Behavioral sensitivity of the European blind cave salamander, Proteus anguinus, and a Pyrenean newt, Euproctus asper, to electrical fields in water.

In the urodeles Proteus anguinus and Euproctus asper, thresholds of an overt avoidance response to weak electrical field stimuli (continuous sine-waves) were measured as a function of frequency. Thresholds down to 0.1 mV/cm (30 nA/cm2) were found in P. anguinus and 2 mV/cm (600 nA/cm2) in E. asper at 'best frequencies' (B.F.) of 20-30 Hz, but sensitivity covered a total frequency range of below 0.1 Hz to 1-2 kHz, with up to 70 dB higher thresholds. Average thresholds of 1 mV/cm in P. anguinus and 40 mV/cm in E. asper were more than 30 dB apart and significantly different. Both species were sensitive to galvanic DC-pulses, clicks, and noise bursts with intensities of about the same order of magnitude. Specimens of the transparent catfish, Kryptopterus (Siluridae) reacted in the same frequency range as found for Proteus and Euproctus, and had still lower thresholds, down to 0.02 mV/cm (1.5 nA/cm2). The biological significance and possibly still ongoing evolution of the electrical sense in urodeles is interpreted in terms of comparative sensory physiology and more recent, still speculative, evolutionary diversification during and since the Pleistocene.