Monogamy is necessary but not sufficient for eusociality in African mole-rats (Rodentia: Bathyergidae)

Eusociality, the highest level of social organization, is rare among vertebrates and is best exemplified by two African mole-rat species (Bathyergidae). The lifetime monogamy hypothesis suggests that monogamy enhances genetic relatedness within colonies, favoring the evolution of cooperative behaviors and eusociality. While strongly supported in eusocial insects, its role in vertebrates remains unclear. We evaluated this hypothesis in the Bathyergidae to determine the role of monogamy in the evolution of eusociality in vertebrates. We evaluated two predictions: (1) eusociality should be restricted to monogamous lineages, i.e., monogamy is a pre-condition of eusociality; and (2) factors additional to monogamy are required for eusociality to evolve. To test these predictions, we inferred a time-calibrated phylogeny for most species of Bathyergidae and combined it with mating system and sociality data to estimate ancestral states and assess evolutionary correlations. We inferred an ancestral monogamous state for social and eusocial African mole-rats. One of the evolutionary transitions with the highest rate of change was from monogamy + solitary to monogamy + social. Our results are consistent with monogamy representing a necessary prerequisite for the evolution of obligate eusociality, while also indicating that additional ecological and life-history factors are required for eusociality to evolve and intensify.