DIFFERENCES IN FORAGING BEHAVIOR BETWEEN WATER SHREWS - NEOMYS ANOMALUS AND NEOMYS FODIENS

The modes and efficiency of foraging in the terrestrial and aquatic ha bitats in water shrews Neomys anomalus Cabrera, 1907 and N. fodiens (P ennant, 1771) were compared in order to investigate if these species c an avoid competition fur food when they occur syntopically. Seven indi viduals of N. anomalus and five of N. fodiens, caught in the Bialowiez a Primeval Forest, were tested individually in the terrarium of size 3 x 0.5 m, containing a 0.25-m-wide 'stream' with flowing water of an a verage depth 25 cm. Six experimental variants, simulating different ha bitat conditions, were established. Each animal was tested in a given variant during 3 succeeding days for 6 h a day. In total, 738 h of shr ews' behaviour were recorded in darkness using 2 infra-red sensitive v ideo-cameras. Results obtained on four N. fodiens tested with similar methods (648 h; Ruthardt 1990) were included for comparison N. anomalu s swam and dived significantly shorter than N. jodiens, and they did n ot take food under water, even when there was no food on land. N. fodi ens found and took food placed under water and foraged quite efficient ly here. They found on average 17.7% of food portions placed under wat er in the most similar to natural conditions and 19.4% when there was no food on land. In both species foraging time on land was much longer than in water. The presence of natural structures increased duration and efficiency of foraging, but this influence was stronger in N. anom alus than in N. fodiens. These results and literature data suggest tha t in the wild: (1) both species forage in shallow water and in muddy g rounds of wet habitats (wading foraging mode), and also in drier terre strial habitats (epigeal and hypogeal foraging); (2) only N. fodiens f orage in deep water (aquatic foraging); (3) the competition for food b etween N. anomalus and N. fodiens may be very weak, when potential aqu atic prey are available.