Discrimination of pure-tone intensities by the California sea lion

The ability of the California sea lion (Zalophus californianus) to detect intensity differences of 16-kHz pure-tone pulses was determined in a yes-no signal detection task with trial by trial feedback, using a modified method of limits similar to the “staircase method.” Additionally, three prethreshold experiments were performed to determine the effect of stimulus presentation strategy on discrimination ability. Results of the prethreshold experiments suggest that a “collapse” method (i.e., symmetrically reducing the intensity of the louder tone pulse in a pair and increasing the intensity of the softer) resulted in smaller threshold estimates than either an ascending or descending staircase method. Under the collapse method, a difference threshold estimate of 3.19 dB was found for 16-kHz pure-tone pulses. These results support a previous hypothesis based of behavioral underwater sound localization experiments and sound-skull measurements by Moore and Au [J. Acoust. Soc. Am. 58, 721–727 (1975)]. These experiments also indicated that a sea lion which persistently biased its responses by making a low rate of false alarms in an absolute threshold task [Schusterman, J. Acoust. Soc. Am. 55, 845–848 (1974)] did not bias its responses in a difference threshold task.