Blood has a pretty distinct smell, but its ability to draw carnivorous animals is often overblown. Sharks, for example, can’t actually detect a single drop of blood in the ocean from miles away (in certain conditions, the best they can do is sniff out blood at one part per million). And, while we're at it, bears aren't attracted to menstruating women or their tampons.
But underneath the hype and myths, there’s some truth, says Matthias Laska, a Swedish biologist who studies animals’ sense of smell. Predatory mammals do seem use the scent of blood to track down wounded prey, though not with the accuracy we often give them credit for. And on the other side of the predator-prey divide, creatures low on the food chain respond to the scent of blood from other animals of the same species like a warning signal, becoming more vigilant or fleeing from an area when they pick up the scent.
While blood seems to be an important scent to animals, scientists aren’t entirely sure which of its molecular ingredients contribute to that smell and which ones spur the behaviors and reactions that they’ve seen in different species. To start figuring that out, one of Laska’s students, Shiva Krishna Rachamadugu, separated, identified, and analyzed the odor compounds in a batch of pig’s blood and found 28 different smelly substances. One of those, a compound called trans-4,5-epoxy-(E)-2-decenal, stood out for having the metallic odor that people normally associate with blood (the human nose is especially sensitive to it, too, and people can detect it at just 0.078–0.33 parts per trillion).
To see if this was the special ingredient in blood that attracts carnivores, Laska and a team of scientists from Sweden and Germany wanted to test it out on live animals, so they partnered with the Kolmården Wildlife Park to use some of its large carnivores as guinea pigs. The Swedish zoo gave the team access to a few dozen Siberian tigers, African and Asian wild dogs, and South American bush dogs. They smeared wooden logs with four different scents—trans-4,5-epoxy-(E)-2-decenal, horse blood, iso-pentyl acetate (an odor compound found in fruit that has a “banana-like” smell), and a near-odorless solvent—placed them in the animals’ enclosures, and watched how they reacted over the course of a few weeks.
All four species interacted (sniffing, licking, biting it, etc.) with the horse blood and blood compound-scented logs two to three times as much as they did the fruity or odorless ones. There wasn’t wasn’t much difference, though, in how often they played with the two logs that smelled like blood. The smell of trans-4,5-epoxy-(E)-2-decenal alone was just as interesting to them as the smell of actual blood.
There aren’t many other examples of animals responding to a single odor component the same way they do to the “whole,” real smell of something. Studies of apes and monkeys have shown that they don’t associate isolated odors of fruit with actual food. Likewise, single components from the urine and body odor of predators don’t cause the same alert responses in some prey species as the full, natural smell. While the study couldn’t tell Laska whether or not the dogs and tigers definitely associated trans-4,5-epoxy-(E)-2-decenal with prey or perceived it as “blood-like,” they spent as much time investigating it as they did the real thing, and even guarded those logs the same way they did their leftover food. Trans-4,5-epoxy-(E)-2-decenal, the researchers think, may be a “character impact compound” in mammal blood, a key odor compound that “defines” its smell to a predator’s nose.
Laska’s team now wants to work on finding other odor compounds in blood that get the same reaction on their own. They also want to see if trans-4,5-epoxy-(E)-2-decenal could be a blood character impact compound for other species, and whether it’s attractive to other predators like wolves and acts as a danger signal to prey animals. If it is, the odor might eventually be used as a repellant for mammal pests like mice, or something to break up the monotony of zoo life for carnivores.
For now, the study has something Kolmården Wildlife Park and other zoos can learn from: animals seem to like smelly logs. A scented hunk of wood, the researchers say, makes a cheap, easy toy for captive carnivores to keep them entertained and active.