Horses’ interest in non-social odours varies with age and pregnancy in the Habituation/Dishabituation test paradigm

In spite of the highly developed olfactory apparatus of horses, implying a high adaptive value, research on equine olfaction is sparse. Our limited knowledge on equine olfaction poses a risk that horse behaviour does not match human expectations, as horses e.g. might react fearful when exposed to certain odours which humans do not consider as frightening. The benefit of acquiring more knowledge of equine olfaction is therefore twofold; 1) it can aid the understanding of horse behaviour and hence reduce the risk of dangerous situations occurring, and 2) there may be unexplored potential of using odours in several practical situations where humans interact with horses, i.e. to improve management routines and the situation for the horses. This study investigated the olfactory sensitivity of 35 Icelandic horses who were presented with four odours: peppermint, orange, lavender and cedar wood in a Habituation/Dishabituation paradigm. The response variables were sniffing duration per presentation and behavioural reaction (licking, biting, snorting, and backing), and we moreover analysed the data for any potential effects of age, sex and gestational stage. Results showed horses were able to detect and distinguish between all four odours and showed increased interest for peppermint (Wilcoxon signed rank test: orange vs peppermint: V = 68, P < 0.001, lavender vs peppermint: V = 20, P < 0.001, cedar wood vs peppermint: V = 47, P < 0.001). More horses expressed licking behaviour when presented to peppermint compared to cedar wood and lavender (Fishers Exact test: peppermint vs lavender & peppermint vs cedar wood: OR = 4.40, P = 0.0068). Young horses (age 0-5 years) sniffed cedar wood for longer than old horses (Anova: F = 10.03, p-value = 0.004), and pregnant mares sniffed lavender less than non-pregnant mares (Wilcoxon signed rank test: pregnant vs not pregnant: W = 93, P = 0.02), whereas sex had no effect (Wilcoxon signed rank test: p = 0.4). The results showed that the test paradigm was meaningful for testing olfactory sensitivity in horses, and olfactory interest of horses varied with age and gestational status but not sex.