Of mice, cows, smells and men

Animals, endowed with sensory organs different from ours, do not perceive the world in the same way as we do. If our sensoriality is predominantly audiovisual, olfaction prevails as much as vision and hearing in most other mammals. In order to live as well as possible with the animals around us, whether our pets or farm animals, it is necessary to understand the sensory bases of the human-animal relationship. For this, the question arises of the contagion of emotions from humans to animals, but also from animals to humans, in order to respect or even improve their well-being, but also ours.

The issue of animal welfare has taken on increasing importance and is at the heart of concerns about the future of animal husbandry. In 2018, the National Agency for Food, Environmental and Occupational Health and Safety (ANSES) proposed a definition of animal welfare:

“An animal’s welfare is the positive mental and physical state related to the satisfaction of its physiological and behavioral needs, as well as its expectations. This state varies according to the perception of the situation by the animal. »

ANSES also emphasizes that “positive human actions towards animals (welfare) are an essential prerequisite for animal well-being”. The human-animal relationship is therefore a key component of animal well-being, but also of that of the breeder. Indeed, a relationship based on calm relationships and with unstressed animals reduces the risk of accidents. The breeder is therefore less stressed on a daily basis, and a virtuous circle is established. The evaluation of this relationship necessarily involves taking into account the sensory perception that the animal has of the human.

Olfaction, a sense of importance in mammals

Surprisingly, the influence of human olfactory cues on farm animals has so far been little considered, despite olfaction being a dominant sensory modality in mammals. As a result, the role of olfaction is potentially massive, and this from the earliest stages of development. It supports and facilitates the establishment of the first social interactions and selective attachment relationships. In sheep, for example, newborn lambs seek acquired odors in utero compared to new smells.

Farm and laboratory animals are also able to perceive the emotions of other congeners via olfactory signals, inducing behavioral and physiological changes. For example, cows will take longer to eat from a bucket or explore a new object if they are exposed to urine odors from stressed peers.

Finally, in a prey-predator relationship, animals are able to olfactorily identify animals of different species. For example, rodents (the prey) that are put in the presence of cat or fox faeces (their predators) will show fear behaviors, such as “freezing” (the fact of being petrified and not moving). or avoidance of these faeces, but also secrete stress hormones like cortisol.

These studies show the importance of olfactory communication between animal species (whether within the same species or between animal species), but generally not with humans. However, the importance of olfaction in human-animal interactions is also beginning to emerge.

Olfactory communication between humans and domestic animals

Companion animals or leisure animals such as dogs or horses can discriminate “emotional” human body odors, that is to say samples from transmitters exposed to a given emotional state (fear or joy). In the presence of human fear scents, Labradors and Golden Retrievers showed fear behaviors: they had a faster heart rate and stayed closer to their owner. Conversely, in the presence of human odors of joy, their heart rate was slower and they showed joyful behaviors towards unfamiliar people by approaching and interacting with them. Horses, on the other hand, are more vigilant (they raise their heads more often and longer) in the presence of human scents of fear than in the presence of scents of joy.

Dogs react to the crying of a human baby by increasing their own level of stress: this is called emotional contagion.
Crazybananas/flickr, CC BY-NC-ND

Interestingly, these studies showed that the perception of these emotional odors activated in the recipient animals a response congruent with the emotional state of the human transmitter, according to a form of emotional contagion similar to that observed between humans. For example, when we hear a child crying, our secretion of cortisol (stress hormone) increases: this is a sign of empathy, and the emotion of sadness or fear is transmitted from the child to the adult who hears it. But it is also transmitted to the dog, which will also increase its level of cortisol.

What about farm or laboratory animals?

These different results thus suggest that olfaction could influence the establishment and quality of the human-animal relationship, and thus impact the well-being of both parties. The question of my research work is therefore the following: “As the human-animal relationship is a key factor in animal and human well-being, are farm and laboratory animals able to perceive human emotions via olfactory? »

One of my first studies aimed to test whether a stressed human odor modifies the behavior of farm animals (like cows) and laboratory animals (like mice). Two sweat odors were collected from 25 engineering school students (14 women, 11 men, aged 19 to 23): a “stress” odor after an exam and a “non-stress” odor after lessons. Two experiments to discriminate between these odors were conducted: one on 20 male mice under controlled conditions and the other on 10 cows on the farm.

Mice defecated more in the presence of the stress odor and cows spent more time smelling the non-stress odor. Increased defecation can be seen as a marker of stress in animals, but also in humans (for example, we may want to urinate more frequently before an assessment or interview). Conversely, the fact of interacting longer with an object (smelling it, touching it or manipulating it) can be considered as a marker of interest and not of stress in the animal.

Thus, mice and cows seem to perceive and react differently to the smells of human emotions. Mice seem to show a fear response to the smell of human stress instead. Cows, on the other hand, seem to show a preference for the non-stress odor, but without seeking to avoid or run away from the stress odor. These preliminary results may indicate different levels of attachment between animals and their breeder, but also different husbandry and handling practices.

Studies should continue in these animals to determine soothing human emotional odors to improve the human-animal relationship and their well-being. One could imagine using human odors of joy to soothe animals during stressful events with humans they do not know, for example during transport or at the time of slaughter. Genetic selection of animals could also be considered, selecting animals least reactive to human stress odor.

Our next work will focus on testing different human emotional odors (such as joy and stress) in sheep. Sheep are indeed accessible farm animals, generally curious and expressive. The management of a sheep herd also involves many manipulations in contact with humans (calving, identification, weighing, shearing, hoof trimming), which exposes the animals to human olfactory cues, which they can probably also detect remotely. The sheep model is therefore particularly interesting to further our studies.

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