A Single Smelly Compound Sparks Carnivores’ Lust for Blood

When given scented wooden blocks soaked in this single chemical, captive carnivores go wild

siberian tiger
Konrad Wothe/Minden Pictures/Corbis

Smells are notoriously hard to pin down, describe and identify. But most people agree that the smell of fresh blood has a distinct, metallic tang. You might assume this comes from the iron in our blood, but an organic compound—a type of aldehyde—is to blame.

In a new study, researchers found that this single component drew the interest of tigers and wild dogs just as much as the scent of fresh horse blood. Whatever other complicated smells were in the horse blood did not make it more intriguing to the carnivores; this one compound by itself was enough to attract their full attention.

The fact that blood carries a distinct stench is well known and has even given rise to a few myths. Bears are not drawn to menstruating female campers. Sharks cannot detect a single drop of blood in a vast ocea. But their ability to home in on blood in the water is still impressive: they can detect the equivalent of 10 drops of blood in the average home pool. It makes sense for the smell of blood to be so potent. A carnivore can use the scent to track wounded prey, and the prey species can use it as an alert that danger is close.

Still, finding the one molecule that our brains latch onto among all the scents in any given substance is a complex task. "You have to start by making a best guess," Matthias Laska of Linköping University in Sweden told Science

Laska and his colleagues knew that, in isolation, a molecule called trans-4,5-epoxy-(E)-2-decenal smells like blood to humans, so they started testing that compound on carnivores. They tossed wooden blocks soaked in the aldehyde into the pens of four different carnivores at the Kolmården Wildlife Park in Sweden. 

The Siberian tigers, South American bush dogs, African wild dogs and Asian wild dogs loved the blocks. They sniffed and bit the blocks, carried them around and even toyed with the apparently deliciously scented objects. They were just as interested in the compound alone as they were in blocks soaked in horse blood, the researchers report in PLOS One. Blocks drenched with a sweet "artificial banana" smelling compound and a scentless solvent weren’t nearly as interesting.

All four groups of carnivores loved their bloody-smelling blocks. Credit: Nilsson et al (2014) PLoS One

The study offers up some interesting questions to examine next, writes Puneet Kollipara for Science:

“Other animals and other olfactory systems might have evolved an alternative strategy,” [Laska] says. He hopes to answer that question by doing similar studies on other blood compounds and other carnivorous species, such as wolves. Laska even has a student performing a similar study using mice instead of carnivores. “We want to see if blood elicits escape behavior in prey species,” he says.

The carnivores the team did study liked the scented block so much that the researchers think that the tigers and dogs deserve to have them all the time. It would be a cheap, easy way to give captive carnivores fun toys. It’s certainly a better idea than, say, recommending the aldehyde as a component in a new line of perfume called "Tasty."

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