Aggression is a polymorphic trait that occurs in subterranean blind mole rats of the Spalax ehrenbergi superspecies in Israel (NEVO et al., 1975, 1986; NEVO, 1991). We found Egyptian mole rats in the isolates to be pacifistic. This supports our evolutionary theory of aggression in Spalax which predicts that aggression should decrease in the desert habitat (NEVO et al., 1986), presumably to minimize overheating, water and energy expenditure. The described behaviour is a pre-requisite for social evolution. We hypothesize that pacifistic behaviour in Spalax isolates in North Africa has been adaptively selected for survival in the harsh Sahara desert ecology.