AGONISTIC INTERACTIONS BETWEEN RESIDENT AND IMMIGRANT SYMPATRIC WATERSHREWS - NEOMYS-FODIENS AND N-ANOMALUS

Behavioral mechanisms which control resident-immigrant relations in wi ld populations of Neomys fodiens (Pennant, 1771) and N. anomalus Cabre ra, 1907 were studied by quantitative analysis of agonistic interactio ns between animals inhabiting the enclosure for at least 1 week ('resi dents') and animals newly introduced ('immigrants'). Tested animals (4 6 N. fodiens and 24 N. anomalus) were observed in one- and two-species groups or pairs in enclosures of 275 x 135 or 135 x 135 cm built in a laboratory room. During 214 hours of direct observation, social behav iour and patterns of hiding place usage were registered. Individual oc cupation of the nest boxes, a great number of conflicts among fodiens- 'residents', and a large number of conflicts between fodiens-'immigran ts' and -'residents', and suggest that strong territorial competition in breeding females and strong competition for females in adult males occur between residents and immigrants of the wild N. fodiens-populati ons. Group occupation of the nest boxes, a very low degree of aggressi veness among anomalus-'residents' and the tolerant interactions betwee n anomalus-'immigrants' and -'residents' suggest that, in wild populat ions, N. anomalus are gregarious and inhabit given areas in groups. Th e low number of interspecific conflicts between 'residents' suggests t hat in the two-species stabilized groups, behavioral mechanisms exist, which allow N. anomalus to avoid conflicts with the more aggressive N . fodiens. Since the number of conflicts between fodiens-'residents' a nd anomalus-'immigrants' is much greater than between 'residents' of t he two species, it seems that these mechanisms are active learning rat her than simple habituation. The decrease in the number of interspecif ic conflicts within 1-3 days suggests high efficiency of these mechani sms.