Comparative studies of two species of mole-rat are helping to clarify
the ecological correlates of mammalian eusociality. Both species live
in social groups composed of close kin, within which breeding is restr
icted to one female and one to three males. They inhabit xeric areas w
ith dispersed, patchy food and unpredictable rainfall. During droughts
, they can neither expand their tunnel systems nor disperse. In brief
periods after rain the animals must cooperate and dig furiously to loc
ate rich food patches. By living in groups, arid-zone mole-rats can ta
ke full advantage of windows of opportunity when conditions are right
for burrowing. Thus, ecological factors and kin selection have apparen
tly interacted in the evolution of eusociality in these species.