After the discovery of eusociality in the naked mole-rat, it was proposed that inbreeding and high colony
relatedness in this species were the major underlying factors driving cooperative breeding in African molerats.
By contrast, field and laboratory studies of the eusocial Damaraland mole-rat (Cryptomys damarensis)
have raised the possibility that this species is an obligate outbreeder, although the build-up of inbreeding
over several generations could still occur. Using microsatellite markers, we show that most breeding pairs
in wild colonies of the Damaraland mole-rat are indeed unrelated (R = 0.02 ± 0.04) and that mean colony
relatedness (R = 0.46 ± 0.01), determined across 15 colonies from three separate populations, is little
more than half that previously identifed in naked mole-rats. This finding demonstrates that normal familial
levels of relatedness are suffcient for the occurrence of eusociality in mammals. Variation in the
mean colony relatedness among populations provides support both for the central role played by ecological
constraints in cooperative breeding and for the suggestion that inbreeding in naked mole-rats is a response
to extreme constraints on dispersal. Approaches that determine the relative importance of an array of
extrinsic factors in driving social evolution in African mole-rats are now required.