ECOPHYSIOLOGICAL CONSTRAINTS ASSOCIATED WITH AGGRESSION, AND EVOLUTION TOWARD PACIFISM IN SPALAX-EHRENBERGI

Evolution from a solitary way of life to sociality involves changes in the modalities of within-population aggression. This paper is concern ed with environmental causes of variation in aggression within the com plex of mole-rat species Spalax ehrenbergi. We focus on physiological constraints associated with aggressive behaviour, i.e. those resulting from emotional arousal and thus stress. Aggression, social stress sen sitivity and wine osmolarity were studied in males of two chromosomal species of Spalax ehrenbergi, 2n = 52 (occurring in cool humid areas) and 2n = 60 (occurring in warm arid zones). Males with 2n = 52 reached a higher level of within-population aggression during a 3-round tourn ament (an average, 488 +/- 140 acts) than males with 2n = 60 (163 +/- 54 acts). The species with 2n = 52 displayed a higher corticosterone r esponse after a social encounter (in average, 5.15 +/- 0.54 mu g/100 m l) than that with 2n = 60 (3.0 +/- 0.3 mu g/100 ml) whereas resting le vels of this hormone were low and not different between the two specie s. Urine osmolarity was significantly higher in males with 2n = 60 (33 1.9 +/- 18.6 mmol/kg) than males with 2n = 52 (267.3 +/- 10.9 mmol/kg) . Considering the overall general trend urine osmolarity tended to be negatively related to the level of aggression, while corticosterone le vels tended to vary positively with aggressiveness (Fig. 3). Our resul ts allow further discussion of the hypothesis that high levels of aggr ession could be selected against in arid habitats due to their physiol ogical correlates (decrease in water economy, increased corticosterone leading to potential energy mobilisation and loss). We propose that b ehavioural ecophysiology could have contributed to the radiation of th e S. ehrenbergi superspecies into increasingly arid environments, and may favour evolution toward social tolerance.