Ttdfjt

You just need to have the appropriate chemireceptor which is triggered by CO molecules.

Sensitivity is less of an issue: our nose can detect certain molecules at very low concentrations, like H₂S, the molecule responsible for the smell of rotten eggs, which can be smelled at 0,0047 ppm.

Since CO reacts better with hemoglobin than oxygen and CO₂, a suitable receptor can be based on a modified version of hemoglobin, with the reaction triggering the nervous pulse to the brain.

The affinity between hemoglobin and carbon monoxide is approximately 230 times stronger than the affinity between hemoglobin and oxygen so hemoglobin binds to carbon monoxide in preference to oxygen

Once the brain has its impulse, it will smell the gas.

Are you open to symbiosis? There are microorganisms that actively metabolise Carbon Monoxide. A creature that harbours colonies of such bacteria may not be able to directly smell atmospheric Carbon Monoxide but they could get feedback on it's local concentration based on the activity level of those colonies. This would be by way of sensing metabolic byproducts from those bacteria.

They can just smell it like anything else.

"Odorless" just means "human noses didn't happen to evolve the ability to smell this particular chemical."

If humans or other animals evolved in an environment where being about to smell carbon monoxide was beneficial, they'd just be able to smell it.

People are talking about particular mechanisms, which is fine and all, but I suspect you don't care about the exact biochemical pathway activated when you smell, say, a cheeseburger.

It's not the same as "smelling" it, and as far as I can find, there are no studies testing this ability, but there are countless stories about cats (and sometimes dogs) who saved their human families (and dogs) from carbon monoxide poisoning.

In these cases, the CO levels were high enough to cause severe symptoms in humans, but they were asleep and didn't notice. The cat noticed and woke at least one person up.

...The time around 1 a.m. when everyone was sound asleep.

“All of the sudden Gracie, I heard she was pounding, knocking,
knocking, knocking at the door,” Shanahan said. “And so I got out of
bed and to stop her from pounding at the door, and I looked to my left
and Annette was there in the chair.”

“I was hanging onto the arm of the chair, and I thought I was dying,”
his wife Annette said.

“And she called 911, and all she could say was ‘can’t breath.'”

When firefighters arrived, they discovered lethal levels of carbon
monoxide in the home. It was caused by a malfunction in the hot water
heater.

Annette and Kevin were taken to the hospital, where they spent the
night.

Had it not been for Gracie — well, they’d rather not think about that.

There are many more stories. In several the family has a working CO alarm, which sounds after the cat has already woken them.

We humans can detect CO quite well. But we've been conditioned by a couple centuries of living indoors with combustion to ignore mild symptoms. Even manufacturers don't make detectors for low-level exposures that may not be immediately life-threatening, but certainly do affect your health.

Most studies are about acute exposures to high levels of CO, at least 30-50 ppm over a few hours (and all the charts about "safe" exposure levels say "for healthy adults"). But health effects can occur with chronic exposure to low levels, like 10 ppm, especially in children and people with pre-existing health conditions.

Evidence that exposure to low concentrations of carbon monoxide can
affect a number of organ systems is accumulating. It is, perhaps,
easiest to explain effects on the heart in subjects with incipient
myocardial ischaemia. Less easy to explain are effects on the central
nervous system; that these effects may not be accurately predicted on
the grounds of blood COHb concentration does, however, seem
increasingly clear. Whether long term exposure to low concentrations
of carbon monoxide can produce long lasting effects on the brain does
not yet seem to be settled. If such effects do occur, the impact on
public health may be large: many homes are heated with gas appliances
and a significant number by solid fuel; failures are inevitable and
known to be common, and thus a significant number of people must be
being exposed to levels of carbon monoxide in excess of those found in
ambient air. Even if only a modest proportion of those exposed
sustained effects, the impact on public health may be significant.
These findings may have implications for the setting of occupational
exposure limits. The Health and Safety Executive recommend a limit of
30 ppm, which can cause COHb levels to rise above 2.5% in less than
one hour. However, it should be noted that the evidence for low level
effects of carbon monoxide does not arise from occupational exposure
studies. The patterns of exposure of people exposed in their homes may
be quite different from those exposed occupationally.

We humans may not recognize CO as such, but we do usually know something is wrong with our bodies. Most people ignore that or get belittled by doctors when they try to get help. Or they might get a diagnosis that doesn't acknowledge the source, or even attempt to test for it. If tested, it's dismissed as an issue, because the prevailing wisdom is that CO below 30-50 ppm isn't dangerous (in some cases, the thresholds are even higher).

Animals can also be trained not to "bother" humans with complaints. But generally they know something is wrong and don't care what humans think about it. Many will go to great lengths to alert their humans as well.

In your creature design, take animals that have intelligence levels similar to cats and dogs and add in something specific that CO can do. If it's a world with lots of CO around in pockets, this could be something an animal evolved to detect easily.

For example, have the lack of blood flow CO causes that turns gums red also produce heat. Uncomfortable levels of heat or a burning sensation. It doesn't matter what, as long as it's exclusive to CO exposure. This can be inside the mouth and nose. Or it could be on the paw pads or anus or someplace that other animals can see the change to bright red (make sure their eyes can detect that color change). The animals might also be able to touch noses to noses (or butts) and feel the heat.

These are all signs to raise the alarm, scruff the children, and get the hell out.

Have a look at the Wikipedia page on CO detectors: https://en.wikipedia.org/wiki/Carbon_monoxide_detector

Especially the sections about Biomimetic and Electrochemical sensors could be used here. Biomimetic sensors try to emulate hemoglobin which darkens in the presence of CO. To emulate that process they use Cyclodextrines which can totally be produced biologically.

The Electrochemical sensors work like a fuel cell that digests CO to produce minimal amounts of electricity. I don't see why a very simple version of that couldn't be grown in an animal.

The question is what kind of circumstances would be necessary for an animal to evolve something like that? Usually, CO is not an issue when you are outside of buildings or caves, both of which are places that wild animals don't live in.

To the best of my internet researching ability, animals cannot 'smell' or otherwise detect CO.

They can't detect it? But they can! CO is even used as a neuromodulator and gasotransmitter.

Soluble guanylyl cyclase (sGC) is able to detect both nitric oxide and carbon monoxide, and that is an enzyme we all have. Although CO is a polar molecule and thus not effectively detected with membrane-bound receptors, an intracellular receptor could be made to work. It turns out that sGC is able to fully discriminate between NO and CO, so it would be an ideal chemoreceptor to "smell" carbon monoxide.

Guanylyl cyclase is one of the candidates for CO and NO's surprising behavior as a neuromodulator. While sGC is more sensitive to NO, it has mechanisms to detect which one it is bound to. Because sGC is a signalling molecule, it can communicate the fact that it has bound to CO to the rest of the cell, leading to a neuronal response. In an olfactory neuron, it could be used to selectively detect CO.

See also NO and CO differentially activate soluble guanylyl cyclase via a heme pivot-bend mechanism.

• 
  

  From Wikipedia, s.v. Sentinel species:

  
  
  

  
  

  The idea of placing a canary or other warm blooded animal in a mine to detect carbon monoxide was first proposed by John Scott Haldane, in 1913 or later. Well into the 20th century, coal miners brought canaries into coal mines as an early-warning signal for toxic gases, primarily carbon monoxide. The birds, being more sensitive, would become sick before the miners, who would then have a chance to escape or put on protective respirators.

  
  
  

  Canaries were iconically used in coal mines to detect the presence of carbon monoxide. The bird's rapid breathing rate, small size, and high metabolism, compared to the miners, led birds in dangerous mines to succumb before the miners, thereby giving them time to take action." (Wikipedia)

  
  

  
  


• 
  

  From the Canadian Institute of Mining, Metallurgy and Petroleum (CIM):

  
  
  

  
  

  Small animals like cana­ries proved useful for detecting poisonous gases because of their rapid breath­ing rate and high metabolism, making them more sensitive to the effects of poisonous gas. When exposed to low levels of CO, a canary has difficulty breathing and becomes quite visibly agitated and unsteady. Miners knew there was poisonous gas around when the bird began to sway on its perch or ­collapse.

  
  
  

  Regular miners rarely brought canaries with them into the mines on their shifts. The birds were, instead, primarily used by rescue crews following explosions resulting from regular detonations during mining, sparks from mining equipment, or the open flames of the miners’ carbide lamps. Combustion in the mines produced CO, which could kill miners through asphyxiation.

  
  
  

  Canaries were brought underground in cages about the size of a lunchbox, made out of a durable, transparent material known as Perspex. The handles doubled as a small oxygen canister, and if a canary collapsed from exposure to CO, the miner could cover the ventilation holes and open the oxygen canister to revive the bird. It was not in anyone’s interest to let the birds die, and miners were known to grow fond of them and treat them like pets, whistling to them as they worked. Some even carried extra oxygen bottles with them especially for the canaries just in case they needed a refill. (Correy Baldwin, "Who brought the canary into the coal mine?", in CIM Magazine, November 01, 2014)

  
  

  
  

The creature cannot smell carbon monoxide directly, but is extremely sensitive to the other gasses that usually accompany it.

This is, after all how we humans generally manage to avoid breathing too much CO most of the time. We can't smell CO directly, but we tend to avoid breathing anything that has a lot of the bad-smelling combustion products that are normally emitted along with the CO. Automobile exhaust fumes (for example) are deadly because of the CO, but they also smell pretty bad because of all the other combustion products in there. If you are standing in a garage that has a dangerous concentration of exhaust fumes, you will probably notice the smell and get out of there quickly.

However, some sources of CO produce much less odor than others, so there will be some cases where human noses are not good enough to notice the smell in time. In a situation where CO poisoning is a major risk, it would make perfect sense to train some animal to detect the subtle scent that accompanies dangerous levels of CO. Some animals, like dogs, pigs, and rats, have a vastly better sense of smell than us, and could be trained to recognize the danger signs long before humans could.

Mosquitos can detect carbon dioxide from a distance, and follow the "scent". Shouldn't be that much different for an animal and carbon monoxide.

In the real world, the main difference is that carbon dioxide occurs naturally, while carbon monoxide pretty much doesn't, so animals haven't evolved the ability to detect it. (Also, carbon dioxide sinks while carbon monoxide rises, so that would make it less likely for a ground-based or near-the-ground animal to detect carbon monoxide in general.)