International Journal of Comparative Psychology

Pigs may express a startle response and then freeze after they have heard

an auditory stimulus. The objective of this study was to induce startle-freeze behaviour, describe it and to seek potential variables that might influence it. A startle-freeze response lasting 1 to 12 seconds was produced in 36 pigs 5-6 weeks old penned in 12 groups (3 pigs per group), using a cymbal (26 cm radius) to produce a sound of 1 10 dB intensity. This was repeated at 4 minute intervals for 8 stimuli over 28 minutes. The duration of the startle-freeze response decreased over presentations and all pigs had ceased to respond by the seventh stimulus. The frequencies of social interaction, ingestion and lying or sitting behaviours before freezing decreased after the freeze period, and standing or walking increased. The behaviours shown before and after immobilisation were not independent (x\ = 101.9, p < 0.001). The social status of each group was determined from videotape records. The dominance rank of the pig was significantly related to the onset of immobilization; the most dominant pigs responded to the noise first, often 1 to 2 seconds ahead of lower ranked pigs (x\ = 15.74, p < 0.05). In conclusion, pigs show a clear startle-freeze response to a novel auditory stimulus, they habituate rapidly and the onset of startle-freeze behaviour depends upon their social status.