate lineage [6,7,8]
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