When suckling piglets are isolated from the sow and litter-mates they vocalize a great deal. Sows also call when isolated from their piglets. In one experiment, we found that isolated piglets doubled their call rate in response to playback of sow calls, indicating that piglets are directing calls towards the sow. In a second experiment, we found that variation in the piglet isolation calls related to aspects of the piglet's condition: piglets isolated for 13 min in an enclosure kept at approximately 14°C called more, used higher frequency calls and longer calls, than litter-mates isolated in an enclosure kept at 30°C. These differences in vocal behaviour were significant at 8 and 12 min of isolation, but not at 1, 2 or 4 min. These results for pigs correspond to those found with other species, indicating that characteristics of isolation calls can provide information about a young animal's need for supplemental heat that can be supplied by the parent. The results also agree with a previous study on pigs, showing that features of these vocalizations vary with the piglet's need for other resources such as milk.
Weary, D. M., Ross, S., & Fraser, D. (1997). Vocalizations by isolated piglets: a reliable indicator of piglet need directed towards the sow. Applied Animal Behaviour Science, 53(4), 249-257.