Development of hearing and vocalization in a marsupial, the Northern Quoll, Dasyurus hallucatus

The development of hearing was studied in the Northern Quoll, a nocturnal marsupial carnivore whose young are born into a pouch in an immature state after about 21 days in utero. Startle responses to noise bursts of 105 dB sound pressure level first appeared at 60 days after arrival in the pouch, but only to occasional stimuli; forelimb rather than whole body twitches were evoked. The latter were elicited regularly at 67 days onward. Auditory brainstem response (ABR) audiograms were measured during the period when startle responses first appeared. At 68 days responses were elicited between 1 and 16 kHz with thresholds in excess of 55 dB. At 81-88 days responses occurred over the adult range at lower thresholds than observed in the adult. The waveform of the ABR increased in complexity and decreased in latency during development. Pouch-young emitted acoustically well-defined isolation cells when separated from their mothers. Fourier analysis of these calls revealed peak energy at 8-11 kHz irrespective of the sex of the pouch-young. The isolation call first became evident at about 35 days, when the young were deliberately removed from the nipple, and ceased being emitted beyond about 80 days, some 5 or so days after the eyes open, when the young leave the pouch for substantial amounts of time. The spectrum of the call was similar at all ages within this 45 day period, and its peak frequency correlated closely with the best frequency in the adult quoll's ABR audiogram. At approximately 80 days of age the calls developed major low frequency components, similar to those measured in adult vocalizations, and the contributions of frequencies between 8-11 kHz waned in importance. The results indicate that the onset of hearing in quolls occurs at a relatively early time in the course of their development, but prior to this time the pouch-young emit characteristic isolation calls easily detectable by the mother.