Sensory processes in the control of isolation-induced ultrasonic vocalization by 2-week-old rats.

The presence of a passive (anesthetized) littermate greatly reduced the ultrasonic vocalization of 2-wk-old rat pups placed suddenly in an unfamiliar test box, without affecting other behavior. Isolated pups maintained high levels of vocalization over the 6-min test period while rapidly reducing other behavioral activities. A series of experimental surrogate models allowed analysis of the sensory modalities necessary and sufficient for the littermate effect. Pups did not respond to a single cue or to a pattern of different modalities, but they gave graded responses to several sensory sensory modalities in a cumulative fashion. Extent of body contact and reduction of ultrasound were positively correlated across surrogates, but not across individuals in most conditions. Pups made anosmic with ZnSO4 did not reduce vocalization and continued to show high levels of body contact. Experiments manipulating the olfactory properties of the littermate, however, raised the question whether olfactory denervation exerted its effects by depriving the pup of cues or by secondary effects on arousal and/or motivational systems.