A horse's sense of smell is incredibly keen, often alerting them to any sign of danger before it can be seen. Researchers in France, led by Plotine Jardat, sought to determine whether horses could differentiate human emotion by smell, as well.
To test the theory, the scientists created a habituation-discrimination test, which presents an odor in two successive trials, then presents that odor and a new, novel odor simultaneously. Researchers expect the duration of sniffing to decrease in the first phase, called the habituation phase, then increase when introduced to the new odor in the discrimination phase.
During the test, the scientists observed whether a horse preferred to use an ear, eye, or nostril to evaluate the sample. Auditory and vision are contralateral, meaning they might look or hear out of the left eye or ear, but the input is processed on the right side of the brain. Smell is ipsilateral, meaning it occurs on the same side of the body.
Previous studies have shown that domestic mammals use the right brain hemisphere for negative or intense stimuli and the left hemisphere for familiar or positive stimuli.
The team used 30 Welsh mares to see whether they could discriminate between human scents related to joy or fear. They collected sweat via cotton pads placed in the armpits of 18 women and 6 men who had been shown a 20-minute video that encouraged fear or joy. One day later, the video viewing was repeated. Participant pads and shirts were then frozen in air-tight bags for up to six weeks; they were thawed at room temperature before the study began.
Once the samples were unfrozen, they were presented to the horses on a 5-foot pole so there was no co-mingling of scents. Three pads of one scent (joy or fear) and one of the opposite scent were used in total. The first odor was offered twice (habituation), then the same odor and a novel odor were offered simultaneously (discrimination), 20 inches apart.
The results show that horses used their left nostril more than their right during the habituation phase, which indicated that the scent was familiar or positive. During the discrimination phase, horses used their right nostril more, indicating a negative or intense stimuli.
The researchers concluded that horses can discriminate between human odors related to fear or joy, and that they can emotionally respond to the scents. They feel this information may affect how horses interact with their owners, riders or caretakers.
Read more at EquiManagement.
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