A dispersive morph in the naked mole-rat

CLOSE inbreeding is known for a variety of small mammal species 1–4 for which a high probability of mortality during dispersal makes helping and delayed maturation a relatively secure fitness option 5 . Prolonged inbreeding, however, is usually associated with lowered fitness 6,7 , and it has been shown that most highly inbred small mammals 8 and social insects 9 have inbreeding-avoidance mechanisms that promote some degree of outbreeding. However, previous field and laboratory research on the naked mole-rat ( Heterocephalm glaber ) suggested that this cooperatively breeding rodent is highly inbred 10–12 , with new colonies forming by fission 13 . Here we report the discovery of a dispersal phenotype that may occasionally promote outbreeding in naked mole-rats. These dispersers are morphologically, physiologically and behaviourally distinct from other colony members. They are laden with fat, exhibit elevated levels of luteinizing hormone, have a strong urge to disperse, and only solicit matings with non-colony members. These findings suggest that, although rare, a dispersive morph exists within naked mole-rat colonies.