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.

"XR Mark Test" Reveals Sensorimotor Body Representation in Toddlers

The mark test is a popular test for self-recognition. Although the extent to which self-recognition can be assessed remains controversial, the test elicits visually guided, self-oriented, and spontaneous reaching movements. In this study, we demonstrated that this self-oriented reaching is suitable for estimating sensorimotor body representation in toddlers. We developed a non-verbal task (Bodytoypo) to assess the localization of body parts by gamifying the mark test and conducted it with thirty 2- and 3-year old children. Specifically, we detected the children's skeletal data in real-time, displayed virtual marks on various parts of their body, and estimated their reaction time and accuracy of body part localization. Subsequently, a statistical-based automated analysis using 2-D image processing and conventional frame-by-frame coding were performed. The results revealed developmental changes in the children's reaching strategies. A few errors were observed around the face. A reduction in the error rate for joint and movable areas was observed in children between the ages of 2 and 3 years. An analysis of movement trajectories using a combination of image processing and machine learning algorithms showed that 2-year-olds acquired visually guided reaching (feedback control) from ballistic exploratory reaching and 3-year-olds acquired rapid and predictive reaching (feedforward control) from visually guided cautious reaching. It was also found that the accuracy of localization could be predicted by examining the coordination of body parts. Evaluation of the developmental changes in self-oriented reaching reveals new possibilities for the mark test and development of body representation.

Investigating Behavioral Responses to Mirrors and the Mark Test in Adult Male Zebra Finches and House Crows

Earlier evidence suggests that besides humans, some species of mammals and birds demonstrate visual self-recognition, assessed by the controversial “mark” test. Whereas, there are high levels of inter-individual differences amongst a single species, some species such as macaques and pigeons which do not spontaneously demonstrate mirror self-recognition (MSR) can be trained to do so. We were surprised to discover that despite being widely used as a model system for avian research, the performance of zebra finches (Taenopygia guttata) on the mark test had not been studied earlier. Additionally, we studied the behavioral responses of another species of passerine songbirds (Indian house crows; Corvus splendens) to a mirror and the MSR mark test. Although a small number of adult male zebra finches appeared to display heightened responses toward the mark while observing their reflections, we could not rule out the possibility that these were a part of general grooming rather than specific to the mark. Furthermore, none of the house crows demonstrated mark-directed behavior or increased self-exploratory behaviors when facing mirrors. Our study suggests that self-directed behaviors need to be tested more rigorously in adult male zebra finches while facing their reflections and these findings need to be replicated in a larger population, given the high degree of variability in mirror-directed behaviors.