| Naked mole-rats present a unique opportunity in vertebrates to study evolution of eusociality as characterized by reproductive altruism, overlap of adult generations and lifelong philopatry. Earlier studies of the apparently eusocial naked mole-rats assumed that they are facultative inbreeders with very limited dispersal and link their extreme social behavior to high within colony relatedness. However, more recent work indicates that naked mole-rats actively avoid inbreeding and may frequently disperse aboveground, bringing this assumption into question. Here we compile evidence of aboveground dispersal in naked mole-rats including visual observations and pitfall trap captures as well as the occurrence of small nascent colonies. Endocrinological examinations of wild animals confirm earlier laboratory findings of the immediate establishment of a breeding pair in groups separated from the original breeders. We also document differences in testosterone and progesterone levels between breeding and non-breeding animals in the wild. In addition we examine possible correlates of dispersal assuming that yearly attrition from the colony is indicative of dispersal, including correlates of weight, body mass index, hormone levels, and nocturnal rhythmicity. Another hypothesis related to the evolution of eusociality, the aridity food distribution hypothesis, assumes that naked mole-rats forage at random in large groups that benefit from cooperative foraging in unpredictable, arid environments. However, we show that naked mole-rats do not dig at random and can orient themselves efficiently in the field, thus improving their probability of surviving by themselves. One of the more challenging factors for animals living in an underground burrow is the almost complete lack of ventilation, which manifests itself in low availability of oxygen and elevated levels of carbon dioxide. We compared hematocrit and hemoglobin data with a selection of surface dwelling rodents and laboratory housed Heterocephalus. We found very little plasticity in this trait in naked mole-rats. These results indicate that individuals dispersing aboveground don't experience a change in blood physiology during their run. We conclude that dispersal is in fact a frequent occurrence in naked mole-rats and question previous notions of individuals of this species staying within the colony until they die, as required by the definition of eusociality in mammals. |
