We investigated whether domestic sow discriminates one day old piglets based on their individual odours or based on some odour common to the whole litter. We manipulated the contacts between sow and piglets during the first 24 h post partum in such a way that produced 4 types of piglets, as combinations of two factors: relatedness and familiarity. We used ten pairs of sows individually housed in farrowing pens in which parturitions were induced on the same day. When the manipulations were finished, each sow was subjected to two testing situations. First it was a 5 min choice test in which the sow could show her preference for the 4 types of anaesthetized (immobile and mute) piglets: Own Familiar, Own Unfamiliar, Alien Familiar and Alien Unfamiliar. Second the sow was subjected to three 2.5 min preference tests with the following combinations of dyads of awake piglets: Own Familiar vs Alien Unfamiliar; Own Familiar vs Own Unfamiliar; Alien Unfamiliar vs Alien Familiar. We recorded the frequency and duration of visits and the grunting rate of sow with respect to each piglet type, in the test with awake piglets we also recorded piglets' vocalisation and movement. In the test with anaesthetized piglets we found a significant effect of piglet type on the duration of visits (F
(3,16) = 3.94, p = 0.014). The significant difference between Own Unfamiliar and Own Familiar piglets indicates that sows use the individual odours of piglets to discriminate between own and alien ones. The number of visits to individual piglets was also affected by piglet type (F
(3,16) = 2.71, p = 0.055), with sows visiting Own Familiar piglets significantly more often than Alien Unfamiliar ones. In none of the three preference tests with awake piglets did the sows show significant preference for a specific type of piglet. However the sows' visits in the piglets was affected by the piglets' activity. In periods when both the piglets were moving the sow spent a greater proportion of time by visiting them (Students t-test, N = 11, mean = 0.12, p = 0.0014) then during periods when only one or none of the piglets was moving. Sows also spent a greater proportion of time visiting the piglets when they were vocalising (Students t-test, N = 11, mean = 0.129, p = 0.019) than when they were not. The study shows that early recognition of piglets by sows is probably mediated by the individual odour of each piglet and that kinetic and acoustic cues augment the motivation to reunite with the piglets and this may then override the willingness to discriminate among them.