Swiss primatologist Christophe Boesch once commented on a picture of a chimpanzee skull, pondering, "What goes on in the chimpanzee's mind when they see such a sight in the forest?" We might not yet be able to suss out what chimps are thinking regarding their own mortality, but they do show a strong preference for the faces and skulls of their fellow chimps, according to a recent paper published in the journal Royal Society Open Science. They share this attraction to faces with humans, and the recognition of chimp skulls may be a form of face pareidolia (the ability to perceive faces in inanimate objects).

Chimpanzees are known to share some unusual traits with elephants, including being capable of recognizing themselves in the mirror and showing interest in injured or deceased members of their own species. In fact, elephants have been observed showing interest not just in elephant corpses, but in bones and tusks. It has also been suggested that elephants "visit" the bones of their deceased relatives.

A 2006 study of African elephants conducted by scientists at the University of Sussex found that the elephants showed more interest in the skulls (or tusks) of their own species than the skulls of other animals (such as a buffalo or rhinoceros). However, the study found no evidence that the elephants could recognize the remains of close kin, concluding that this observed behavior is due simply to their general attraction for elephant remains.

Numerous studies have been conducted that show chimpanzees' reactions to deceased chimps, such as revisiting corpses and showing signs of mourning. For instance, some chimpanzee mothers will carry their dead infants for days or even months. A 2020 study reported on two cases of a chimpanzee community clustering around a skeleton, suggesting that perhaps the chimps were drawn to the remains because they still bore some resemblance to a living chimp. (There were confounding factors, however. The dead chimps in both cases had actually been killed by fellow chimpanzees, and this is just one of a wide range of observed reactions to chimpanzee skeletons.)

André Gonçalves and his colleagues at Kyoto University noted several studies showing that chimpanzees pay close attention to the face when encountering dead members of their species. Chimps are known to respond strongly to faces in general, much like humans. So they decided to investigate further to see if chimpanzees treat skulls similarly to how they treat faces.

The study was conducted using an experimental booth housed in a testing room. Chimps were shown sets of images, and eye-tracking equipment monitored their eye movements (a common indicator of attention). The subjects were allowed to sip juice through a nozzle to reduce head movement during the experiments.

The team recruited 10 adult chimpanzees for the experiments, all housed at the Primate Research Institute in Kyoto, Japan, although three chimps ended up dropping out and being excluded from the final study. A chimp named Pendesa had a cyst on her brain that affected her field of vision. Another, named Mari, couldn't comfortably complete the tasks because of difficulty breathing through her nose, making sipping juice uncomfortable. And the third, Pan, just wanted to be her own chimp, abandoning the experiment halfway through "for no apparent reason" and refusing to return despite coaxing from her handlers.

In the first experiment, the subjects viewed 180 images of four species (cat, chimp, dog, and rat), shown four at a time simultaneously in each corner of the computer screen for six seconds. Each group of four images showed either a skull, a face, or stones in the shape of a skull, and those three types were also shown in three different orientations (diagonal, frontal, and lateral).

A second experiment followed a similar protocol, except only the chimpanzee images were used: a face, a skull, and skull-shaped stones shown diagonally, frontally, and laterally. The intent was to determine whether the chimp faces were the driving factor in drawing the subjects' attention. Finally, in a third experiment, only the images of chimpanzee skulls were used, in the frontal and diagonal orientations. This helped determine whether the chimps' attention was drawn specifically to facial cues, such as teeth.