The degradation of distance discrimination in big brown bats (Eptesicus fuscus) caused by different interference signals.

The ability of two big brown bats (Eptesicus fuscus) to discriminate the distance to an electronically synthesized "phantom" target by echolocation was tested in the presence of interfering signals presented slightly before the target echo. Interfering signals were chosen to have differing degrees of similarity to the typical echolocation emission used by the bat in this task (which was the signal used to create the phantom target), and we predicted that the degree of disruption of ranging would be proportional to the similarity of the interference to the target echo. This prediction was not confirmed; rather, all interference signals not identical to the target echo increased the threshold to about twice that found with no interference. When the interference was identical to the target echo, the threshold increased to about 4 times that with no interference. When each bat was presented with phantom target "echoes" appropriate for the other bat, its range discrimination threshold increased about ten fold, and in this case the degree of interference of different signals was related to their similarity to the target echo, not to their similarity to the bat's "normal" signal. We suggest that Eptesicus may suppress interference by a more sophisticated strategy than simple linear matched filtering.