Anyone who has ever taken a dog on a walk knows that they love to sniff. Even when there's nothing we can see, they can sense the presence of something with their super sniffers. It's built into their DNA on a very fundamental level. Seriously—there's evidence to suggest that dogs' sense of scent is so impressive, it's almost like a second sight.
But, not everything dogs choose to sniff is, well, something we as humans would ever choose to sniff. And, since dogs' sense of smell is about 100,000 times better than ours, it seems like they should be about 100,000 times more repelled by stinky smells than we are. But they aren't, as anyone who has ever watched in disgust as their dog sniffed a pile of another animal's feces or a dead bird can attest to.
Why are dogs drawn to bed smells?
Far from being repelled, dogs actually seem attracted to gross smells. But why? One theory is that they have a biological imperative to cover their own scent. In the wild, dogs (and their wolf ancestors) have to hunt for food—but they also have to worry about being hunted. Not only does sniffing everything around help them find food, but covering their scent with less-appealing scents (like those of dead animals) might also serve as a line of protection against predators. Even though your dog lives the life of luxury with you, those instincts are still alive inside of him.
Another reason is that your dogs is learning when he sniffs poop. A pile of foreign feces is like an encyclopedia to a dog. Smelling it allows him to take in and process a ton of information.
"For the dog, feces are a source of a huge amount of information. They tell dogs about the individual who deposited the feces, their dominance status, relatedness, sex and so on," — Peter Hepper, head of the school of psychology at Queen’s University in Belfast.
What's more, feces is a way of marking territory for a dog, so smelling feces is a way for your dog to figure out whose turf he's on.
"One key function for wild canines is delineation of territory," Hepper told Gizmodo. "Dogs defecate around territory boundaries and key paths to mark ownership with feces."
Our definition of "bad" might be the problem.
Another popular explanation among experts is that we're framing this question wrong. By assuming that humans are right about what smells bad or that what smells bad to us is universally repulsive, we're ignoring the role our own evolution has played in the equation.
"Humans perceive ‘bad’ odors through either some inbuilt evolutionary acquired mechanism to prevent harm, e.g. repulsion by feces to prevent disease, or through learning," Hepper explained.
Alexandra Horowitz, author Inside of a Dog and Being a Dog, pushes that theory even further, explaining that even though some smells repel humans for biological reasons, others we are socialized to dislike.
"So-called 'bad' and 'good' smells are products of our culture. As young children we are ambivalent toward smells like poo and stinky feet: we have to be taught that these are 'bad,'" she explained. "Dogs, by contrast, are in but not of our culture. They do not inherit our value system (unless we explicitly train them in its rules) and so are left with their own canine tendencies....For dogs, there seem not to be good nor bad (with a few exceptions) smells; smells are just the way the world looks. Smells are just information."
How does a dog's nose work?
Another very important piece of the why-dogs-love-gross-smells puzzle is found in the biology of how their noses work.
According to Donald Wilson, author of Learning to Smell: Olfactory Perception from Neurobiology to Behavior, many animals have a second olfactory system that's specifically designed to tackle heavy odors.
"Dogs have a structure called the vomeronasal organ, and that works not by just inhaling, but it's actually sort of like a little pump," he explained to Gizmodo. "It's in the nose, or sometimes it's in the roof of a mouth. They can bury their noses into another dog's butt, or into urine, or poop, [and with] their vomeronasal can really suck up these heavy molecules and extract more info out of it than we can."
According to PBS, dogs possess up to 300 million olfactory receptors in their noses (comparatively, humans have about six million) and the part of their brains devoted to analyzing scents is 40 times bigger than in humans, proportionally speaking. In other words: They are built to sniff.
It's not just dogs' brains and ability to process odors that are different from us. Their noses also work in a fundamentally different way. When humans inhale through our noses, we smell and breathe through the same airways — dogs don't. A piece of tissue in dogs' noses separates smelling from breathing.
"We found that when airflow enters the nose it splits into two different flow paths, one for olfaction and one for respiration," Brent Craven, a bioengineer at Pennsylvania State University explained.
How to stop your dog from sniffing gross things
Because this behavior is biologically hardwired into dogs, it's hard to stop all together. However, training your dog on the "leave it" command and mastering that will allow you to curtail the behavior and stop it when you see it happening.