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Abstract
The developmental course of ultrasonic signaling by neonatal rat pups (Rattus norvegicus) was monitored under varying degrees of hypothermic stress. Signal rate, peak frequency, duration, and complexity of signals were analyzed separately for two different laboratory stocks, Sprague-Dawley and Wistar/Furth. Additionally, preliminary data were obtained on maternal retrieval latencies during the 1st 4 days post-partum and compared with the emission rate of neonatal ultrasounds. On the basis of these and other data, there is evidence that mothers play an important role in mediating many early experience effects. The data suggest that rate of neonatal ultrasounds may provide an index of some aspects of rate of maturity; that there may be different periods of maximum sensitivity to external stimulation in various stocks of laboratory animals; and that differential maternal responsiveness in different stocks of laboratory animals may correlate with differences in neonatal ultrasonic calling. Implications for comparative, genetic, and developmental studies are discussed.