There are three types of rats that are important for you to know because in this part of the world you are bound to meet at least two of them. The most common is the black rat, full name Rattus rattus. It is a relatively small mammal found across most of sub-Saharan Africa, Europe, Asia and Australia, but not perhaps surprisingly South America.
The second type of rat is the mall rat. Of indeterminate size, the mall rat is found around the world where there are shopping malls and so is common in North America and Europe.
The third kind of rat is in the eyes of the describer a small human being capable of the worst behaviour, the betrayer, an informer who deserts his friends or associates. All three types of rats cause problems in society. Both Rattus rattus and the rat that betrays his friends are opportunistic survivors and live with or very near human beings.
They can cause substantial losses of food; in the case of the black rat is associated with infectious disease; while in the rat-betrayer, mental illness is a distinct possibility. It is in this context that the phrase “to smell a rat” has real meaning. The origin of the idiom is obscure but arose perhaps from the observation of a cat not being able to see a rat, but scratching about a place trying to reach it perhaps behind a little hole.
In navigating day-to-day life we rely heavily on our visual sense, our sense of hearing and much less so on our sense of smell. Research demonstrates that people can distinguish several million different colours and about 300,000 audio tones but science has yet to discover the dimensions of the sense of smell. Nevertheless our sense of smell helps us both enjoy life and warn us of the dangers of life. The smell of our favourite food, or person or even fresh air makes us happy.
On the other hand the smell of rotting food, a fire or diesel fumes puts us on the alert that things are not quite right. Unlike vision and hearing, smell is a very direct sense.
In order to smell something molecules from that thing must reach your nose. So the only things that you can smell are things that give off some volatile chemical that will travel in the air to your nose. Try smelling the can of insecticide (do not spray it) or some piece of cold metal, no smell! That means when you smell something rotten, some of the rotten stuff has entered your nose.
There are two pathways for the body to detect a smell. The first is when the volatile molecules, be it of fresh bread, coffee, enter the nose. Deep within the nose are specialised sensory cells, tiny nerve fibres covered with a thin layer of mucus, called olfactory sensory neuron, which are connected directly to the brain.
The winners of the 2004 Nobel prize for medicine found that there are about 1,000 olfactory receptor genes that encode the receptors and that each olfactory neuron expresses one odour receptor. The second pathway is when the same molecules enter through the mouth and communicate with the nose via the back of the throat.
When we chew food, the aromas in the chewed food are detected through this route, which is why when we have a flu or throat infection the sense of smell is diminished. The sense of taste is therefore intimately connected with the sense of smell, though smell is 10,000 times more sensitive than the sense of taste. Either pathway depends on the working of the nose and an extensive library within the brain to appreciate the smells.
Any loss in the sense of smell lowers our quality of life and a loss is often a sign of disease but can also occur with age. About one per cent of people have a smell disorder ranging from anosmia, the inability to smell anything at all to hyposmia, where there is a reduced ability to smell. Increasing age diminishes the sense of smell.
One study showed that 11 per cent - 25 per cent of persons over 60 years had a reduced sense of smell. Conditions that can affect smell range from Parkinson’s disease, Alzheimer’s disease, diabetes and hypertension. Of course physical damage to the nose or brain can affect the sense of smell. Tobacco smokers inflict considerable damage to their own sense of smell and well-being and for any cigarette smoker fresh air is a description found only on paper.
In the end if you smell a rat, it is possibly a sign of the company you keep, the environment you live in or both. This might seem defeatist but from an individual and public health perspective the key thing is that you and others around you must be able to smell the rat and then do something about it.