Are naked and common mole-rats eusocial and if so, why?

Eusociality in mammals is defined in the present paper by the following cri teria: reproductive altruism (which involves reproductive division of labor and cooperative alloparental brood care), overlap of adult generations, an d permanent (lifelong) philopatry. We argue that additional criteria such a s the existence of castes, colony size, reproductive skew, and social cohes ion are not pertinent to the definition of eusociality in mammals. Accordin g to our definition of mammalian eusociality, several rodent species of the African family Bathyergidae can be considered eusocial, including the nake d mole-rat (Heterocephalus glaber), Damaraland mole-rat (Cryptomys damarens is): and several additional, if not all, species in the genus Cryptomys. Fu rthermore, some species of social voles (like Microtus ochrogaster) may als o fulfill criteria of mammalian eusociality. Understanding the evolution of eusociality in mole-rats requires answers to two primary questions: (1) Wh at are the preconditions for the development of their eusocial systems? (2) Why do offspring remain in the natal group rather than dispersing and repr oducing? Eusociality in mammals is by definition a special case of monogamy (more specifically: monogyny one female breeding), involving prolonged pai r bonding for more than one breeding period. We argue that eusociality in m ole-rats evolved from a monogamous mating system where cooperative brood ca re was already established. A tendency for group living is considered to be an ancestral (plesiomorph) trait among African bathyergid mole-rats, linki ng them to other hystricognath rodents. A solitary lifestyle seen in some g enera, such as Bathyergus, Georychus, and Heliophobius, is assumed to be a derived trait that arose independently in different lineages of bathyergids , possibly as a consequence of selective constraints associated with the su bterranean environment. In proximate terms, in eusocial mole-rats either pu berty is assumed to be developmentally delayed so that under natural condit ions most animals die before dispersal is triggered (e.g., in the case of H eterocephalus) or dispersal is induced only by an incidental encounter with an unfamiliar, yet adequate sexual partner (e.g., in the case of Cryptomys ). Ultimately, a combination of strategies involving either dispersal and/o r philopatry can be beneficial, especially in a highly unpredictable enviro nment. If genetic relatedness among siblings is high (e.g., a coefficient o f relatedness of 0.5 or more), then philopatry would not invoke an apprecia ble loss of fitness, especially if the cost of dispersing is higher than st aying within the natal group. High genetic relatedness is more likely in a monogamous mating system or a highly inbred population. In this paper, we a rgue that the preconditions for eusociality in bathyergid mole-rats were a monogamous mating system and high genetic relatedness among individuals. We argue against the aridity food-distribution hypothesis (AFDH) that suggest s a causal relationship between cooperative foraging for patchily distribut ed resources and the origin of eusociality. The AFDH may explain group size dynamics of social mole-rats as a function of the distribution and availab ility of resources but it is inadequate to explain the formation of eusocia l societies of mole-rats, especially with respect to providing precondition s conducive for the emergence of eusociality.