 1999 John Wiley & Sons, Inc CCC 1012-1630/99/030195-10 Stefan M. Brudzynski Department of Psychology, Brock University, St. Catharines, Ontario, L2S 3A1 Canada Priscilla Kehoe Megan Callahan Neuroscience Program, Trinity College, Hartford, CT 06106 Sonographic Structure of Isolation-Induced Ultrasonic Calls of Rat Pups Received 15 April 1997; accepted 8 September 1998 ABSTRACT: Sonographic analysis of isolation-induced calls of 10- to 17-day-old Sprague- Dawley rat pups showed that average acoustic parameters of calls changed with pups’ age. Average call duration increased with age from 80 ms to over 140 ms in 15-day-old pups. The peak frequency increased from approximately 50 kHz to an average of 64 kHz in 17-day-old pups, and the average bandwidth increased from 34 to 45 kHz in 17-day-old pups. Analysis of the sonographic structure of pup calls additionally revealed a tendency to produce two or more alternating sweeps of sound frequency in each call. Development of sweeps is the most typical feature of pup calls with a dominant call type resembling “U” or inverted “U” shape in the sonogram. Number of “U” or inverted “U” call types significantly increased with pups’ age. It is concluded that pups developed and strengthened those acoustic features of distress calls which play a role in intraspecific communication and maximize pup survival.  1999 John Wiley & Sons, Inc. Dev Psychobiol 34: 195– 204, 1999 Keywords: ultrasonic vocalization; isolation calls; distress calls; rat pups; vocal communi- cation; sonographic analysis Infant rats emit ultrasonic vocalization in a number of distress situations such as separation of the neonates from the mother, acute isolation from the nest, unfa- miliar environment or odors, thermal or tactile stimuli, etc. (Allin & Banks, 1971, 1972; Conely & Bell, 1978; Gardner, 1985; Ha ˚rd & Engel, 1988; Hofer & Shair, 1978, 1980; Insel, Hill, & Mayor, 1986; Insel, Miller, Gelhard, & Hill, 1988; Kehoe & Blass, 1986a, 1986b; Miczek, Tornatzky, & Vivian, 1991; Noirot, 1968, 1972; Okon, 1971; Oswalt & Meier, 1975; Sales & Pye, 1974). Behavioral and physiological studies of the mother – pup relationship clearly indicate that Correspondence to: S. M. Brudzynski Contract grant sponsor: Natural Sciences and Engineering Re- search Council of Canada Contract grant number: RGPIN 98 emission of ultrasonic calls by pups in dangerous or potentially dangerous situations plays an adaptive role in activating maternal response directed to the endan- gered pup(s). Pup ultrasonic isolation calls elicit searching and retrieval behaviors by the dam and shorten the response latency to transport litters from an endangered nest (Allin & Banks, 1972; Brewster & Leon, 1980; Smotherman, Bell, Starzec, Elias, & Zachman, 1974). The calls elicit not only maternal search behavior but have been shown to act as direc- tional cues for the dams (Brunelli, Shair, & Hofer, 1994). The isolation calls also contribute to an increase in anogenital licking by the dam following retrieval and induce prolactin release in lactating females (Brouette-Lahlou, Vernet-Maury, & Vigouroux, 1992; Terkel, Damassa, & Sawer, 1979; Voloschin & Tra- mezzani, 1984). There is also evidence that the rat pup