Human odors appear to convey emotional information to horses, although scientists who have reviewed evidence related to this chemosignaling describe the results as tentative.
How can this be? Has evolution delivered chemical fingerprints with meaning across species?
Gün Semin and his colleagues, writing in the open-access journal Animals, say human body odors contain chemosignals that make species-specific communication possible.
Studies have been conducted on dogs and horses exposed to human chemosignals produced when happy and fearful.
Dogs showed behaviors consistent with the human emotions, they said, while horses showed differences in the activation of their autonomic nervous system, depending upon the odor.
“These results are leading the way for further studies on human-animal communication through emotional chemosignals,” they said.
The authors say that while human body odors contain chemosignals that can be sensed by other people, such communication is without communicative intent and is generally below the threshold of consciousness.
Human recipients of these chemosignals produced during emotional conditions can mirror that emotional state. But what about chemical signaling between species?
Research findings in the likes of dogs and horses – species with interdependence with humans that goes back thousands of years – indicate the possibility of a path to open our understanding of inter-species emotional communication via chemosignals, they say.
“Although horses and dogs arise from progenitors with a very different behavioral ecology, namely prey and predators, they have converged in acquiring similar skills in communicating with humans.”
In a communicative context, dogs have developed a high sensitivity to human gestures, being able to also learn human words. They can perceive and respond to human emotions through visual and auditory signals.
“Similar skills have been achieved by horses,” they note. “They are able to integrate different sensory systems to individually recognize both conspecifics and humans in a cross-modal recognition and to combine different facial cues of conspecifics to gather information on the environment.
“Moreover, horses can also communicate their emotions and understand the facial expressions of both horses and humans.”
Furthermore, some recent findings suggest that physiological measurements, such as heart-rate variability, taken from humans and horses show a synchronization of these indicators as a function of the type of contact they had.
The review team noted the findings of a recent study by Biagio D’Aniello and his colleagues which showed that a happy human odor evoked “happy behaviors” in dogs, while a human fear odor triggered behaviors indicative of dog fear states.
More recently, Antonio Lanata and his colleagues had been able to show a similar contagion effect with a small sample of horses who were exposed to happiness and fear odors.
These results, described by the review team as highly tentative, converge with the research reported by D’Aniello, the review team noted.
In the horse study, human body odors triggered systematic sympathetic and parasympathetic changes, showing that the body odors carry the emotional information to horses.
“The data that they obtained reveal that the human chemosignals induced a similar emotional status in horses.
“However, although these findings could open the way to understanding the behavior of horses when interacting with humans, they remain highly speculative at this stage and invite replication with an adequate sample size.”
The review team said the findings of such studies raise questions. “An important one is what drives emotional transfer between and within species?
“One of the two candidate answers is that the odors produced in emotional contexts (of fear and happiness) contain distinctive chemical compounds that invariably activate the same responses across the two species and thus have a pheromone type quality.
“The second possible answer is that the responses to the odor are acquired in the process of socialization and the two species acquire a sensitivity to the specific emotion-induced odors.
“If this second process is responsible, it is nevertheless the case that there is a very distinctive emotion quality to the odor compound to the extent that in the study examining the transfer of emotional states from humans to dogs the volatiles were — as we noted — collected in Portugal and administrated to the pet dogs in Italy.”
They continued: “The extraordinary significance of olfactory communication discussed here relies on the argument that these constitute evolutionarily conserved signals driving biological sensory systems critical for survival and reproductive success.
“Their special power is to be found in their significance in promoting survival chances, especially where visual and acoustic senses are restricted. It is in the absence of these sensory functions that odors become uniquely powerful by providing silent and invisible warning signals with long-lasting effects.”
The review team comprised Semin, from Utrecht University in the Netherlands; Anna Scandurra and Biagio D’Aniello, from the University of Naples Federico II in Italy; and Paolo Baragli and Antonio Lanatà, from the University of Pisa, also in Italy.
Inter- and Intra-Species Communication of Emotion: Chemosignals as the Neglected Medium
by Gün R. Semin, Anna Scandurra, Paolo Baragli, Antonio Lanatà and Biagio D’Aniello
Animals 2019, 9(11), 887; https://doi.org/10.3390/ani9110887