Sex difference in ultrasound distress call by rat pups.

Ontogenic changes in ultrasound production by isolated rat pups were compared in male and female pups of litters of various sex compositions. In both sexes, the peak of emission of sound around 50 kHz was on days 4-5, while 40 kHz sound production peaked on days 6-7. By days 4-5 each pulse is an inverted V-shape with frequencies between 40 and 55 kHz. At the peak of sound production, pulses with a slight downsweep from 40-45 kHz were prominent, and in the preweaning period, pulses of a relatively stable frequency of 35-40 kHz were emitted. There was a general tendency for sound pulses of female pups to be of short duration compared with those of male pups. The ontogenic changes in the ultrasonic distress call are less variable throughout all female pups of litters consisting of 2, 4, 6 or 8 female pups in a litter of 8. In contrast to this, it was found that male pups emit ultrasound more vigorously than female pups from days 2-3 to days 12-13 when male and female pups coexisted in the same litter. The activity of sound production of male pups which had no female littermates was similar to that of female pups. These findings suggest that the interaction of some kind between male and female pups produces the sex differences in the ultrasonic distress calls as early as several days after birth in the rat.