Pygmy Marmosets (cebuella Pygmaea) Do Not Differentially React to Images of Conspecifics Varying in Their Ocular Appearance

Variation in eye pigmentation across primate species has been proposed to serve as a cue to emotional or temperamental traits, potentially shaped by processes like domestication or selection against aggression. We tested whether pygmy marmosets (Cebuella pygmaea) respond differently to conspecific facial images that vary in peri-iridal pigmentation. Specifically, we examined whether darker sclera pigmentation elicits more distress or aversive reactions, consistent with the hypothesis that eye coloration serves as a signal of underlying temperament. Contrary to predictions, subjects did not exhibit differential behavioural responses to light-sclera, dark-sclera, or control stimuli. These results align with emerging evidence that challenges the presumed association between ocular pigmentation and temperament or aggression levels in primates. We discuss potential explanations, including species-specific differences in tolerance to eye contact, generalized arousal to experimental contexts, and the need for more controlled comparisons. While our findings do not support a signaling function for eye pigmentation in this context, they contribute to a broader reassessment of the proposed links between morphology and socio-emotional traits in primates.