Horse odor exploration behavior is influenced by pregnancy and age

Abstract

In spite of a 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 behavior does not match human expectations, as horses might react fearful when exposed to certain odors, 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 behavior and hence reduce the risk of dangerous situations, and (2) there may be unexplored potential of using odors in several practical situations where humans interact with horses. This study investigated behavior and olfactory sensitivity of 35 Icelandic horses who were presented with four odors: peppermint, orange, lavender and cedar wood in a Habituation/Dishabituation paradigm. The response variables were sniffing duration per presentation and behavioral reaction (licking, biting, snorting, and backing), and data were analyzed for potential effects of age, sex and pregnancy. Results showed that habituation occurred between successive odor presentations (1st vs. 2nd and 2nd vs. 3rd presentations: P < 0.001), and dishabituation occurred when a new odor was presented (1st vs. 3rd presentations: P < 0.001). Horses were thus able to detect and distinguish between all four odors, but expressed significantly longer sniffing duration when exposed to peppermint (peppermint vs. orange, lavender and cedar wood: P < 0.001). More horses expressed licking when presented to peppermint compared to cedar wood and lavender (P = 0.0068). Pregnant mares sniffed odors less than non-pregnant mares (P = 0.030), young horses (age 0-5 years) sniffed cedar wood for longer than old horses (P = 0.030), whereas sex had no effect (P > 0.050). The results show that horses' odor exploration behavior and interest in odors varies with age and pregnancy and that horses naïve to the taste of a substrate, may be able to link smell with taste, which has not been described before. These results can aid our understanding of horses' behavioral reactions to odors, and in the future, it may be possible to relate these to the physiology and health of horses.

Keywords: enrichment; horse-human relationship; nose-work; olfaction; scent; sensory ability; smell; trigeminal nerve.

FIGURE 1

(A) Illustration of an odor bucket with wire mesh lid, ballast rock and filter paper (with odor sample). The odor bucket measured 35 × 16 × 27.5 cm. (B) Illustration of the test situation. The odor bucket was placed on the aisle floor 25 cm from the grating to the horse’s pen. Sniffing duration was measured as from when the horse’s muzzle was within the length of one horse muzzle (12 cm) away from the bucket.

FIGURE 2

(A) Illustration of sniffing duration per horse during odor presentations (1st, 2nd, 3rd) over time regardless of specific odor. (B) Illustration of sniffing duration per horse during odor presentations (1st, 2nd, and 3rd) for all odors, orange representing orange, light blue representing peppermint, purple representing lavender and light gray representing cedar wood. For both (A,B) the boxes represent the 25, and 75% quartiles, the thick line inside the box represent the median and the dashed lines illustrate the range.

FIGURE 3

Illustration of the total sniffing duration (seconds) of each odor per horse. Orange represent orange, light blue represents peppermint, purple represents lavender and light gray represents cedar wood. The boxes represent the 25, and 75% quartiles, the thick line inside the box represent the median and the dashed lines illustrate the range.

FIGURE 4

The total number of horses expressing licking, biting, snorting and backing separately when presented with each of the odors 1st, 2nd, and 3rd, time. Orange represent orange, light blue represents peppermint, purple represents lavender and light gray represents cedar wood.

FIGURE 5

Illustration of sniffing duration (seconds) per odor (orange, peppermint, lavender and cedar wood), divided by pregnant (blue) and non-pregnant (red) females. The boxes represent the 25, and 75% quartiles, the thick line inside the box represent the median and the vertical error bars illustrate the range.