A chemical scent contained in the spilt blood of mammals is irresistible to predator species craving fresh kill, but provokes a very different reaction in animals that are hunted.
Researchers have found that E2D, a molecular component of animal blood, is as effective as actual blood in luring wolves, tigers, and wild dogs, but produces aversion in prey species who may associate it with danger and death, not dinner.
While smells constitute a range of chemical signals to various species, the researchers say E2D stands alone by eliciting significant behavioural responses in different animals like this.
“This finding is unique, as it is the first demonstration of a single chemical cue with the dual function of informing both approach and avoidance in a predator-prey predicted manner across taxonomically distant species,” says psychologist Artin Arshamian from Radboud University in the Netherlands.
“Importantly, it is the first known chemosignal that affects human and non-human animals alike.”
E2D, aka trans–4,5-epoxy-(E)–2-decenal, forms when fats in blood break down due to exposure to the air. No open wounds, no E2D.
The molecule was first identified by researchers in 2014, who isolated it from pig’s blood, then tested its olfactory appeal with three species of wild dog and some Siberian tigers thrown in for good measure.
In short, the predators couldn’t get enough – sniffing, licking, biting, pawing, and toying with wooden logs slathered with E2D and its scent. For the most part, the team observed the same behaviour in the animals as when real blood was used in the experiments.
In the new research, the team wanted to see what the effect of E2D was on less bloodthirsty organisms, reasoning its metallic odour wouldn’t go unnoticed.
“We hypothesised that prey species would be under evolutionary pressure to become sensitive to E2D,” biologist Johan Lundström from the Karolinska Institute in Sweden told AFP, “to help them avoid an area where a bloodbath is going on.”
And that’s exactly what the results seem to suggest. In new experiments, the researchers found E2D attracted wolves and also blood-sucking flies.
But when they exposed mice to the substance, the animals displayed flight behaviour and tried to avoid the chemical, much like they did with blood.
In humans, an experiment with forty volunteers saw the participants lean back slightly when E2D was pumped in their faces – a recoil gesture that researchers interpret as a sign of aversion to odour.
What’s more, it didn’t take much of the scent to trigger a negative response.
“Humans are able to detect E2D at concentrations of less than one part per trillion,” one of the team, zoologist Matthias Laska from Linkoping University in Sweden told AFP.
“This is uncommon. For the majority of odourants which have been tested with humans, the detection threshold is in the parts-per-million or billion range.”
Of course, one of the big differences between humans and prey species like mice is that we’re actually pretty deadly these days, hunting and eating basically anything else that moves.
But it wasn’t always that way, and our aversion to the smell of blood carried on the wind may hint at a distant past, in which E2D signalled a threat: we could end up on the menu if we didn’t vacate the area fast.
“Our finding in humans fits in with the paleontological data showing that early primates were small-bodied insectivores,” Lundström explains in a press release.
“There is no question that humans are opportunistic predators, but we probably evolved from a prey species and some aspects of this trait linger on.”
The findings are reported in Scientific Reports.