Mirror self-recognition in gorillas (Gorilla gorilla gorilla): a review and evaluation of mark test replications and variants

Mirror self-recognition (MSR), widely regarded as an indicator of self-awareness, has not been demonstrated consistently in gorillas. We aimed to examine this issue by setting out a method to evaluate gorilla self-recognition studies that is objective, quantifiable, and easy to replicate. Using Suarez and Gallup’s (J Hum Evol 10:175–183, 1981) study as a reference point, we drew up a list of 15 methodological criteria and assigned scores to all published studies of gorilla MSR for both methodology and outcomes. Key features of studies finding both mark-directed and spontaneous self-directed responses included visually inaccessible marks, controls for tactile and olfactory cues, subjects who were at least 5 years old, and clearly distinguishing between responses in front of versus away from the mirror. Additional important criteria include videotaping the tests, having more than one subject, subjects with adequate social rearing, reporting post-marking observations with mirror absent, and giving mirror exposure in a social versus individual setting. Our prediction that MSR studies would obtain progressively higher scores as procedures and behavioural coding practices improved over time was supported for methods, but not for outcomes. These findings illustrate that methodological rigour does not guarantee stronger evidence of self-recognition in gorillas; methodological differences alone do not explain the inconsistent evidence for MSR in gorillas. By implication, it might be suggested that, in general, gorillas do not show compelling evidence of MSR. We advocate that future MSR studies incorporate the same criteria to optimize the quality of attempts to clarify the self-recognition abilities of gorillas as well as other species.