Features of the auditory development of the short-tailed Brazilian opossum, Monodelphis domestica: evoked responses, neonatal vocalizations and synapses in the inferior colliculus.

The onset of hearing in anesthetized South American opossums (Monodelphis domestica) was determined by the measurement of evoked potentials to click stimuli from the vertex of the skull immediately over the inferior colliculus. Evoked potentials were first recorded at postnatal day 24 at a threshold of 83 dB SPL; thresholds declined over subsequent weeks to below 58 dB at 40 days. Isolation calls emitted by the pups had stereotypic spectra with peaks at near 13 kHz and an octave higher. Such calls declined in frequency by day 32 and were not emitted at day 40. The peak frequency of the calls matched very closely the best frequency of hearing of adult Monodelphis. The number of synapses in the inferior colliculus increased at day 26; when plotted in relation to the number of cells, synaptic density increased steeply from day 27 after the animal had begun to hear. This suggests that environmental sound has a potent effect on the development of synapses in the auditory system.