Mandrill mothers associate with infants who look like their own offspring using phenotype matching

Behavioral discrimination of kin is a key process structuring social
relationships in animals. In this study, we provide a first example of
discrimination towards non-kin by third-parties through a mechanism of
phenotype matching. In mandrills, we recently demonstrated increased
facial resemblance among paternally-related juvenile and adult females
indicating adaptive opportunities for paternal kin recognition. Here, we
hypothesize that mothers use offspring’s facial resemblance with other
infants to guide offspring’s social opportunities towards
similarly-looking ones. Using deep learning for face recognition in 80
wild mandrill infants, we first show that infants born to the same father
or conceived during the tenure of the same alpha male resemble each other
the most, independently of their age, sex or maternal origin, extending
previous results to the youngest age class. Using long-term behavioral
observations on association patterns and controlling for matrilineal
origin, maternal relatedness and infant age and sex, we then demonstrate
that, as hypothesized, mothers are spatially closer to infants that
resemble their own offspring more, thereby facilitating associations among
similar-looking infants. Using theoretical modeling, we describe a
plausible evolutionary process whereby mothers gain fitness benefits by
promoting nepotism among paternally related infants. 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 social species, including humans.