Maternal vocalisations and other sounds in the fetal lamb's sound environment.

In a first experiment a miniaturised radio hydrophone was implanted inside the amniotic sac in three pregnant ewes and recordings were made from 3 weeks before the lambs were born until they had emerged; these recordings showed that the sound of the mother's voice was slightly louder when picked up by the hydrophone inside the amniotic sac than when picked up by a microphone beside her flank and that sounds recorded during labour included long and loud low frequency sounds associated with contractions, and an increase, compared with before labour, in the incidence of sounds produced by the maternal cardiovascular system and by breathing. Over the last 3 weeks of gestation the attenuation of sounds from outside the mother decreased at frequencies between 500 and 4000 Hz, but not below or above those frequencies. In a second experiment the bleats of 23 pregnant ewes were recorded; their lambs were taken at birth and tested with the sound of either their own mother's bleats, or with bleats from an alien ewe. Heart rate changes which occurred during playback of maternal and alien bleats differed significantly, but only on the occasion when each lamb heard its first bleat, postnatally.