Gaze in context: non-human eyes can be more salient under ecologically-relevant conditions

Primate eyes vary strikingly in pigmentation, yet the drivers of said variation are strongly debated. Recent revisions of the Cooperative Eye Hypothesis (CEH) propose that the human eye’s sclerae evolved to enhance gaze communication specifically under challenging conditions of visibility. We tested this idea under ecologically realistic conditions by presenting observers with a live model wearing contact lenses that simulated either a human-like or a chimpanzee-like eye. At a university lab, observers judged gaze direction at different viewing distances and lighting levels. Contrary to expectations, chimpanzee-like outperformed human-like eyes in dim lighting and close-viewing conditions. Human-like eyes yielded the highest accuracy under bright, far-viewing conditions, consistent with a long-distance signaling advantage. Our results demonstrate that ecological visual constraints shape the potential informativeness of distinct ocular configurations. We hypothesize that species-typical eye appearances may be tuned to their species-typical visual ecology.