Citizen science study exploring visual cues to kinship in chimpanzee faces.
Kin discrimination is the different treatment of kin (individuals who are biological relatives), as opposed to non-kin, primarily by engaging in prosocial behaviour that benefits them. In doing so individuals across taxa enhance their inclusive fitness. Kin discrimination is enabled by kin recognition, facilitated by behavioural cues such as prior association, and perceptual similarities between phenotypes. Phenotype matching requires a correlation between phenotypic and genotypic similarity, and can occur via matching yourself to others, or matching of two other related individuals. Matching occurs in diverse species, including humans, and across sensory domains.
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. The full experimental set up is available as an Open Materials page in Gorilla.sc under Chimp_faces (https://gorilla.sc/openmaterials/110651)
Code and Data will be made available in this repository. May 2022: R code updated, Updated data files added (ChimpFaces - Data for GLMMs_May2022-newages; ChimpFaces -Data GLMMs with no exclusions_May2022-newages)