Behavioral measures of auditory thresholds in developing tree shrews (Tupaia belangeri).

The development of hearing in tree shrews (Tupaia belangeri) was tested through behavioral measures in the range of 0.3-60 kHz from the first postnatal thresholds detectable through to those of weanlings. The onset of hearing was defined using unconditioned startle responses, while, later on, pinna reflexes and head movements were used as behavioral indicators of sound perception. Startle responses could be evoked from day 16 after birth (DAB) onwards, indicating that the hearing system is capable of transferring airborne sound, although no reactions to tones were recorded at this age. The first unconditioned reactions to tones were found at DAB 18 in the range of 1-5 kHz (range of mother-infant call). Responses were restricted to the range of greatest sensitivity in adults (1-10 kHz). An increase in sensitivity was detected from DAB 18-38. The lowest tested frequency (0.3 kHz) reached adult levels earlier than higher frequencies. No further improvement of auditory thresholds could be observed from DAB 38 (end of nutritional weaning) to DAB 42. Results were related to recent findings on the development of acoustical behavior and of the peripheral and central auditory system of tree shrews and other mammals.