Two manipulations were performed on domestic piglets to determine whether differences in calling during periods of separation from the mother can indicate differences in need. In both cases, the aim was to manipulate the piglet's need for the sow's attention. In the first manipulation a 'thriving' piglet (i.e. the piglet with the heaviest weight and most rapid weight gain) and a 'non-thriving' one (lightest and slowest weight gain) were selected from each of 15 litters. The two piglets were removed from the sow and litter and recorded for 13 min in separate isolated enclosures. For the second manipulation, two piglets of intermediate weight and weight gain were selected from each of the same 15 litters, and were removed from the sow during nursing under one of two conditions. The 'unfed' piglet was removed just before the milk ejection and the 'fed' one just after receiving milk. Both were recorded as in the first manipulation. 'Non-thriving' and 'unfed' piglets called more and used more high-frequency calls, longer calls, and calls that rose more in frequency than their 'thriving' and 'fed' litter-mates. By means of a playback experiment, the assumption that sows respond to these piglet calls was tested. Sows were more likely to vocalize and approach the loudspeaker during playback of the piglet isolation calls than during playback of white noise. It is argued that if a piglet's calls provide reliable information about its need for the sow's resources, then this calling can be used as a measure of its welfare. These results are consistent with theoretical models of honest signalling.
Weary, D. M., & Fraser, D. (1995). Calling by domestic piglets: reliable signals of need?. Animal Behaviour, 50(4), 1047-1055.