Chimpanzees are very interested in their conspecifics' skulls, but what goes on in the chimpanzee's mind when they encounter those skulls remains unanswerable

Staring death in the face: chimpanzees' attention towards conspecific skulls and the implications of a face module guiding their behaviour. André Gonçalves, Yuko Hattori and Ikuma Adachi. Royal Society Open Science, March 2022. https://doi.org/10.1098/rsos.210349

Abstract: Chimpanzees exhibit a variety of behaviours surrounding their dead, although much less is known about how they respond towards conspecific skeletons. We tested chimpanzees' visual attention to images of conspecific and non-conspecific stimuli (cat/chimp/dog/rat), shown simultaneously in four corners of a screen in distinct orientations (frontal/diagonal/lateral) of either one of three types (faces/skulls/skull-shaped stones). Additionally, we compared their visual attention towards chimpanzee-only stimuli (faces/skulls/skull-shaped stones). Lastly, we tested their attention towards specific regions of chimpanzee skulls. We theorized that chimpanzee skulls retaining face-like features would be perceived similarly to chimpanzee faces and thus be subjected to similar biases. Overall, supporting our hypotheses, the chimpanzees preferred conspecific-related stimuli. The results showed that chimpanzees attended: (i) significantly longer towards conspecific skulls than other species skulls (particularly in forward-facing and to a lesser extent diagonal orientations); (ii) significantly longer towards conspecific faces than other species faces at forward-facing and diagonal orientations; (iii) longer towards chimpanzee faces compared with chimpanzee skulls and skull-shaped stones, and (iv) attended significantly longer to the teeth, similar to findings for elephants. We suggest that chimpanzee skulls retain relevant, face-like features that arguably activate a domain-specific face module in chimpanzees' brains, guiding their attention.

5. Conclusion and future directions

We began this study with the central assumption that chimpanzee skulls are perceived like degraded chimpanzee faces and that they would likewise be subjected to the same biases. We proposed three working hypotheses: H1a, chimpanzees look longer at conspecific stimuli versus non-conspecific stimuli (conspecific stimuli > non-conspecific stimuli); H1b, chimpanzees look longer at frontal/diagonal conspecific stimuli versus laterally presented conspecific stimuli (frontal ≈ diagonal > lateral); H2, within conspecific stimuli, chimpanzees look longer at chimpanzee faces followed by skulls and stones (face > skull > stone) and H3, just as elephants direct their attention towards elephant tusks, likewise chimpanzees look longer at conspecific teeth versus other facial regions (teeth > eye ≈ nose). Overall, we found support for all three hypotheses. For H1a, chimpanzees exhibited significantly longer looking durations towards conspecific relative to non-conspecific stimuli when types were pooled (see electronic supplementary material, Data). They also looked significantly longer across most types (skull and face) and orientations (frontal and diagonal) except for stone stimuli (looking durations were relatively longer toward frontal chimpanzee stones, but the difference was only significant when compared for dog stones) reinforcing the ‘degraded face assumption'. For H1b, chimpanzees showed significantly longer looking durations for frontal/diagonal conspecific stimuli in comparison with laterally presented conspecific stimuli, again showing similar biases to previous facial research experiments. For H2, with the chimpanzee-only stimuli, the chimpanzees did look significantly longer at the chimpanzee faces compared with chimpanzee skulls and chimpanzee-shaped stones, but this dropped below significance when comparing the chimpanzee skull with the chimpanzee-shaped stone, although the direction of difference fitted our prediction further supporting the ‘degraded face assumption'. For H3, in the chimpanzee skull regions, our prediction that chimpanzees would look predominantly at the teeth compared with other areas was also upheld. They looked significantly longer at the teeth versus the eye socket and the nasal regions of the skull.

The combined results show support for our hypotheses and do suggest a connection between a domain-specific module in the chimpanzee brain directing their attention towards face-like stimuli. This face module evolved and develops within the context of face-to-face interactions (the likely reason all frontal conditions in our experiment, the chimpanzee stimuli received longer looking patterns overall). The skull contains relevant, albeit impoverished face-like features. This relationship is, of course, not incidental, as skulls support faces, but the attention towards skulls appears to be best explained as a by-product of a module originally evolved for decoding facial expressions. Perhaps notably, unlike wild chimpanzees, our captive subjects never interacted with conspecific skeletons. This suggests that, apart from learned associations, similar interest exhibited by their wild counterparts towards conspecific skulls might also be explained by the same recognition mechanism. To further decode the phylogeny of this face–skull relationship, future studies could compare naive human infants' performance in a similar task (1–3-year-olds familiar with human faces, but with no experience of human skulls). Another research avenue would be to replicate McComb et al.’s [12] experiment in the laboratory with the aid of a three-dimensional printer (skulls controlled for size and colour). Finally, neuroimaging studies could further address the precise connection between skull and face stimuli in the brain. The question put forth by Christophe Boesch, at the beginning of our paper, pondering what goes on in the chimpanzee's mind when they encounter conspecific skulls remains unanswerable. In the light of this study, our tentative answer must also, in the end, be phrased as a question: strange, yet familiar?