ABSTRACT
To explain the evolution of sociality the authors propose the hypothesis that every social interaction is based on the contest. The synergetic nature of any interaction determines the permanent involving partners in the contests and leads to increasing social pressure within community. The contestants' interactions inhibit an activity in one of the partners. Two possible evolutionary strategies of adaptation to increasing social density are: 1) an increase in resistance to the activity inhibition; 2) a retaining in memory the results of inhibition, i.e. reducing of ability to restore inhibited activity. Three species of gerbils (Gerbillidae): Mongolian (Meriones unguiculatus), Libyan (M. libycus), and midday gerbil (M. meridianus) were compared. Being similar in their ecology and behavioural repertoire three species differ strongly in extent of sociality: the highest in Mongolian gerbil and the lowest in midday one. The duration of social acts in laboratory groups of gerbils as well as frequency of acts delaying conflicts under natural conditions were assumed to correlate with increase in resistance to social pressure. Both parameters were higher in mongolian and libyan gerbils. The data on dynamics, asymmetry, and stability of social relations indicates more stable retaining in memory the results of interactions in Mongolian and Libyan gerbils than in midday ones but the latter restore inhibited activity more quickly. In the laboratory groups where the available space is compressed the midday gerbils maintain population at a level not lower than Mongolian ones and much higher than M. libycus. Moreover aggression within the group of midday gerbils doesn't result in rapid population decrease as in other two species.
