For large meat eaters, that mouthwatering smell of blood comes down to a single molecule. Researchers working with four large predator species found that one particular component of blood odor was just as enticing as the odor of real blood itself. The study was published in PLOS ONE this week.
"For predators, food scents are particularly attractive, and much of this has to do with blood," Matthias Laska of Linköping University says in a news release. “We wanted to find out which chemical components create the scent of blood.” Recent analyses of the composition of volatiles in mammal blood revealed that the compound trans-4,5-epoxy-(E)-2-decenal is what emits that metallic, blood-like odor quality (to us at least). This aldehyde forms when animal fat breaks down.
To test if this odorant component is what draws predators to blood, Laska and colleagues observed the behavior of four captive species at Kolmården Wildlife Park in Sweden: Asian wild dogs, African wild dogs, South American bush dogs, and Siberian tigers. They presented the animals with half-meter-long spruce logs that were doused with either horse blood or trans-4,5-epoxy-(E)-2-decenal. Then the researchers compared these reactions to their behavior towards logs drenched with a fruity, banana-like odor (isopentyl acetate) and a near-odorless solvent (diethyl phthalate) as controls. The animals were exposed to one of the four scents a day in their regular enclosure; they did this five times for a total of 20 experimental days per species.
The logs saturated with the aldehyde were just as attractive as those containing real blood. The predators sniffed, licked, bit, pawed, and toyed with both “bloody” logs, and there were no significant differences between the number of interactions between the aldehyde-laced and blood-soaked logs. They’d also guard and rest close to (sometimes on top of) the odorized logs -- as if they were bones leftover from a meal. The two non-prey-associated logs aroused little interest.
In this image (clockwise from upper left), you can see an African wild dog sniffing at an odorized wooden log, two Asian wild dogs biting one, a South American bush dog performing “flehmen” (a lip curl, followed by an inhalation), and a Siberian tiger toying with (in this case, carrying) the log. The tiger was the most persistent, while the South American bush dogs lost interest over time.
"How this has developed through evolution is an interesting question,” Laska says. “Perhaps there is a common denominator for all mammalian blood." He plans to follow up with how prey animals like mice react to blood odor.
Images: Linköping University (top), 2014 Nilsson et al., PLOS ONE (middle)