Group suckling cohesion as a prelude to the formation of teat order in piglets

During the lactation period, piglets experience intense social interactions with their littermates until they establish a reliable teat order on the mother's udder. Here, we examined group suckling cohesion in piglet littermates, an order mechanism that refer to the maintenance of significantly similar inter-individual distances on the udder and operates prior to the establishment of teat order. We analysed the suckling positions of 160 piglets from 16 litters, the distances between individuals on the udder in each suckling, and the stability of their suckling positions during lactation. Teat order stabilised by day 10 (PSuck = 0.62) and remained relatively stable (≈0.60) throughout the rest of lactation. Littermates tended to maintain significantly similar distances from one another on the udder throughout lactation [r = 0.30 (p < 0.05) to 1.00 (p < 0.0001)], although suckling stability remained incomplete. Partial analysis of suckling stability (examining each litter and period separately) revealed that the teat order did not always remain stable throughout lactation. However, group suckling cohesion was normally rigid and was unaffected by interruptions in teat order. Group suckling cohesion was an aspect of piglets' development of fixity on particular teats and thus was a part of the establishment of a teat order. Significantly higher mortality (71% of all recorded cases; p < 0.05) occurred in litters in which neither group suckling cohesion nor a stable teat order was established during the first days of lactation. Such litters were produced by sows with significantly lower parity (p < 0.05). A late start to suckling cohesion and a late establishment of teat order appear to compromise survival.