Genetic and developmental influences on infant mouse ultrasonic calling. II. Developmental patterns in the calls of mice 2-12 days of age.

Infant house mice, Mus musculus, produce ultrasonic calls that reliably lead to retrieval by adult mice. While individual differences in calls have been demonstrated both among and within species, the influences of age and sex on call characteristics have not been systematically investigated in mice. This study examined the influences of age, sex, and genotype (inbred versus hybrid) on the rate, length, and frequency characteristics of the calls of 486 male and female mice from 2 to 12 days of age. Rate of calling followed a shallow inverted U-shaped function across days. Call lengths decreased and call frequency characteristics increased, in a linear manner, with age. Females emitted fewer calls, with a smaller bandwidth, at some ages than males. Hybrid pups produced more calls of greater length and a lower frequency than inbred pups. These results indicate the presence of cues that could allow adult mice to behave differentially toward pups as a function of their age and sex.