Stimulus discrimination and surface wave source localization in Crocodilians.

Juvenile Nile crocodiles (Crocodylus niloticus) and spectacled caimans (Caiman crocodilus) use water surface waves for the detection of prey, usually insects trapped at the water surface. This prey detection relies on mechanosensors, the integumentary sensory organs. We found by go/no go conditioning that C. niloticus and C. crocodilus can discriminate surface waves that differ in frequency. On average, frequency difference thresholds were about 4-5 %, e.g. C. niloticus and C. crocodilus distinguished a 40 Hz surface wave from a 38,5 Hz surface wave stimulus. C. niloticus and C. crocodilus also discriminated between single-frequency surface waves (15 Hz or 40 Hz) and surface waves that showed an abrupt frequency change (e.g. from 15 to 16.5 Hz or from 40 Hz to 38.5 Hz). The threshold for the abrupt frequency changes averaged 3-9 %. Additionally, Nile crocodiles differentiated also between a single-frequency water surface wave and a water surface wave that was amplitude modulated. C. niloticus also determined the direction (mean error angle between 13,7° and 16,6°) to a surface wave stimulus. Furthermore, the distance covered by the Nile crocodiles increased slightly with increasing source distance. This was true whether a single-frequency (15 Hz or 40 Hz, relative distance error between 36 and 37%) or a multi-frequency (band width 1 - 80 Hz, relative distance error 25%) surface wave stimulus was offered. Even if the rewarded stimulus (40 Hz) was superimposed by an unrewarded surface wave some distance determination was observed (relative distance error between 30 and 62%).