Hearing in the crepuscular owl butterfly (Caligo eurilochus, Nymphalidae)

Tympanal organs are widespread in Nymphalidae butterflies, with a great deal of variability in the morphology of these ears. How this variation reflects differences in hearing physiology is not currently understood. This study provides the first examination of the hearing organs of the crepuscular owl butterfly, Caligo eurilochus. We hypothesize that (1) its hearing may function to detect the high-frequency calls of bats, or (2) like its diurnal relatives it may function to detect avian predators, or (3) it may have lost auditory sensitivity as a result of the lack of selective pressures. To test these hypotheses we examined the tuning and sensitivity of the C. eurilochus Vogel’s organ using laser Doppler vibrometry and extracellular neurophysiology. We show that the C. eurilochus ear responds to sound and is most sensitive to frequencies between 1-4 kHz, as confirmed by both the vibration of the tympanal membrane and the physiological response of the associated nerve branches. In comparison to the hearing of its diurnally active relative, Morpho peleides, C. eurilochus has a narrower frequency range with higher auditory thresholds. We conclude that hearing in this butterfly is partially-regressed, and may reflect a trade-off between hearing and vision for survival in low light conditions