Chimpanzee's in Black: Visual Search for the Conspecific Body Silhouette by Chimpanzees (Pan troglodytes)

Like faces, we obtain various information from the body, such as species, individual identity, age, sex, action, and emotional state. Bodies are processed in a specialized manner, similar to faces. Furthermore, it is known that faces capture attention and cause efficient search in primates including humans (i.e., shallower response-time slopes with increasing set size). In this study we aimed to examine from a comparative cognitive perspective whether bodies have a similar effect. To this end, we conducted a visual search task using body silhouette stimuli with chimpanzees and humans as participants. The results showed that chimpanzees detected body silhouettes among a set of miscellaneous object silhouettes more quickly and accurately compared to the other stimulus categories. These results could not be explained by physical features such as pattern complexity and suggest that chimpanzees categorically perceive the bodies of their own species. In contrast, the body-specific inversion effect, which could be evidence for specialized processing of bodies, was not observed. Furthermore, in a generalization test using silhouettes of quadrupedal animals, which were novel but similar in pattern to chimpanzees, and silhouettes of humans, which were familiar but not quadrupedal, their performance was better with the silhouettes of quadrupedal animals than with the familiar human silhouettes. These findings contrast with those in humans, who efficiently searched for both chimpanzee and human silhouettes, and suggests species differences in the effect of familiarity (expertise).