Horses recognise human fear by the smell of sweat

Horses are able to "smell" human fear by smell alone - and this affects their behaviour and physiological responses.

This conclusion was reached by a group of French researchers, whose work was published in the journal PLOS One.

Scientists have previously shown that dogs can detect human stress by smell. The team from the National Institute for Research in Agriculture, Food and Environment (INRAE) decided to test whether a similar mechanism operates in horses - animals that have been around humans for thousands of years.

To obtain "emotional" odour samples, the researchers collected sweat from 30 volunteers using cotton pads. The first set of samples were taken when the participants watched the horror film Sinister and experienced fear. The second was taken later, while watching positive and funny videos, when the mood was more calm and joyful.

The pads were then placed in a special elastic muzzle (made of Lycra) and put on 43 Welsh-bred mares. The animals were randomly divided into three groups: "fear" (sweat odour after a horror film), "joy" (sweat odour after positive videos) and control (clean pads with no human odour).

The horses were then given four tests: grooming, reaction to a suddenly opening umbrella, human proximity, and learning a new object. When the animals "sniffed fear," they were more likely to show signs of anxiety: they flinched more, looked at unfamiliar objects for longer, showed more pronounced spikes in heart rate, and were less likely to approach or touch a human.

The key point of the experiment is that the horses did not see frightened people and could not orientate themselves by facial expression or body language: the effects are attributed specifically to chemical signals in the smell of sweat (their composition was not analysed separately).

The authors note that the results may be important for the practice of working with horses. If a person's emotional state is communicated through odour, even when they are trying to 'keep a straight face', this could influence training, care and general animal welfare.