Chimp_Faces

Facial phenotype matching has been suggested as a possible mechanism for both paternity recognition in fathers and paternity signalling and has been showed in primates, such as mandrills (Charpentier et al., 2020); however, evidence for its presence in humans and other apes remains mixed. Chimpanzees are capable of recognizing individuals by their faces (Parr & de Waal, 1999), and show similar visual face scanning to humans, with a preference for faces over backgrounds and bodies, and a bias towards central facial features (Kano & Tomonaga, 2010). While the pressure to detect or conceal specific relationship-types may vary between chimpanzees and humans, chimpanzees may also benefit from the ability to signal kinship through phenotype-matching. Captive, trained individuals were able to match images of unfamiliar mother-son pairs Parr & de Waal, 1999 and all types of parent-offspring images at above chance (Parr, Heintz, Lonsdorf, & Wroblewski, 2010). High performance on mother-son and father-daughter pairs in particular suggests visual kin-discrimination may provide a mechanism to avoid inbreeding (Parr et al., 2010). However, as only mature (adult and subadult) chimpanzees were used and only parent-offspring relationships were tested, there were limitations to the hypotheses that could be explored. For example, if visual kin-discrimination functions to impact infant survivorship, as has been argued in humans (Christenfeld and Hill, 1995), it must be detectable in infant faces. Similarly, inbreeding avoidance should also select for kin-detection between mature siblings, in particular paternal siblings who have limited cues available from prior association. While testing these mechanisms in chimpanzee subjects would be preferable, it is only possible to do so in captivity where selective pressures are limited and mating opportunities are carefully controlled, impacting the expression of genotypes and phenotypes. However, human participants have been shown to be reliable judges of visual markers of chimpanzee kinship (Vokey et al., 2004; Alvergne et al., 2009).

Here we use a citizen-science approach to test human detection of facial similarity in a community of wild chimpanzees. We test relationships across five age groups from infant to adult, and four kinship types: fathers and their offspring, mothers and their offspring, paternal half-siblings and maternal half-siblings.

Stimuli consisted of 180 in-colour photographs of wild East African chimpanzees (Pan troglodytes schweinfurthii), living in the Budongo Forest Reserve, Uganda. Images dated from 1995 to 2018. All targets, answers, and most foils were individuals from the Sonso community, who were habituated to human observation during the 1990s (Reynolds, 2005), and where extensive records of parentage are available. Images of individuals from the neighbouring Waibira community, where habituation began in 2011 (Samuni et al., 2014), were sometimes included as foils.

Chimpanzees were assigned to one of the following age categories defined by Reynolds (2005): individuals 15yrs and females >14yrs as adult. We further split the Infant age category into Infant 1:

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