The domestic pig has complex nursing and suckling behaviour, consisting of several distinct phases of suckling by the piglets and a characteristic pattern of grunting by the sow. A review of observations and experimental findings suggests cause-and-effect relationships between the different elements of sow and piglet behaviour, and the relationship of the behaviour to milk ejection. Characteristic features of pig nursing and suckling behaviour are interpreted as an adaptation promoting even distribution of milk among a number of precocious litter-mates.
Fraser, D. (1980). A review of the behavioural mechanism of milk ejection of the domestic pig. Applied Animal Ethology, 6(3), 247-255.