Laboratory observations on a field-collected colony of 40 Heterocephalus have shown that only a single female breeds. The remaining individuals constitute two or three castes, each containing both sexes and distinguishable by size differences and the tasks they perform. These features, together with long life-spans, overlap of generations, cooperative brood care, and possible age polyethism provide parallels with the eusocial insects.

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Close inbreeding is known for a variety of small mammal species for which a high probability of mortality during dispersal makes helping and delayed maturation a relatively secure fitness option. Prolonged inbreeding, however, is usually associated with lowered fitness, and it has been shown that most highly inbred small mammals and social insects have inbreeding-avoidance mechanisms that promote some degree of outbreeding. However, previous field and laboratory research on the naked mole-rat (Heterocephalus glaber) suggested that this cooperatively breeding rodent is highly inbred, with new colonies forming by fission. 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.

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The African mole-rats (family Bathyergidae) are subterranean hystricomorph rodents occurring in a variety of habitats and displaying levels of sociality which range from solitary to eusocial, making them a unique mammalian taxonomic group to test ecological in£uences on sociality. Here, we use an extensive DNA-based phylogeny and comparative analysis to investigate the relationship between ecology, sociality and evolution within the family. Mitochondrial cytochrome-b and 12s rRNA trees reveal that the solitary species are monophyletic when compared to the social species. The naked mole-rat (Heterocephalus glaber) is ancestral and divergent from the Damaraland mole-rat (Cryptomys damarensis), supporting previous ¢ndings that have suggested the multiple evolution of eusociality within the family. The Cryptomys genus is speciesrich and contains taxa exhibiting di¡erent levels of sociality, which can be divided into two distinct clades. A total of seven independent comparisons were generated within the phylogeny, and three ecological variables were signi¢cantly correlated with social group size: geophyte density (p50.05), mean months per year of rainfall greater than 25 mm (p50.001), and the coe¤cient of rainfall variation (p 0.001). These results support the food^aridity hypothesis for the evolution of highly social cooperative behaviour in the Bathyergidae, and are consistent with the current theoretical framework for skew theory.