New chemistry research from the University of Maryland seems to indicate that rodents combine food smells with breath odor to obtain information about safe food supplies. When you consider the life of a mouse, and really begin to understand what limited resources they have to work with – they’re not exactly the smartest creatures alive, and have to eat a large amount of food each day – you realize that mice must have developed some sort of signalling system. A secret mouse-only code, that they can use to point their friends and family towards a good source of safe food.
Researchers at Marylands School of Medicine believe that they have unraveled this rodent code: it has to do with the smell of food on a mouses breath. Now, for humans, the aroma of food on someones breath isn’t exactly what we would describe as appetizing. Our mouths are absolutely teeming with bacteria, which begin to feed instantly upon our eating a mouthful of food. When the bacteria begin to break down the various food components, often the byproducts of that bacterial digestion are compounds containing nitrogen and sulfur and they don’t smell the best. Smelling another persons breath is normally not something that we associate with hunger stimulation.
However, for mice (which lack a more sophisticated language), various aromas present on the breath of another mouse can be a fantastic indicator of what is attractive and safe to eat. It’s a classic case of “monkey see, monkey do” – monkeys in the wild often avoid brightly colored food until they see another monkey eat it and survive. It’s only then that they feel comfortable eating the red berries. This is behavior not too far removed from our own, and it’s very understandable – if you live in a world of questionable food sanitation, you wait until someone else takes a mouthful of that pile of foodscraps before you dive in yourself. It could be poisoned, after all – humans are tricky that way.
The article summarizing this research (published in the journal Current Biology) shows that one component of mouse breath is the organic molecule named carbon disulfide. Contrary to what you might expect given the name, carbon disulfide has a slightly sweet, pleasant aroma – almost like ether, if you’ve ever smelt that. It’s a byproduct of mouse metabolism, and is excreted by their respiration. So, a mouse who has just enjoyed a good meal will have a breath that is laced with this distinctive carbon disulfide aroma. They will also have scent traces of their previous meal on their breath. Another mouse who sniffs this combination of aromas will be able to recognize (for example) that: A) This mouse has just eaten a meal and survived; and B) The meal was a piece of cheese.
This olfactory signalling is so overpowering, so hardwired into a mouses behavior, that this combination of odors translates into a near-irrestible “eat this” signal for the mouse. The researchers found that special cells in the mouse nose, which are called GC-D cells, are responsible for the signalling. A mouse that smells cinnamon on another mouses breath will then choose cinnamon-scented food over any other flavor that might be present. Mice are a little, well, simple and so it doesn’t even have to be another mouse – if they sniff a cotton ball that’s been soaked in cinnamon and carbon disulfide, they’ll immediately set out to find this cinnamon meal that they just know is going to be delicious and safe. However, mice that were engineered to not have any GC-D cells did not copy their companions behavior. They had lost the ability to put two and two together, and had to just take a leap of faith when it came to finding a safe source of food.
It’s extremely interesting to note that this social cue for mice is so strong that even a mouse that has eaten poison will continue to eat more of the poison if it smells the poisoned food on another mouses breath. This indicates that the benefits of this scent signalling to mice must be extremely high, as it outweighs any other normal considerations. Scientists theorize that it may be because the mice are nocturnal, and therefore the types of visual cues that might be useful for a human are lacking for a mouse. Rodents have to learn to rely on their other senses; in some cases, they may rely a little too much on their other senses. Researchers are already putting this discovery to good use, as they have demonstrated that lacing a poisoned mouse bait with a few drops of carbon disulfide draws four times as many mice as a regular poisoned bait. This has implications for pest control and is extremely simple to implement, as carbon disulfide is cheap and readily available.