Sonographic characterization of ultrasonic vocalisations emitted by infant laboratory mice Mus musculus [abstract]

Young mice and rats separated from their mother and littermates produce ultrasonic vocalisations during the first two or three weeks of postnatal life. Various conditions such as handling, hypothermia, olfactory or tactile stimulation elicit ultrasonic emission from the immature rodent. These vocalisations are also affected by either changes in social variables or by administration of psychoactives. Although it is known that pup vocalisations stimulate a prompt expression of maternal behaviours such as nest-building, searching and retrieving, the communicative role of infant ultrasonic vocalisations is still a matter of investigation. A fine-grained analysis of ultrasonic vocalisations per se, as well as the study of the different contexts in which calls are emitted, may contribute to a better understanding of their possible role in adult-infant intraspecific communication. Eight-day-old outbred CD-I mice were isolated from their mother and littermates and randomly exposed to one of five different experimental conditions, namely: i) odour from the nest; ii) social isolation; iii) low temperature; iv) tactile stimulation; v) odour from a conspecific adult male. Spectrographic analysis of the structure of the calls revealed that ultrasonic calls by neonatal mice are differently shaped (although conforming to five main stereotyped types: constant frequency, modulated frequency, lequency steps, composed, short) and that, when the sound-structure typology and emission frequency interval are considered, the context under which vocalisations are emitted strongly influences type and frequency of call production. Furthermore, the present study helps us to better understand the ultra- sonic production behaviour of the mouse, increasingly the most common rodent species in biomedical laboratories, and it can be highly adequate for the development of ethological models aimed at teratological or toxicological investigations. Finally, our results may be useful for the evaluation of regulatory guidelines (OCSE, WHO, EU, etc.) using functional endpoints for assessment of early behavioural deficit.