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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.

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Scatterbrained
Everything You Ever Wanted to Know About Dogs
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YouTube

Dogs: They’re cute, they’re cuddly … and they can smell fear!

Today on Scatterbrained, John Green and friends go beyond the floof to reveal some fascinating facts about our canine pals—including the story of one Bloodhound who helped track down 600 criminals during his lifetime. (Move over, McGruff.) They’re also looking at the name origins of some of your favorite dog breeds, going behind the scenes of the Puppy Bowl, and dishing the details on how a breed gets to compete at the Westminster Kennel Club Dog Show.

You can watch the full episode below.

For more episodes like this one, be sure to subscribe here!

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Animals
Sploot 101: 12 Animal Slang Words Every Pet Parent Should Know
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For centuries, dogs were dogs and cats were cats. They did things like bark and drink water and lay down—actions that pet parents didn’t need a translator to understand.

Then the internet arrived. Scroll through the countless Facebook groups and Twitter accounts dedicated to sharing cute animal pictures and you’ll quickly see that dogs don’t have snouts, they have snoots, and cats come in a colorful assortment of shapes and sizes ranging from smol to floof.

Pet meme language has been around long enough to start leaking into everyday conversation. If you're a pet owner (or lover) who doesn’t want to be out of the loop, here are the terms you need to know.

1. SPLOOT

You know your pet is fully relaxed when they’re doing a sploot. Like a split but for the whole body, a sploot occurs when a dog or cat stretches so their bellies are flat on the ground and their back legs are pointing behind them. The amusing pose may be a way for them to take advantage of the cool ground on a hot day, or just to feel a satisfying stretch in their hip flexors. Corgis are famous for the sploot, but any quadruped can do it if they’re flexible enough.

2. DERP

Person holding Marnie the dog.
Emma McIntyre, Getty Images for ASPCA

Unlike most items on this list, the word derp isn’t limited to cats and dogs. It can also be a stand-in for such expressions of stupidity as “duh” or “dur.” In recent years the term has become associated with clumsy, clueless, or silly-looking cats and dogs. A pet with a tongue perpetually hanging out of its mouth, like Marnie or Lil Bub, is textbook derpy.

3. BLEP

Cat laying on desk chair.
PoppetCloset, Flickr // CC BY-NC-ND 2.0

If you’ve ever caught a cat or dog poking the tip of its tongue past its front teeth, you’ve seen a blep in action. Unlike a derpy tongue, a blep is subtle and often gone as quickly as it appears. Animal experts aren’t entirely sure why pets blep, but in cats it may have something to do with the Flehmen response, in which they use their tongues to “smell” the air.

4. MLEM

Mlems and bleps, though very closely related, aren’t exactly the same. While blep is a passive state of being, mlem is active. It’s what happens when a pet flicks its tongue in and out of its mouth, whether to slurp up water, taste food, or just lick the air in a derpy fashion. Dogs and cats do it, of course, but reptiles have also been known to mlem.

5. FLOOF

Very fluffy cat.
J. Sibiga Photography, Flickr // CC BY-NC-ND 2.0

Some pets barely have any fur, and others have coats so voluminous that hair appears to make up most of their bodyweight. Dogs and cats in the latter group are known as floofs. Floofy animals will famously leave a wake of fur wherever they sit and can squeeze through tight spaces despite their enormous mass. Samoyeds, Pomeranians, and Persian cats are all prime examples of floofs.

6. BORK

Dog outside barking.
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According to some corners of the internet, dogs don’t bark, they bork. Listen carefully next time you’re around a vocal doggo and you won’t be able to unhear it.

7. DOGGO

Shiba inu smiling up at the camera.
iStock

Speaking of doggos: This word isn’t hard to decode. Every dog—regardless of size, floofiness, or derpiness—can be a doggo. If you’re willing to get creative, the word can even be applied to non-dog animals like fennec foxes (special doggos) or seals (water doggos). The usage of doggo saw a spike in 2016 thanks to the internet and by the end of 2017 it was listed as one of Merriam-Webster’s “Words We’re Watching.”

8. SMOL

Tiny kitten in grass.
iStock

Some pets are so adorably, unbearably tiny that using proper English to describe them just doesn’t cut it. Not every small pet is smol: To earn the label, a cat or dog (or kitten or puppy) must excel in both the tiny and cute departments. A pet that’s truly smol is likely to induce excited squees from everyone around it.

9. PUPPER

Hands holding a puppy.
iStock

Like doggo, pupper is self-explanatory: It can be used in place of the word puppy, but if you want to use it to describe a fully-grown doggo who’s particularly smol and cute, you can probably get away with it.

10. BOOF

We’ve already established that doggos go bork, but that’s not the only sound they make. A low, deep bark—perhaps from a dog that can’t decide if it wants to expend its energy on a full bark—is best described as a boof. Consider a boof a warning bark before the real thing.

11. SNOOT

Dog noses poking out beneath blanket.
iStock

Snoot was already a dictionary-official synonym for nose by the time dog meme culture took the internet by storm. But while snoot is rarely used to describe human faces today, it’s quickly becoming the preferred term for pet snouts. There’s even a wholesome viral challenge dedicated to dogs poking their snoots through their owners' hands.

12. BOOP

Have you ever seen a dog snoot so cute you just had to reach out and tap it? And when you did, was your action accompanied by an involuntary “boop” sound? This urge is so universal that boop is now its own verb. Humans aren’t the only ones who can boop: Search the word on YouTube and treat yourself to hours of dogs, cats, and other animals exchanging the love tap.

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