Faced with the odor molecule e2d in the blood, humans react more like prey than predators.
In mammals, "smells" from the body—such as body odor, urine, feces, or blood—may attract predators closer; for prey, they may be a warning.
Throughout this long evolutionary history, such smells have continued to convey important messages between predators and prey. Usually such a flow of information is limited to a particular predator-prey relationship, but the "smell of blood" is a relatively common smell cue. Most studies have found that mammalian predators use the smell of blood to lock in injured prey, while those species that are prey produce avoidance and alertness to the smell of blood of the same species or different species. This phenomenon suggests that in the blood, perhaps an ancient and evolving chemical message has been preserved.
Since the smell of blood is a mixture of volatile substances, mammals in general rarely react the same to any single volatile substance. In 2014, a study by Swedish biochemists in collaboration with German medicinal chemistry experts found that a volatile substance in the blood, simply e2d, "trans-4,5-epoxy-2-decenal(trans-4, 5-epoxy-(e)-2-decenal", has the same attraction to mammalian predators as it does to blood. The study used four predators, including: the Asian jackal (cuon alpinus), the African wild dog (lycaon pictus), the South American bush dog (speothos venaticus), and the Siberian tiger (panthera tigris altaica), to observe their response to four odors, namely soaked in mammalian blood, e2d, fruity fragrance (isoamyl acetate) and odorless (diethyl phthalate, dep) of wooden strips. The latter two are the control group. For the wooden strips in the experimental group, whether it is blood or e2d, predators will have behaviors such as sniffing, licking, biting, slapping, playing, etc., and there is no significant difference between the two.
E2d is produced by lipid peroxidation, and this physiological response is found in all mammals, so it is speculated that a variety of volatile substances, including e2d, should be widely present in the blood of all mammals. Therefore, it is speculated that e2d is likely to be an odor molecule that affects the behavior of mammals in blood.
In October 2017, a multidisciplinary research team (neuroscience, brain cognitive behavior, psychology, physiology, chemistry, biology) in collaboration with Sweden and the United States published a study in the journal Scientific Reports, according to which this "blood-like" effect of e2D on mammals exists even for species that differ greatly in taxonomically, and humans are no exception.
The study first used three taxonomically distinct species, including the blood-sucking stomoxys calcitrans (invertebrates), wolves (canis lupus) (mammalian predators), and mice (mus musculus) (mammalian prey). As the researchers expected, the flies and wolves responded to e2d in the same way as sticks of wood soaked in real blood; similarly, mice behaved in evasion in the face of e2d.
The research team further targeted humans as experimental subjects. First, the response when a human subject came into contact with the e2d molecules sent by a blowpipe was tested through the "strain gauge force plate" and the "galvanic skin response (gsr)"; the results found that the subjects had a retrogressive movement and increased sweating in the palms of their hands, meaning that humans were "disgusted" by the smell. Next, the research team tested whether the e2d molecule would increase human alertness and attention by means of a psychological experiment (after receiving a stimulus, selecting different ones according to the instructions) to increase human alertness and attention; the results were also positive.
That is, the smell of blood for humans is the same as for prey in mammals. But why do humans react like "prey"?
According to the study's first author, artin Arshamian, a postdoctoral researcher, humans' response to blood (e2d) is likely to be traced back to the ancient ancestors of humans, namely early primates. Primates at that time fed mainly on insects and were indeed prey to other carnivores. "Humans today are undoubtedly predators, but we probably evolved from species of prey, and some of the habits that belong to prey have been preserved." Dr. Arshamian said.