Behavioral mechanisms which control resident-immigrant relations in wild populations of Neomys fodiens (Pennant, 1771) and N. anomalus Cabrera, 1907 were studied by quantitative analysis of agonistic interactions between animals inhabiting the enclosure for at least 1 week ('residents') and animals newly introduced ('immigrants'). Tested animals (46 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 behaviour and patterns of hiding place usage were registered. Individual occupation of the neat boxes, a great number of conflicts among/bdiens-'residents', and a targe number of conflicts between /bdiens-'immigrants' and -'residents' 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. /odtercs-populations. Group occupation of the nest boxes, a very low degree of aggressiveness among anoma/iis-'residents' and the tolerant interactions between anom.a/«s-'immigrants' and -'residents' suggest that, in wild populations, N. anomalus are gregarious and inhabit given areas in groups. The low number of interspecific conflicts between 'residents' suggests that 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 /bdiercs-'residents' and arioma/us-'immigrants' is much greater than between 'residents' of the two species, it seems that these mechanisms are active learning rather than simple habituation. The decrease in the number of interspecific conflicts within 1-3 days suggests high efficiency of these mechanisms.
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