Development of a scale to assess pigs’ immediate behavioural reaction when encountering novel odours

Abstract

It is currently not possible to predict how pigs will perceive novel odours, or whether certain odours might evoke avoidant or exploratory behavioural reactions. This study aimed to develop a reaction scale to assess the immediate behavioural reactions of pigs when encountering an odour for the first time. A secondary aim was to compare if reactions to different odours varied consistently among pigs, which could indicate that the reaction scale could be used to assign odour valence. The experiment included 184 growing-finishing pigs tested in 92 pairs of opposite-sex littermates. All pig pairs were presented with three out of a total of twelve different odours, in a balanced order. The odour samples consisted of six drops of essential oils (undiluted) on filter paper in a plastic container. Each of the three odours was presented along with an odourless control (distilled water). The odour and control samples were available to the pigs during 1-minute trials. The pigs’ behavioural reactions were recorded six seconds after each pig had first approached the sample. The reaction was scored on a scale from 1 to 6, where 1 was exploratory behaviour (e.g., sniffing) and 6 was active avoidance behaviour (e.g., leaving, head shaking). An ordinal mixed-effects model was fitted to the data and showed no effect of sex, age, or odour triad on reaction scores, but an effect of treatment with odour eliciting significantly more exploratory behaviour (lower reaction score) than control (P < 0.001). Pigs expressed large individual differences in reactions to 10 of the 12 odours, but a consistent pattern of significantly more exploratory reactions to vanilla and blood orange (both odours: P < 0.05), which could suggest that pigs may perceive these odours as particularly pleasant. Reaction scores were significantly lower (i.e., pigs performing more explorative behaviour) for the first odour tested compared with subsequent odours (P ≤ 0.04) irrespective of the specific odour. The 6-point scale effectively described explorative and avoidance behaviours that may reflect the valence of the pigs’ reactions. To our knowledge, this is the first scale specifically developed to assess pigs’ immediate behavioural reactions to novel olfactory stimuli in a commercial production environment. The development of the reaction scale and the findings provide useful guidance for future research on odour perception. The observed decrease in interest over time should be taken into account when repeated testing is used.