![wild animal mothers and babies matching wild animal mothers and babies matching](https://i.ytimg.com/vi/UO3nc0KIhw4/maxresdefault.jpg)
Males, the dispersing sex, are only temporary residents in these groups, and the highest-ranking male sires a large proportion of infants born into different matrilines each year ( Charpentier et al., 2005 Charpentier et al., 2020). Mandrills are non-human primates from Central Africa that live in large matrilineal societies, characterized by family units of philopatric, maternally-related and highly nepotistic females. A crucial question is therefore how an individual may recognize unfamiliar kin when it cannot match phenotypes to itself or to other templates. This mechanism has been rarely demonstrated in the wild (but see Levréro et al., 2015 and Holmes, 1986 Mateo, 2003 for lab studies). Using familiar kin as templates to recognize unfamiliar kin also requires particular conditions, including the presence of relatives during template formation. Alvergne et al., 2009 DeBruine et al., 2008 Parr, 2011 Sheehan and Nachman, 2014), in natural contexts, animals other than humans have probably limited access to cues regarding their own facial traits (but see Hauber and Sherman, 2001 Hauser et al., 1995). For example, although face recognition is a crucial prerequisite for visual communication and therefore for the maintenance of social relationships in many species ( Sheehan and Tibbetts, 2011), including our own lineage (e.g. through smell in rodents Mateo and Johnston, 2003 Mateo, 2010), which may be challenging in some situations. Self-referent phenotype matching requires individuals to evaluate their own phenotype (e.g. This mechanism, based on learning processes of, for example, odors, sounds, or visual cues, allows individuals to recognize kin based on phenotypic resemblance either with self (‘self-referent phenotype matching’ Hauber and Sherman, 2001 Mateo and Johnston, 2003) or with other kin used as templates ( Sherman et al., 1997). Kin selection sometimes necessitates kin recognition, which can operate through phenotype matching ( Penn and Frommen, 2010). Empirical observations of diverse interactions arising from kin selection have been pervasively reported in natura and constitute the foundations of many studies on social evolution ( Clutton-Brock, 2002). Kin selection is an evolutionary process promoting traits that provide fitness benefits for genetic relatives of individuals expressing them ( Hamilton, 1964). This mechanism, that we call ‘second-order kin selection’, may extend beyond mother-infant interactions and has the potential to explain cooperative behaviors among non-kin in other social species, including humans. In support of this mechanism and using theoretical modeling, we finally describe a plausible evolutionary process whereby mothers gain fitness benefits by promoting nepotism among paternally related infants. We then discuss the different scenarios explaining this result, arguing that an adaptive maternal behavior is a likely explanation. Using long-term behavioral observations on association patterns, and controlling for matrilineal origin, maternal relatedness and infant age and sex, we then show, as predicted, that mothers are spatially closer to infants that resemble their own offspring more, and that this maternal behavior leads to similar-looking infants being spatially associated. Using deep learning for face recognition in 80 wild mandrill infants, we first show that infants sired by the same father resemble each other the most, independently of their age, sex or maternal origin, extending previous results to the youngest age class. Here, we hypothesize that mandrill mothers use offspring’s facial resemblance with other infants to guide offspring’s social opportunities towards similar-looking ones. In mandrills, we recently demonstrated increased facial resemblance among paternally related juvenile and adult females indicating adaptive opportunities for paternal kin recognition. In this study, we provide evidence for discrimination towards non-kin by third-parties through a mechanism of phenotype matching. Behavioral discrimination of kin is a key process structuring social relationships in animals.