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African wild dogs (shown) were one of four groups of carnivores that chewed or played with wooden blocks covered with a single molecular ingredient of blood just as much as with blocks coated with real horse blood.

African wild dogs (shown) were one of four groups of carnivores that chewed or played with wooden blocks covered with a single molecular ingredient of blood just as much as with blocks coated with real horse blood.

Ulrica Skilberg

Here's what draws carnivores to blood

What is it about the scent of blood that makes carnivores go crazy? It may all come down to a single molecule, according to a new study. Tigers and wild dogs were drawn to this compound as much as to blood itself. The findings shed light on how animals recognize complex substances through smell and could even help curb some human phobias.

Like footprints in the snow, the scent of blood can guide meat-hunting animals to wounded prey. Yet blood, like many substances with strong smells, contains myriad molecular ingredients, only some of which may play a role in attracting predators. In general, it’s tough to pin down the role that all molecules in a substance might play, says Matthias Laska, a zoologist at Linköping University in Sweden. “You have to start by making a best guess.”

For Laska and his colleagues, that guess started with a molecule called trans-4,5-epoxy-(E)-2-decenal. The molecule—a so-called aldehyde—carries the telltale metallic stench of blood and is found in pigs’ blood and perhaps in all mammalian blood, Laska says.

To find out if the molecule is really what attracts predators, Laska and colleagues tested how 40 meat-eating mammals from four species would react to its scent. The researchers partnered with Kolmården Wildlife Park, a zoo in Sweden, which gave them access to Siberian tigers (Panthera tigris altaica), South American bush dogs (Speothos venaticus), African wild dogs (Lycaon pictus), and Asian wild dogs (Cuon alpinus) for the study.

The researchers soaked wooden blocks with one of four substances: the aldehyde, horse blood, the fruity-smelling molecule isopentyl acetate, or a nearly odorless solvent. After throwing these substance-laced blocks to the animals, the researchers observed and noted how the creatures responded and interacted with the blocks.

The researchers recorded thousands of interactions between the animals and the blocks in 20 days of experiments per species. On average, all four carnivore species played with the aldehyde-laced blocks just as much as with the blood-laced blocks, the team reports today in PLOS ONE. Moreover, the animals interacted with the blood- and aldehyde-laced blocks twice to four times as much as they did with blocks laced with the other two substances.

Laska says he was surprised that his team’s initial best guess worked so well. “This is one of the things you can only dream of and hope for that happens not so often in your experiments,” he says.

Still, he warns that the findings may not apply to all carnivorous animals and their olfactory systems. “Other animals and other olfactory systems might have evolved an alternative strategy,” he 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.

“It’s an elegantly done study that’s trying to answer a basic question: whether one single molecule can trigger a complex behavior as well as a complex odor does, and it answers that question beautifully,” says Johan Lundström, an experimental neuropsychologist at the Karolinska Institute in Stockholm. Lundström notes the findings could have implications for human health, as some people fear blood’s smell or link it to a traumatic past event. By understanding which components of blood trigger responses in people, researchers could generate better treatments for mental issues such as blood-related post-traumatic stress disorder or phobias.