Ultrasonic vocalizations and ontogenetic development in California mice (Peromyscus californicus)

This experiment investigated the time-course of behavioral and physical development in 18 male and 18 female pups from 18 litters of California mice. From 1 to 23 days of age animals were observed for a 6-min observation period every other day, followed by a neurobehavioral testing session. Ultrasonic vocalizations (UVs), coordinated movements, neuromotor indicators and physical parameters were measured. The production of UVs peaked during the first week of development, remained stable from 9 to 15 days of age, and decreased abruptly thereafter. Females vocalized more than males during the first 9 days after birth, although this difference only was statistically significant on the third day. There was an inverse relationship between the emission of UVs and coordinated movements. The frequency of UVs dropped significantly 1 day after the pups had displayed surface righting, were able to hear, and had begun opening their eyes compared with 1 day before. In conclusion, it appears that there is a sex difference in the emission of UVs, which is more intense at the beginning of development. Furthermore, the ontogenetic patterns of changes in the emission of UVs, by both males and females in California mice, are inversely related to the development of coordinated movements, such as rearing, locomotion and self-grooming.