Why Humans Are Scared of Blood but Animals Are Not

Different species evolved extreme reactions to blood in an effort to keep them alive, one theory suggests. MAXIM MARMUR/AFP/Getty Images

The scent of blood is very polarizing, sending some animals into a hunger frenzy while making others turn away in disgust and fear. A new study suggests it's not the blood itself, but rather a single molecule called E2D that causes these primal reactions to blood in mammals.

With the odd exception, most humans are not drawn to the scent of blood. In fact, we're usually repulsed by it. That's because our instinct uses scent to warn us away from blood as it can be a sign of danger. Other animals, however, such as wolves and horse flies, are drawn to the scent of blood since it may signify their next meal, AFP reported.

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In a new study now published online in Scientific Reports, researchers from the Karolinska Institute in Sweden pinned a molecule in blood called E2D as the culprit behind these strong reactions to blood's scent. In their research, the team isolated the molecule and created a synthetic version of it. Although blood is a combination of a list of components and molecules, wolves and flies exposed to E2D alone treated it as the "real thing" and licked the synthetic version as they were trying to eat it.

Next, the team tested E2D on rodents, and noted they had a much different reaction. Instead of being drawn to the molecule, they recoiled from it in fear.

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These reactions fit into the hypothesis that predator animals evolved an attraction to E2D and prey evolved a fear. But where do humans fit? As the ultimate apex predator we should have a similar reaction to wolves, but anyone who has been exposed to a fresh kill knows the scent instills anything but excitement.

In a simple smell test, the researchers had human volunteers smell E2D, along with other scents, while standing up. This way they could see the physical reaction the scent caused. Results revealed that all volunteers had an immediate stress reaction to the scent of E2D. This included sweating and tilting back on their heels slightly, both natural reactions to danger.

While it's not entirely clear why humans would have such adverse reactions to blood, the study suggested one hypothesis.

"Although humans are thought to be opportunistic predators, palaeontological data indicate that early primates"—our distant relatives—"were small-bodied insect eaters," the study reads, suggesting that our evolutionary ancestors were far more often the prey than the predators. As a result, we evolved to be more akin to mice than wolves in our reaction to blood's scent.

The results are a testament to just how far we've come evolutionarily speaking, and proof that it wasn't so long along that we were on the opposite end of the food chain.

Why Humans Are Scared of Blood but Animals Are Not | Health