About mice, cows, smells and men

Animals, endowed with sense organs different from ours, perceive the world differently than we do. If our sensory is predominantly audiovisual, then in most other mammals the sense of smell predominates in the same way as sight and hearing. To live as best as possible with the animals around us, whether our pets or farm animals, it is necessary to understand the sensory basis of human-animal relationships. This raises the question of transferring emotions from people to animals, and also from animals to people, in order to respect or even improve their well-being, but also ours.

The issue of animal welfare is becoming increasingly important and is at the center of concern about the future of livestock production. In 2018, the National Agency for Food, Environment and Occupational Health (ANSES) proposed a definition of animal welfare:

“The welfare of an animal is a positive mental and physical state associated with the satisfaction of its physiological and behavioral needs, as well as its expectations. This state changes depending on the perception of the situation by the animal. »

ANSES also emphasizes that “positive human action towards animals (welfare) is an important precondition for animal welfare.” Thus, the human-animal relationship is a key component of animal welfare as well as that of the breeder. Indeed, relationships based on calm relationships and with relaxed animals reduce the risk of accidents. In this way, the breeder is subjected to less stress on a daily basis and a virtuous circle is established. Assessing this relationship necessarily involves taking into account the sensory perception of the person by the person.

Smell, sense of importance in mammals

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

Farm and laboratory animals are also able to perceive the emotions of other relatives through olfactory cues, causing behavioral and physiological changes. For example, cows will take longer to eat from a bucket or explore a new facility if they are exposed to urine odor from stressful peers.

Finally, in prey-predator relationships, animals are able to olfactorily identify animals of different species. For example, rodents (prey) that are in the presence of cat or fox feces (their predators) will exhibit fear behaviors such as “freezing” (petrifying and immobility) or avoiding these feces. but also release stress hormones such as 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 smell in human-animal interactions is also beginning to emerge.

Olfactory communication between humans and animals

Companion or recreational animals such as dogs or horses can distinguish “emotional” human body odors, i.e. patterns from transmitters undergoing a particular emotional state (fear or joy). In the presence of scents of human fear, Labradors and Golden Retrievers exhibited fear behavior: their heart rate increased and they stayed closer to their owner. Conversely, in the presence of human smells of joy, their heart rate was slower and they displayed joyful behavior towards strangers by approaching and interacting with them. Horses, on the other hand, are more alert (raising their heads more often and longer) in the presence of human scents of fear than in the presence of joyous scents.

Dogs react to the crying of a human baby by raising their own stress levels: 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 that coincided with the emotional state of the human transmitter, consistent with a form of emotional contagion similar to that observed between humans. For example, when we hear a baby cry, we increase the secretion of cortisol (stress hormone): 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 passed on to the dog, which also raises cortisol levels.

What about farm or laboratory animals?

Thus, these different results suggest that the sense of smell may influence the establishment and quality of human-animal relationships and thus affect the well-being of both parties. Thus, the question of my research paper is: “Since human-animal relationships are a key factor in animal and human well-being, are farm and laboratory animals able to perceive human emotions through smell? »

One of my first studies was to test whether the smell of a stressed person affects the behavior of farm animals (eg cows) and laboratory animals (eg mice). Two odors of sweat were collected from 25 engineering students (14 women, 11 men, aged 19 to 23): “stressful” after an exam and “non-stressful” after class. Two experiments were carried out to distinguish between these odors: one on 20 male mice under controlled conditions, and the other on 10 cows on a farm.

Mice were more likely to defecate in the presence of a stressful odor, while cows spent more time smelling a non-stressful odor. Frequent defecation can be seen as a marker of stress in animals, but also in humans (for example, we may want to urinate more often before an assessment or an interview). Conversely, the fact of a longer interaction with an object (sniffing it, touching it, or manipulating it) can be seen as a marker of interest rather than stress in the animal.

Thus, mice and cows seem to smell and react differently to human emotions. The mice seem to show a fear response to the smell of human stress instead. Cows, on the other hand, seem to have a preference for a non-stressful odor but do not try to avoid or run away from a stressful odor. These preliminary results may indicate different levels of affection between animals and their breeders, as well as different management and care practices.

Further research on these animals should be continued to identify soothing human emotional odors in order to improve human-animal relationships and their well-being. One can imagine the use of human smells of joy to calm animals during stressful situations with people they don’t know, such as during transportation or during slaughter. Animal genetic selection can also be considered, selecting animals that are least responsive to the smell of human stress.

Our next work will focus on testing various human emotional odors (such as joy and stress) in sheep. Sheep are truly accessible farm animals, usually curious and expressive. Sheep herd management also involves many human contact manipulations (calving, identification, weighing, shearing, hoof trimming) whereby the animals receive human olfactory cues that they can probably also detect from a distance. Therefore, the sheep model is of particular interest for our further research.

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