Horses’ exploration of smell and their interest in odors varies with age and pregnancy, researchers have found.
Horses have a highly developed olfactory apparatus. Despite this, research into their sense of smell is sparse, Maria Vilain Rørvang and her fellow researchers noted in the journal Frontiers in Behavioral Neuroscience.
It is clear, they said, that horses are sensitive to, and affected by, odors in their surroundings. “These abilities are mentioned in equitation science books, but are also often ignored in practice.
“How horses respond to odors is important as it plays a key role in their everyday life, and hence their welfare, ” they said. Their reaction to odors – and the ability to predict those reactions – are crucial for humans to ensure safety when handling and training horses.
An odor may be neutral to the horse, they said, but it might also elicit either avoidance behavior or have an attractive effect depending on the horse’s perception of it.
The current limited knowledge on equine smells poses a risk that horse behavior does not match human expectations, they said, as horses might react fearfully when exposed to certain odors, which humans do not consider as frightening.
Learning more about equine smell could therefore boost our understand of horse behavior and reduce the risk of dangerous situations. There may also be unexplored potential in using odors in several practical situations where humans interact with horses.
In their study, Rørvang joined with Klára Nicova and Jenny Yngvesson to investigate the behavior and smell sensitivity of 35 Icelandic horses.
The horses were exposed to four odor oils – peppermint, orange, lavender and cedar wood – in an experimental protocol.
All are complex odors, which were chosen as they are natural and non-toxic, cheap, accessible, and easy to standardize. The four were chosen from 36 possible odors as the researchers considered they would be novel to the horses, since none of them were found in their feed, hay or as ingredients in any products used on the farm, such as creams or soaps.
Moreover, they hypothesized, based on human perception, that the four odors would be perceived as different to each other.
Each odor (10 drops of oil on a piece of filter paper) was placed in a weighted bucket, which had a mesh material placed over the top. Each bucket was then placed within reach of the horses from their stables. Their interactions with each bucket were monitored.
Each odor was presented three times in a row for one minute, with a break of two minutes in between. After the first odor bucket had been placed within reach three times, the horse had another two-minute break before being presented with the next odor bucket, with a different scent.
Sniffing duration varied among the horses, while behavioural reactions involved mostly licking and biting, while snorting or backing away were seen only rarely. Responses between the horses were examined, as were the potential effects of age, sex and pregnancy.
The analysis showed that the horses became habituated to each individual odor as the three trials progressed, with a significant decrease in sniffing duration per presentation. Interest was renewed when a fresh odor was presented, and sniffing times jumped up again.
“Horses were thus able to detect and distinguish between all four odors,” the study team reported. However, they spent significantly longer sniffing when exposed to peppermint.
More horses expressed licking when presented with peppermint compared to cedar wood and lavender. The horses, they said, may have perceived peppermint odor as edible, although none of the horses were understood to have ever been exposed to it before in treats or feed.
Pregnant mares (eight of the study horses were in foal) sniffed odors less than non-pregnant mares, the authors found.
Young horses (those under 5) sniffed cedar wood for longer than older horses. Sex had no effect on the response of the horses, the researchers found.
“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.”
Odors, they said, may constitute a source of environmental enrichment for horses either directly as pleasant scents, or secondarily as new scents to already existing enrichment materials.
Rørvang and Yngvesson are with the Department Biosystems and Technology at the Swedish University of Agricultural Sciences; Nicova is with the Department of Ethology, part of the Institute of Animal Science in Prague, Czechia.
Rørvang MV, Nicova K and Yngvesson J (2022) Horse odor exploration behavior is influenced by pregnancy and age. Front. Behav. Neurosci. 16:941517. doi: 10.3389/fnbeh.2022.941517