You may not
enjoy the smell of your dirty laundry, but your brain knows and appreciates that it’s yours. A new study reveals a key way we detect our own scent and distinguish that scent from others’.
Smell is a powerful thing. Many species use it to communicate (think dogs sniffing their introductions) or attract mates (the Stickleback fish is a good example of this one). Humans may not be as overtly smell-dependent, but our brains actually use this sense more than you might think.
Communication by smell comes down to a thing called the major histocompatibility complex, or MHC. Every creature with a backbone has MHC molecules on the surface of its cells. These molecules act like bouncers, carefully controlling the balance of proteins inside the cell. When new proteins come a-knocking, the MHC checks their IDs to determine if they are okay to enter the cell (recognized as self) or get kicked out (non-self). This keeps the riffraff at bay, but can also cause the body to reject unrecognized things like transplanted organs.
You’ll never get to take a deep breath and smell the roses in Earth’s orbit. The distinct lack of air there means you’d die a gruesome death sans space helmet, probably without smelling a thing.
Ah, but what about once you get back in your ship? As many places around the Web have been discussing recently, astronauts have said that upon coming back from space walks and taking off their gear, a certain specific scent seems to hang in the air…some think it smells like charred steak, or maybe like something metallic.
Here’s how astronaut Don Pettit put it nearly ten years ago:
Each time, when I repressed the airlock, opened the hatch and welcomed two tired workers inside, a peculiar odor tickled my olfactory senses. At first I couldn’t quite place it. It must have come from the air ducts that re-pressed the compartment. Then I noticed that this smell was on their suit, helmet, gloves, and tools. It was more pronounced on fabrics than on metal or plastic surfaces. It is hard to describe this smell; it is definitely not the olfactory equivalent to describing the palette sensations of some new food as “tastes like chicken.” The best description I can come up with is metallic; a rather pleasant sweet metallic sensation. It reminded me of my college summers where I labored for many hours with an arc welding torch repairing heavy equipment for a small logging outfit. It reminded me of pleasant sweet smelling welding fumes. That is the smell of space.
What sounds like a similar smell—astronauts describe it as something like gunpowder—also emanates from moondust.
Airplane food is notoriously bad. But airlines, in financial free fall over the last decade, have been trying to bring back the luxe food of early flight in business class and first class, to lure in more high-end travelers. Biology is working against them, though. As Jad Mouawad reports for the NYTimes, part of why plane food lacks subtlety is that we can’t actually taste as well when we’re at altitude:
Even before a plane takes off, the atmosphere inside the cabin dries out the nose. As the plane ascends, the change in air pressure numbs about a third of the taste buds. And as the plane reaches a cruising altitude of 35,000 feet, cabin humidity levels are kept low by design, to reduce the risk of fuselage corrosion. Soon, the nose no longer knows. Taste buds are M.I.A. Cotton mouth sets in.
The latest diagnostic tool for oncology comes on four paws and is defined by its very effective nose. In a small study, Japanese researchers found that a dog could detect cases of colorectal cancer by sniffing patients’ breath or stool samples. Previous experiments have shown that dogs can sniff out cases of skin, lung, bladder, and breast cancers; researchers think the tumors give off chemical signals that the dog can detect in bodily substances.
The cancer expert in this case was an eight-year-old black Labrador named Marine who was trained to search for disease traces at the St. Sugar Cancer Sniffing Dog Training Center in Chiba, Japan. She must have been a good student. The research, published in the journal Gut, showed that she had a high success rate:
The Labrador retriever was at least 95 percent as accurate as colonoscopy when smelling breath samples, and 98 percent correct with stool samples, according to the study…. The dog’s sense of smell was especially effective in early-stage cancer, and could discern polyps from malignancies, which colonoscopy can’t. [Bloomberg]
Lead researcher Hideto Sonoda says it would be impractical to use dogs for routine bowel cancer screenings, but adds that further research into dogs’ diagnostic ability could lead to the development of an electronic nose.
Dr Sonoda told the BBC: “The specific cancer scent indeed exists, but the chemical compounds are not clear. Only the dog knows the true answer. It is therefore necessary to identify the cancer specific volatile organic compounds [smells] detected by dogs and to develop an early cancer detection sensor that can be substituted for canine scent judgement. To complete the sensor useful in clinical practice as a new diagnostic method is still expected to take some time.” [BBC]
80beats: In Controversial Scent Lineups, a Dog’s Nose Picks Out the Perp
80beats: New Research Points Toward Artificial Nose Based on Human Smell Sensors
80beats: Sniffing Out Sickness: Mouse Noses Respond to the Urine of Diseased Mice
DISCOVER: Lassie–Get the Oncologist!
DISCOVER: 20 Things You Didn’t Know About… Dogs
Image: flickr / pmarkham
Researchers have found the secret to improving a robot’s sense of smell: Shove frog eggs up its nose. A team at the University of Tokyo has developed a sensor made from a genetically modified frog egg that can help a robot pick out insect smells and pheromones.
As useful as a moth-smelling robot may seem, researchers believe the study published yesterday in Proceedings of the National Academy of Sciences is just one step towards an inexpensive but sensitive chemical detector. Study coauthor Shoji Takeuchi explains that such a device could pick out gases like carbon dioxide:
“When you think about the mosquito, it is able to find people because of carbon dioxide from the human. So the mosquito has CO2 receptors. When we can (extract) DNA (from the mosquito) we can put this DNA into the frog eggs to detect CO2.” [Reuters]
Here’s how they did it.
Step 1 — Get Some Frog Eggs
We knew that bacteria could stink, but new research asks if bacteria can smell. A study published today argues that Bacillus licheniformis found in soil can sense ammonia given off by neighboring bacteria. Though we might turn our noses when we smell the gases given off by neighbors, the bacteria respond to ammonia by building a thick biofilm coating.
The research appears today in Biotechnology Journal. Previous studies have shown that bacteria can sense gases such as oxygen, but this is the first study to argue that bacteria can smell, since ammonia has a scent. Lead author Reindert Nijland explains that bacteria break the pungent gas down for its useful nitrogen, so an ammonia-detector and biofilm response might be a useful survival tool. He even suggests that this might be the earliest example of olfaction in evolutionary history.
“Ammonia is the simplest available nitrogen source,” Nijland said. “All organisms need nitrogen to produce their proteins.” The ammonia is thought to signal both the presence of nutrients and the presence of other bacteria, since the biofilms Bacillus species produce in response to ammonia contain antibiotics that can kill competing bacteria. And the ability to “smell” ammonia “gives bacteria a way to sense nutrients where nutrients are and then migrate towards them,” he said. [The Scientist]
Hounds, pointers, and other dogs bred for their excellent abilities to pick up a scent tend to have longer snouts—but it’s not just that a bigger nose is a better one. Researchers have found that human domestication of dogs has shifted the structure and alignment of some dogs’ brains. And in those varieties with shorter snouts—which humans bred for other reasons, like appearance—the olfactory brain region rotated to a different part of their skull, leaving scientists to question whether we’ve crossed up their smelling abilities (and perhaps more).
Since the first wolf was domesticated an estimated 12,000 years ago, “selective breeding has produced a lot of [anatomical] variation, but probably the most dramatic is in terms of skull shape,” said study co-author Michael Valenzuela [National Geographic].
For this study, which appears in the open-access journal PLoS One, Valenzuela and colleagues examined the brains of 11 dog breeds and found great variation in the size and shape of their skulls. The breeds with shorter snouts had brains that rotated forward by as much as 15 degrees over the generations, the scientists say. That means that the olfactory lobe, as well as other parts of these dogs‘ brains, has shifted position and shape because humans guided their evolution through domestication.
Sure, the planet’s increasing carbon dioxide levels are making the oceans more acidic, but what does that really mean for sea life? We’ve already heard that the ocean’s changing chemistry is damaging corals and interfering with mussels, but that’s just the beginning. It turns out things could get seriously weird.
In a paper published this week in The Proceedings of the National Academy of Sciences, researchers led by Philip L. Munday of James Cook University have given us a concrete example: the increased CO2-levels make some fish purposely swim towards predators.
As part of his experiment, Munday used a Y-shaped maze to force baby clownfish to choose between two paths. One path reeked of rock cod, a natural predator; the other had no danger scents. Munday’s team compared the choices of fish raised in water of varying carbon dioxide concentrations, from today’s levels of 390 parts per million up to future expected levels of 850 ppm.
It’s the essence of instinct: If you take a lab mouse who has never caught a glimpse of a cat and waft a little eau de feline towards it, the mouse will freeze in fear, and will then back away from the source of the odor. Now researchers have pinned down the chemical signals the mice are reacting to–and have shown in the process a fascinating new form of inter-species communication.
Mice have a specialized organ in their noses that picks up chemical signals, called the vomeronasal organ, which helps them detect pheromones emitted by other mice. These mice pheromones have a direct effect on behavior–most obviously in the realms of mating and fighting. In this new study, published in the journal Cell, neurobiologist Lisa Stowers decided to investigate whether the vomeronasal organ was capable of picking up signals from other species as well.
In today’s edition of far-out science, researchers have found evidence that the wafting aroma of food has an effect on an organism’s lifespan–and they’ve demonstrated that interfering with a fruit fly’s sense of smell causes it to live a longer, healthier life. While there’s no guarantee that the trick would work for humans, optimistic researchers suggest that certain odors—or drugs that block us from sensing them—might one day help prevent disease and extend lives [ScienceNOW].
In the past decade, scientists have established a clear connection between extremely low-calorie diets and extended lifespans; studies have demonstrated that yeast, fruit flies, mice, and monkeys on these diets live longer than their peers. While the exact mechanism at work isn’t yet clear, researchers suspect that a near-starvation diet causes an organism’s metabolism to slow down, and triggers other changes that evolved to help organisms survive in times when food was scarce. Now scientists say it may not be just what a creature eats, but also what it smells that has an effect on how long it lives.
In one 2007 study, molecular biologist Scott Pletcher and his colleagues found that completely eliminating fruit flies’ sense of smell caused them to live nearly 20 percent longer than normal flies. They also found that wafting the smell of yeast, a tasty treat for fruit flies, towards flies that were on a low-cal, live-extending diet hastened the death of those flies. This led the scientist to hypothesize that specific odors might be influencing the flies’ lifespans. Luckily, other scientists had identified a receptor in a group of neurons that enable fruit flies to smell carbon dioxide, which signals the presence of a good meal of tasty yeast [ScienceNOW]. So, Pletcher and his team set out to find if the CO2 had anything to do with the duration of the flies’ lives.