On October 7th, 2018 we learned about

Proximity to sick peers causes healthy mice adopt an odor of illness

Being sick is bad enough, but the social stigma that goes along with looking like a runny-nosed zombie certainly doesn’t help things. Humans are generally repulsed by the visible symptoms in sick peers, overlooking other signs of illness like body odor. For a scent-oriented mouse though, the smell of a sick cage-mate is a bit more obvious, and would seemingly be a way for healthy individuals to avoid unnecessary exposure to contagion. However, studies have found that healthy neighbors of sick mice strangely start smelling sick themselves, although at this point its unclear how or why this helps either mouse.

Sniffing out sickness

The first step in testing all this was to expose healthy mice to infected neighbors. Some mice were allowed to have direct contact with their suffering peers, while others were separated by plastic barriers in their cages. None of the healthy mice were ever infected themselves, but their urine and body odor seemed to tell a different story. While the change in smell was nothing a human nose could detect, trained ‘sniffer’ mice and gas chromatography could detect a new, sickly scent.

The ‘sniffer’ mice were trained to move towards the smell of a sick mouse in a Y-shaped maze. When asked to sniff out unwell odors, urine samples from healthy mice smelled sickly around 63 percent of the time. Once it was established that mice could indeed detect the change in the odor of their healthy peers, researchers set out to identify exactly which chemical components were responsible.

Source of the scents

The three main components of a sick scent were all products of pheromones. This was unexpected, as these chemicals were normally associated with male mice looking to assert dominance or attract a mate. It’s then unclear why those pheromones would be tied to smelling like you’re sick. Actually, considering how most sick animals are avoided by their peers to avoid the transmission of pathogens, researchers aren’t sure if there’s any evolutionary advantage to imitating the scent of illness in the first place.

One possibility is that there is no significant benefit to smelling like you’re sick. It’s possible that the healthy mice exposed to sick cage-mates experienced an immune response to prepare them for possible infections. That physiological activity may be creating the pheromone by-products, and that the smell is simply sort of an inconsequential side effect. While evolution generally pushes organisms to be successful or efficient in their environments, smelling sick may be harmless enough that there was no pressure on mice to avoid it.

Source: Healthy animals mimic body odour of sick companions by Katrina Kramer, Chemistry World

On June 27th, 2018 we learned about

How fat cells function as the body’s squishy, insulating, and scalable energy reserve

I know that exercising will help me ‘build’ muscle. I also know that that’s a kind of weird way to describe a systematic tearing and repair of muscle tissue, eventually resulting in more muscle mass overall. I should also be able to ‘burn’ fat or ‘lose’ weight, although those terms are a little more opaque. It sounds like the amount of fat in my body will be reduced, but does that mean smaller fat cells? Fewer fat cells? What does it mean to get fat in the first place, and why do our bodies even bother in the first place?

Storing energy to ensure survival

Fat does a number of jobs in an animal’s body, from providing insulation from the elements to padding or protecting more sensitive anatomy deeper in the body. Most humans aren’t hitting the gym because they’re too comfortable swimming in cold water though. The issue we struggle with today is the way fat can store energy. It’s an ability that has helped organisms survive intermittently-low food supplies for millions of years, but thanks to modern farming and food storage, is basically working better than our bodies really need. As we eat more food than we can use in a day, our bodies try to store extra energy in fat cells a hungrier time that just never seems to arrive.

To store energy, your body packages excess sugars into molecules called fatty acids, stuffing them into fat cells for storage. As your personal energy reserves continue to grow, your body will increase both the size and number of fat cells you’re carrying. As normal deposits are stuffed full, fat cells will even get deposited on muscles and major organs, leaving you with more fatty acids than you’re likely to need in most modern circumstances.

Saving more than your body can spend

Carrying fat obviously doesn’t make you immune from hunger, and your body seems to only tap into these reserves of fatty acids in certain circumstances. Highly aerobic activities, such as fleeing from a predator on foot, is one way to gain access to the energy stored in fat. On a longer timescale, reducing your overall calorie intake can also convince your body to start cracking open those cells to make use of your stored energy. This is done by releasing fatty acids into the bloodstream so your muscles, heart and lungs can literally break them apart to make use of the energy stored in their molecular bonds. The remaining molecular debris, as well as the emptied fat cells, are eventually expelled from the body in both our breath and urine as a waste product. So you sort of ‘burn’ fat, but you’re also breaking and exhaling it.

As efficient a system as that may be, it only seems to work when we regularly dip into our fat reserves. Once we have an excessive number of fat cells, they weirdly become harder to use. These cells, known as adipocytes, are often oversized and produce inflammation-causing hormones. They also end up storing extra energy, and releasing it to our muscles and organs at an abnormally slow rate. In a way, they become too good at their jobs, hindering the original function of fat cells as a way to get through lean times.

Source: How does your body 'burn' fat? by David Prologo, The Conversation

On May 23rd, 2018 we learned about

Two types of taste-buds allow bees to enjoy their nectar more than other insects

My daughter might be part honeybee. While her younger brother seems to think the best way to eat ice cream or candy is to gobble it up quickly, she figures the experience should be extended for as long as possible, even if that means licking a single Skittle for minutes at a time instead of just chewing it up in her mouth. Now, bees don’t necessarily ration our their enjoyment of nectar from flowers in quite this manner, but they do spend a lot more time than other insects slurping up the sugary liquid, eventually collecting more nectar and pollen as a result. While some of that nectar will need to be shared with their hive, the bees aren’t strictly being dutiful about their nectar collection— their mouths are actually adapted to help maximize their enjoyment of every sugary sip in a way my daughter would be envious of.

Like many foods that non-carnivores enjoy, the sugars in nectar activate specialized sensory cells on a bee’s proboscis, or mouth parts, which send a signal back to the brain. This usually coincides with a pleasurable release of neurotransmitters, providing positive feedback for acquiring some calorie-rich food. Over time, this response is diminished, presumably to make sure an animal doesn’t just burn out its taste buds by constantly soaking them in the first source of sugar it can find, which is why a tenth piece of candy might not feel as exciting to eat as the first.

Repeating the first sugary sip

Since bees are gathering nectar and pollen for more than their own consumption, evolution has set them up with an incentive to linger in flowers a bit longer than other insects. In addition to the sweet-sensitive taste receptors, they have secondary nerve cells that intermittently interrupt the process, causing the timer on their enjoyment to repeatedly reset. So by hitting restart on this process over and over, the bee is incentivized to suck up nectar for a longer period of time, maximizing each visit to a flower which can last as long as 10 seconds.

Aside from explaining what keeps bees on a particular flower for extended nectar-slurping sessions, this is also the first time scientists have ever seen taste-oriented nerve cells interact with each other, rather than simply pipe information straight to an animal’s brain.

Source: What gives bees their sweet tooth? by Newcastle University, Phys.org

On April 23rd, 2018 we learned about

Crocodiles can change their skin coloration based on environmental color cues

Few predators would antagonize an adult saltwater crocodile (Crocodylus porosus). Weighing close to a thousand pounds, these huge reptiles have little to fear in their local ecosystems. Their hatchlings, on the other hand, are usually around 2.5 ounces at birth, and need to be a bit more wary about becoming someone’s lunch. To stay safe, these tiny crocs, as well as other members of the Crocodylidae family, have developed the ability to better hide themselves by changing the color of their skin. Through a series of experiments, researchers have been able to isolate the exact mechanisms that allow these scaly predators to obscure themselves according to environmental conditions.

Shading skin according to what they see

The color changes in question are noticeable to the naked eye, assuming you’re comfortable staying in proximity to a crocodile for 60 to 90 minutes. While no crocodile was observed creating new patterning or bright shifts in hue like a chameleon, they could shift their skin from lighter to darker shades of their normal coloration. Members of the Crocodylus genus, like saltwater crocodiles, would make their tails, backs, and heads turn darker in darker environments, while members of the Gavialidae family did the opposite, lightening their backs when placed in dark environments.

In the context of this study, dark environments consisted of black or white tubs of water. It was easy to see the crocodiles change color according to their surroundings, but researchers needed to make sure that this was really triggered by what each animal was seeing, rather than other conditions like body temperature or stress levels. One way to do this was to place a crocodile in one color tub, blindfold it, then move it to the opposite lighting conditions. In those cases, the crocodiles’ coloring didn’t change according to the new environment- they kept the color that matched the last environment that their eyes were able to see. This was fairly conclusive, particularly after factors like temperature and stress hormones were found to be consistent between light and dark tubs. Finally, tests with red lights, which crocodiles don’t see as well as colors like blue, showed that if the crocodile didn’t visually perceive the difference, their skin didn’t react either.

Pinching and stretching pigments

Of course, visual stimulation can only be part of the story. Researchers also examined the crocodiles’ skin to see how it could change color once the animal noticed it was in a dark or light environment. They found that an α-melanocyte-stimulating hormone was released in the body, which triggered changes in cells’ melanosomes. When a crocodile needed to darken, the pigment in the melanosome would spread out, increasing the percentage of each cell that would absorb light. When a crocodile lightened, the same melanosomes would contract into tight packets, leaving more surface area without pigment to absorb light. This expansion and contraction is all that’s needed to achieve the relatively quick but reversible color change seen across multiple species’ skin.

Estimating when this abilities evolved

The species of crocodile that change colors revealed some information about when this ability likely evolved. Alligators, for instance, don’t change colors, which suggests that this ability had not developed when that family split from Crocodylidae 80 million years ago. Outside the genus Crocodylus, two African members of Crocodylidae showed little to no color-shifting ability, which then suggests that this trait evolved after that branch in the family tree occurred 30-40 million years ago. However, Crocodylus diversified a lot around 12 to 17 million years ago, so researchers assume that this trait was likely established by then in order for it to turn up in species that have since become separated from each other.

My third-grader asked: Do adult crocodiles do this too, or is it just the babies? Why do some of them turn the opposite color? Are they sure the pigment isn’t to help prevent sunburns?

The tests were done with baby crocodiles, possibly because they’re a lot easier to move between tubs of water. However, adults would benefit from dynamic coloration, as hiding from prey would aid in the ambush-style hunting crocodiles typically rely on.

The benefits of Gavialidae crocodiles’ reversed coloration wasn’t directly tested in this study. Researchers speculate that it acts like a form of countershading, similar to the way a shark’s white belly makes it harder to see when viewed as a silhouette from below. If true, this further supports the notion that adult crocodiles make use of these abilities to help them surprise their prey.

The sunburn question was likely sparked by Claude, the albino alligator living at San Francisco’s California Academy of Sciences. As an albino, Claude had no pigment it his skin, putting him at greater risk from ultraviolet light damage (and predation! and being spotted by prey!) While it might seem handy to be able to activate built-in sunscreen on command, it’s hard to see how that would benefit an animal more than good protection all the time. Since these animals mostly live in equatorial regions, they wouldn’t really have a pressure to decrease their ultraviolet light protection at any point, making the perception to hormone to melanosomes system needless complicated. Temperature control would seem like a better tool for a cold-blooded reptile to have, but the study directly controlled for temperature changes, and found that it didn’t influence the crocodile’s coloration.

Source: Crocodiles Alter Skin Color in Response to Environmental Color Conditions by Mark Merchant, Amber Hale, Jen Brueggen, Curt Harbsmeier & Colette Adams, Scientific Reports, volume 8

On April 17th, 2018 we learned about

Describing, and disabling, a carnivorous plant’s version of consciousness

“Wait, how do plants know things? Do they know things? They can’t, right?”

My third-grader’s brow furrowed as she ran scenarios over in her head. Plants aren’t the same as animals. They don’t have muscles to move with. They don’t have eyes like we do, and no brain to do any thinking. Nonetheless, they are known to follow the Sun, open and close flowers, and even react to the sound of predators eating their neighbors’ leaves. How was any of this possible without the anatomical gear we depend on to do any of those jobs?

It’s a tough question, and we don’t know all those answers yet. The sound sensory in particular is quite odd, but fortunately, some very reactive plants have helped botanists figure out how a plant can sense and respond to stimuli without nerve cells and a brain to do so. Because carnivorous plants like the Venus flytrap (Dionaea muscipula) and sundew plants (Drosera) have to actively trap their prey, they need to operate in a time-frame that matches the critters they want to catch. To do that, the plants are relying on so-called trigger hairs that essentially give the plant a way to “sense” when something has touched it.

As the titular fly lands on the attractively-scented “lobe” at the end of a leaf, it’s likely to bump into one or more trigger hairs. Unlike your nerve cells, those hairs don’t report back to any brain to trigger further activity. Instead, they create an action potential, which is an electrical charge that builds up in the trigger cell, eventually reaching a threshold where it gets discharged to the next cell, and the next, and the next. Eventually, this signal reaches the plant’s version of a muscle cell, which either expands or contracts to change the water pressure along the joint of the mouth-shaped lobe, causing it to close shut on the bug. With further stimulation of those trigger hairs, the flytrap will start to excrete digestive enzymes to it can actually eat its prey.

Turning off the trigger hairs

Even though trigger hairs aren’t exact matches for animal nerve cells, they’re close enough that they can be used to study the effects of the general anesthetics we use on animals. Even though those drugs are used every day to numb and temporarily paralyze people in surgery, we don’t know exactly how they do it. By using them on reactive plants like Venus flytraps and the “shy plant” Mimosa pudica, researchers are getting closer to understanding the exact cellular mechanisms that make modern surgery possible.

The answer seems to go back to the idea of action potentials, and how they get started in the first place. In both the plants trigger cells and animal nerves, a charge is built up on the outside of the cell membrane or wall. In the membrane are openings called ion channels that open and close when specific molecules are present to unlock them, a bit like a key opening a gate. Charged molecules, ions, can then move into the cell, helping it accumulate a larger charge, until eventually it triggers the release of an electrical charge to kick off another cell.

Paralyzing the plants

When carnivorous plants were subjected to anesthetics like diethyl ether, their trigger cells became unresponsive. The shy plant didn’t curl its leaves. Flytraps didn’t close after being poked. More importantly, no charge was detected at the plants’ trigger cells, indicating that the ion channels weren’t opening, heading off any action potential before it started. A second test with the roots of a mustard plant, Arabidopsis thaliana, found that the lipids, or fat proteins, in the cell membrane were being disrupted, helping researchers narrow their focus even further.

This wasn’t done to numb Venus flytraps, of course. They seem to be quite good at regulating their own activity already; they only close if multiple hairs have been touched, don’t usually close down on their pollinators, and reserve digestive enzymes for when they really have lunch in their clutches. Instead, this work may help us understand and then design better anesthetics for humans and other animals, taking some of the trial and error out of how we temporarily stop each other from sensing the world around us.

Source: We can make plants pass out—with the same drugs that mysteriously knock us out by Beth Mole, Ars Technica

On April 4th, 2018 we learned about

Staying warm in a hot spring lowers Japanese macaques’ stress hormones

Humans have enjoyed bathing in Japanese hot springs, or onsen, since at least 712 AD. The other local primates, macaque monkeys (Macaca fuscata), took a bit longer to catch on, waiting until 1963 to take a dip in the geothermally warmed waters. That first female apparently told her friends though, because the hotel where she took her bath was shortly swarmed with other bathing macaques. Rather than potty train every furry guest, certain springs were designated as exclusive to the macaques, eventually leading the founding of the Jigokudani Monkey Park in Nagano. The hot springs have only grown in popularity since that first bath, but researchers have only recently been able to confirm why the monkeys have adopted this unusual behavior.

To a human bather, the motivation for the bathing monkeys may seem obvious. Judging by the somewhat serene looks on the macaques’ faces, it’s safe to assume that they enjoy their visits to the onsen for the same reasons we do- the warm water is relaxing and pleasant. The fact that the monkeys are more likely to use the hot springs in the winter further supports the idea that they’re just interested in warming up when the weather gets cold. However, only one-third of the female monkeys living in the park seem to bathe, suggesting that this behavior may have a bit more nuance to it than a desire for hot water.

Measuring stress hormones in the monkeys’ scat

While the macaques are now adept at mimicking human bathing in the onsen, they’re not about to answer questions about their motivations. So researchers looked to their lack of hygiene for answers, testing the poop of various individuals for levels of stress hormones like faecal glucocorticoid (fGC). As expected, the warm water helped the monkeys maintain their body temperature, lowering stress levels. Less obviously, dominant females were found to demand more access to the warm pools, but also raise their stress levels more in various conflicts with other monkeys. So every bather benefited, but dominant females felt more of a swing between their stressed and relaxed moments.

So it’s not a huge surprise that our fellow primates enjoy a warm bath in cold weather, but it’s important for wildlife managers to understand. These monkeys have not only adopted behavior modeled by humans, but they’ve also grown accustomed to being fed barley over the winters in the park, partially to keep them in the area for the pleasure of tourists. These are some significant changes in behaviors, which may prove to have implications in the macaques’ health and reproduction. At this point, only 50 or so monkeys are actually bathing on a regular basis, but it’s worth understanding what that means to the macaques if humans are actively protecting this change in their ecology.

Source: Spa therapy helps Japan's snow monkeys cope with the cold, Science Daily

On March 25th, 2018 we learned about

Emotions influence circulation, and therefore coloration, in our faces, helping express how we feel

A perfect poker face may be physiologically impossible. No matter how well someone might try to hide their emotions behind a clenched jaw and vacant stare, they probably won’t be able to control their cardiovascular system well enough to stop changes in blood flow just below their skin. While these shifts aren’t necessarily as dramatic as the pink cheeks many of us get when feeling nervous or embarrassed, they are clear enough to help people read each other’s emotions with up to 75 percent accuracy.

Researchers from Ohio State University found that emotional states changed people’s circulation in their nose, eyebrows, cheeks and chin, and set out to test how much those changes were noticed by onlookers. The slight shifts in color, like a blue-yellow cast around a disgusted person’s lips, are subtle enough that most of us have never consciously noticed them. So computers were used to analyze images and find the patterns that turned up as a person experienced sadness, anger, happiness and more.

Reading emotions without facial expressions

Once those patterns were found, they were used to manipulate photos of people making neutral facial expressions. Those photos were then examined by test participants to see if they could correctly identify what emotion was coloring the otherwise blank expression. Some emotional color-schemes were harder to identify than others, but people definitely detected the correct emotion the more often than not. For example, angry colors were correctly identified 65 percent of the time, while happier hues were spotted 70 percent of the time.

A second phase of testing approached things from the opposite direction. Instead of identifying emotions on blank faces, participants were asked to assess photos were the emotional coloration was purposely mismatched to the expression being made by the face’s muscles. For instance, a rosy chin normally associated with a big smile was digitally added to a photo of someone otherwise expressing despair. Test participants couldn’t put their finger on what was wrong, but they did note that something was “off” with this round of photos, indicating that our brains do look for this kind of information, even if we’re not aware of it.

Communicating with color

These results make sense considering how social and visually oriented human beings are. We spend a lot of time trying to read each other’s faces to form bonds, get signs of danger and more. Even if we don’t take our shifting hues to the extremes of a stressed chameleon, we certainly wouldn’t be the first animal to adjust skin temperatures according to emotional states. Of course, before we pat ourselves on the back for this subtle form of communication, it’s worth noting that the entity that could most accurately read the emotional states expressed by skin coloration was a computer. Once it was trained on what to look for, image analysis was able to identify a happily-colored face with 90 percent accuracy.

Source: Happy or sad, the colour of your face reveals how you feel by Haroon Siddique, The Guardian

On March 1st, 2018 we learned about

Investigating how our brains determine when our bodies should have a drink

Your skin may seem dry, your mouth may be parched, but the experience of thirst is all in your brain. To be more precise, it’s in your ‘thirst center,’ a bundle of brain cells found in your hypothalamus. These neurons are tasked with the job of telling an animal when to get a drink, and just as importantly, when to stop sipping. Weirdly, that second command takes place well before the rest of the body can benefit from any new fluids, which has made untangling the exact functionality of this system a surprising tricky problem.

When the body wants water

The thirst center isn’t making us go for a drink at random. The hypothalamus takes in information about a lot of our physiology, including blood pressure, sodium concentration and more. When these systems are out of balance and more water is needed, one response is to create more of the hormone vasopressin in the pituitary gland, which then makes the kidneys try to reclaim water from any available urine. The second, but probably preferable response is to have the thirst center have us look for a drink.

As obvious as that response may seem, it’s taken years for researchers to isolate which cells were actually in charge of feeling thirsty. To identify exact which parts of the brain were tied to thirst, researchers turned to optogenetics, a technique that uses engineered neurons to be activated by light via implanted fiber optics. In this case, mice had their brain cells monitored and stimulated until they either became thirsty, or artificially had their thirst quenched without needing a drink.

In this process, it became clear that the amount of water an animal drinks is entirely up to the brain. Water won’t do a body much good until 10 to 15 minutes after it’s been ingested, whereas mice’s thirst center stopped them from drinking after only a minute each time. Somehow the brain felt it had had enough liquid, and that it was time to stop drinking.

Drinking disorders

This may sound pointlessly academic, but that’s only because it works so well for most of us. Whatever mechanism the thirst center may be using to measure our water intake, it’s generally helping us obtain a safe amount of water. If we ignore the thirst center, as some marathon runners have done, or the mechanism is damaged, as with people with psychogenic polydipsia, the results can be dangerous or even fatal. Cells swell and burst like balloons as they try to absorb the incoming water, a fate particularly damaging when it occurs in the brain. On the flip side, as people age they often have a weaker thirst response, and may forget to hydrate on a regular basis.

At this point, the investigation into how we feel thirst is actually moving out of the brain. Researchers are interested in muscle cells in the throat that help with swallowing, as there’s a chance that our sip-stopping thirst center may be gauging our water intake based on how quickly we swallow.

Source: Still Thirsty? It's Up To Your Brain, Not Your Body by Jon Hamilton, The Salt

On February 26th, 2018 we learned about

Bat immune systems can safely endure viruses in order to enable flight

Bats adding a new twist to the concept of “compromised immune systems” The term usually refers to organisms that have weakened immune systems, and are at higher risk for infection as a result. Bats, on the other hand, seem to have taken the idea of “compromise” in a new direction, as their immune system has an unusual tolerance for viruses in order to benefit the bats. Not only do the bats seem to survive well enough with extra viruses in their bodies, but a dampened immune response may be a crucial component of their ability to fly.

Permissive of pathogens

The key to bats’ strange immune system are stimulators of interferon genes (STING). Humans, cats and dogs all have our own versions of STING, which allows our bodies to create interferon proteins, which help us fight off invasive viruses. Interferons are powerful, and if STING pathways are active when a virus isn’t present, an animal can face severe consequences in the form of an auto-immune disease. So while our STING pathways are triggered selectively, bats seem to take a very different approach, as their STING pathways are always on, but at a reduced degree of intensity.

This basically means that bats’ immune systems are always activated, but not at full power. As a result, many viruses from rabies to Ebola, can take up residence in a bat’s body, turning it into a viral reservoir. However, the bat seems to suffer no debilitating symptoms from those diseases, letting them be controlled without any dramatic immune reaction one way or another. Researchers describe this dynamic as a “balance” with the bats’ viral load, and as nice as being able to ignore a SARS virus sounds, dealing with pathogens was probably only a happy byproduct of another evolutionary pressure.

First required for flight

It’s suspected that the reason bats needed a constant but dampened STING pathway was to deal with self-inflicted metabolic damage to cells. The energetic demands of flight can cause some DNA to be released into cells, outside the cell nucleus where it belongs. In most mammals, those DNA fragments would be activate STING pathways to produce interferons, leading to a big immune reaction. Since bats need to fly on a daily basis, this arrangement would be problematic. By altering the sensitivity of their interferon reactions, bats were able to cope with these metabolic demands without freaking out their immune systems. Safely harboring viruses then made sense, since their immune systems were already functioning on different level of activity than other mammals.

My five-year-old asked: So do the bats that help farmers have lots of viruses?

Since the STING dampening probably first evolved to enable flight, it’s safe to assume this compromising immune response is present in all bats. Many of the viruses these bats may be carrying require direct contact of some kind, such as with an infected bat’s saliva, so as long as people aren’t handling the animals, there’s not too much risk there. However, bats are more likely to transmit a virus to a domesticated animal, so it’s good to try to inoculate livestock against viruses like rabies whenever possible.

Source: Switched-on bats: hosting viruses is a cost of flying by Tanya Loos, Cosmos Magazine

On February 19th, 2018 we learned about

A runny nose’s excessive boogers are made in our body’s best interest

It’s only been three days since my son’s nose started getting snotty, but that’s long enough to make you wonder why, and how, all this mucus keeps coming out of his nose. At just shy of five-years-old, he’s technically able to handle a tissue himself, but not to the point where he can be expected to be effective in his booger management. Coupled with the sore throat and cough of a nasty cold virus, all this mucus-production feels like a bit of a curse. Of course, it’snot— it’s simply our body’s way of purging pathogens that are trying to take up residence in our respiratory tract.

On any given day, your sinuses are doing double-duty at a minimum. They warm and moisten air before it gets to the sensitive tissue in our lungs, plus captures junk that we don’t really want to be inhaling in the first place. That can be dust, dirt, pollen and of course, viruses and bacteria. Ideally, the layer of mucus that coats the inside of your nose and sinuses is enough to capture these potential irritants, moving the sticky stuff back down your throat with hair-like structures called cilia.

Purging pathogens

As my son’s clogged nose can attest, sometimes things get through. There’s likely to be some resistance in your mucus from benign bacteria, but your immune system revs up when a pathogen start penetrating cell walls in your nose. Proteins called cytokines are released, which then activate T and B cells that will attack the pathogen directly. To help in that battle, the lining of your nose swells and increases its booger production, hopefully creating enough mucus to grab and flush the offending pathogens out of your body. Unless of course you’re five, in which case you’ll probably get the contaminated snot on your hands and spread it far and wide, infecting everyone around you. (Not that we hold that against you, son!)

From the outside, this all looks like a runny nose. The excess fluid in the swollen mucus lining can lead to gross, drippy discharge in a condition known as rhinorrhea. Sometimes the extra mucus just clogs things up, making us feel horribly congested in the process. You can blow your nose to help with that, but violently trying to force the boogers out of your face can actually damage the cilia that help move mucus around. It can also end up sending pathogens deeper into your sinuses, kicking of new infections. So even though a drippy nose is annoying, it’s actually working as intended.

External influences

Of course, sometimes your nose is drippy when it doesn’t need to be. For instance, cold weather can trigger a runny nose in healthy people, albeit for very different reasons than described above. In those cases, the air is probably cold and dry enough to make your mucus linings activate in an effort to keep air properly warmed and moistened on its trip to the lungs. That can lead to extra fluid in your nose that then starts dripping out. Alternatively, there’s a chance that moisture in the air is condensing just inside your nose, forming noticeably large droplets that feel like snot.

Finally, crying can lead to a runny nose because eyes are just filling the place with fluid. As tears drain into your nose, they soften the layer of mucus that’s always present enough to start flowing. That way, your emotional moment can feel a bit sticky too.

My third grader asked: Is the runny nose you get from cold air the reason we call it a ‘cold?’

Basically. A “cold” was first used to describe illness in the 1530s, long before anyone knew to look for the rhinovirus or coronavirus that was actually causing a person’s symptoms. The resulting infection felt enough like the unpleasant effects of being chilled that it easily described what was wrong with someone, even if it did lead to confusion about what actually causes the illness (mostly.)

Source: Why Does Your Nose Run When You’re Sick? by Alexandra Ossola, Popular Science