The nursing behaviour of the sow commonly involves upward rotation of the udder, movements of the head and legs, and rhythmic grunting. Aspects of this behaviour were studied in several experiments with pregnant females, juveniles males, and suckling piglets. Many pregnant females lay down and assumed the nursing posture when their udders were first rubbed by hand. Those which initially appeared agitated in the experimenter's presence often failed to do so until after some habituation to the procedure. When the upper and lower rows of teats were rubbed alternately, the udder was usually rotated toward the stimulation. This rotation, like the rate of grunting and milk ejection, was greatest when the most anterior teats were stimulated. Juvenile male pigs showed very similar behaviour. When housed alone they lay down in response to manual stimulation of the belly, but only after some habituation to the experimenter. They rotated their bellies toward the stimulation, and gave most grunts in response to stimulation of the anterior teats. Sleeping piglets often gave an attenuated form of the nursing posture when their bellies were rubbed gently. Rotation of the belly was the most common response, with the movements of the head and legs apparently occurring as elaborations.
Fraser, D. (1976). The Nursing Posture of Domestic Sows and Related Behaviour. Behaviour, 57(1), 51-63.