Why Does the Scent of Blood Attract Carnivores?

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.

Slow Motion Is the Only Way to Appreciate a Chameleon’s Lightning-Fast Tongue

From the unusual way they walk, to their ability to change color, the evolutionary adaptations of chameleons are pretty bizarre, and some of them remain mysterious even to scientists. Their super-powered tongues, for instance, can dart out so quickly that the movement can barely be seen with the naked eye. But modern high-speed cameras have enabled researchers at the University of South Dakota to observe this appendage at work like never before. The video below, shared over at The Kid Should See This, includes some of that groundbreaking footage, and it's pretty amazing to watch.

Shooting at 3000 frames per second, the camera was able to capture every split-second aspect of the chameleon's tongue strike. Slowed down, the video allows you to see how every component of the process works in harmony: First, muscles in the lizard’s tongue contract like the string of a bow. Then, when that tension is released, the bony base of the tongue shoots forward, pushing the sticky, elastic part toward the chameleon’s prey.

According to Christopher Anderson, one of the scientists who conducted the high-speed camera research, larger chameleons can catapult their tongues forward at distances of one to two times their body length. For smaller chameleons, this distance can reach up to two and a half times their body length. “Small chameleons need to be able to eat more food for their body size than large chameleons,” he tells bioGraphic in the video, “and so by being able to project their tongues proportionately further than these large species, they basically are opening up additional feeding opportunities to themselves that they wouldn’t have if they had a shorter tongue.”

To see one of nature’s greatest hunting tools in action, check out the full video below.

[h/t The Kid Should See This]

There May Be an Ancient Reason Why Your Dog Eats Poop

Dogs aren't known for their picky taste in food, but some pups go beyond the normal trash hunting and start rooting around in poop, whether it be their own or a friend's. Just why dogs exhibit this behavior is a scientific mystery. Only some dogs do it, and researchers aren't quite sure where the impulse comes from. But if your dog is a poop eater, it's nearly impossible to steer them away from their favorite feces.

A new study in the journal Veterinary Medicine and Science, spotted by The Washington Post, presents a new theory for what scientists call "canine conspecific coprophagy," or dogs eating dog poop.

In online surveys about domestic dogs' poop-eating habits completed by thousands of pet owners, the researchers found no link between eating poop and a dog's sex, house training, compulsive behavior, or the style of mothering they received as puppies. However, they did find one common link between the poop eaters. Most tended to eat only poop that was less than two days old. According to their data, 85 percent of poop-eaters only go for the fresh stuff.

That timeline is important because it tracks with the lifespan of parasites. And this led the researchers to the following hypothesis: that eating poop is a holdover behavior from domestic dogs' ancestors, who may have had a decent reason to tuck into their friends' poop.

Since their poop has a high chance of containing intestinal parasites, wolves poop far from their dens. But if a sick wolf doesn't quite make it out of the den in time, they might do their business too close to home. A healthier wolf might eat this poop, but the parasite eggs wouldn't have hatched within the first day or two of the feces being dropped. Thus, the healthy wolf would carry the risk of infection away from the den, depositing the eggs they had consumed away in their own, subsequent bowel movements at an appropriate distance before the eggs had the chance to hatch into larvae and transmit the parasite to the pack.

Domestic dogs may just be enacting this behavior instinctively—only for them, there isn't as much danger of them picking up a parasite at home. However, the theory isn't foolproof. The surveys also found that so-called "greedy eaters" were more likely to eat feces than dogs who aren't quite so intense about food. So yes, it could still be about a poop-loving palate.

But really, it's much more pleasant to think about the behavior as a parasite-protection measure than our best pals foraging for a delicious fecal snack. 

[h/t The Washington Post]


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