Vocal communication in pigs: Who are nursing piglets screaming at?

Vocalizations during competition among nursing piglets were studied to inve stigate their possible effects, functions and implications for welfare. In Expt 1, two experimental piglets in each of 14 litters were temporarily dep rived of milk by covering their preferred teats on the sow's udder. These p iglets spent more time away from their teats than two control piglets, and vocalized frequently in the 2 min before milk ejection. Frequency of vocali zation showed no consistent change over time within nursings; nor did it ch ange in successive nursings despite the fact that hunger presumably increas ed. In Expt 2, tape recordings of intense vocalizations (screams) produced by piglets competing at the udder were played to 22 litters while they were nursing; each litter was played its own recording, a recording from anothe r litter and silence as a control. Of 51 nursings analysed, 14 were termina ted without milk ejection, all during playbacks. When the sow did nurse suc cessfully during a playback, nursing was shorter (138 s) than during the si lent controls (179 s). Both these responses by the sow might be expected to advance the next nursing. Piglets rarely showed any apparent response to s creaming either from their littermates or from the loudspeaker. These resul ts suggest that the calls function mainly as a signal to the sow that some piglets are being excluded from the current nursing episode.