Figure 1. Example experimental stimulus pairs.

Subjects were shown a pair of stimulus images consisting of a conspecific and a heterospecific. Facial traits (nose spots for putty nosed monkeys and eyebrow patches for mona monkeys) were varied across trials, with conspecifics paired with a heterospecific species that shares the facial trait (row 1) and one that does not (rows 2 and 3). Conspecifics were displayed either naturally (rows 1 and 2) or with the facial trait removed (row 3). All subjects participated in all three trial types. Trial order and stimulus image side were counterbalanced across subjects.

Figure 1—figure supplement 1. The experimental apparatus.

(Top) The apparatus was painted to draw interest and placed immediately outside subject enclosures. (Middle) A widescreen laptop and video camera were housed inside the apparatus to display stimulus images and record subject responses. (Bottom) View of subject participating in looking time trial as recorded by the internal video camera.