MASKING PATTERNS IN THE BULLFROG (RANA-CATESBEIANA) .1. BEHAVIORAL-EFFECTS

A psychophysical technique based on reflex modification was used to measure masked auditory thresholds in the bullfrog (Rana catesbeiana). Masked thresholds were converted to critical ratios to describe the frequency selectivity of the bullfrog''s hearing. The bullfrog''s critical ratio function is complex. At low frequencies (100-600 Hz), it increases gradually with tone frequency at a rate of about 4 dB/octave. At high frequencies (1000-2500 Hz), the rate of increase with tone frequency is steeper (9 dB/octave). Critical ratios are lowest at frequencies around 1000 Hz (about 18 dB). Unlike effects previously reported in another anuran species, the green treefrog, the bullfrog''s critical ratio function is not parallel to its pure-tone audiogram. Moreover, the relationship of the bullfrog''s critical ratio function to the spectral composition of its species-specific vocalizations is not as compelling as in the green treefrog.