Diagram of the human olfactory system. Odour molecules (odourants) are captured by special olfactory cell receptors in the nasal mucosa. This causes an electrical impulse that travels to the central nervous system, and we can smell. Source:
We smell even the slightest gas leak, elephants can smell water several kilometres away, and dogs can easily track a criminal even after several days. What are the limits of such abilities? How sensitive can the chemosensors of living organisms be, and who has the best? In this article we try to find the answers to these questions.
Chemoreception, or sensitivity to chemicals, is one of the most ancient methods of sensing. Almost all living organisms on our planet possess: it: bacteria, amoeba, insects, fish, and, of course, mammals. Thanks to their chemosensors, living organisms recognise the presence of certain substances in the environment, which gives them the signal for a change in behaviour. The search for food, shelter, a sexual partner, or simply communication are all possible due to the chemical sensitivity of many animals (1,2).
Moreover, chemoreception is developed in different ways in different animals. Simple organisms (bacteria, protozoa, etc.) generally do not have special chemoreceptor systems, whereas in vertebrates and insects such systems reach the highest level of development: separate complex organs of smell and taste (1). Taste is a 'contact' sense: it requires a sufficiently high concentration of the substance; in other words, in order to taste, we must put food on the tongue. The sense of smell is 'contactless', in that it allows one to obtain information about distant objects. Therefore, it is precisely among the olfactory organs that the most sensitive chemosensor should be found (1,3).
When answering the question of the keenest sense of smell, many would think of the dog first. Indeed, the canine sense of smell sometimes seems incredible to us. Everyone knows how efficiently dogs can find explosives or drugs, rescue people caught in avalanches, and track down criminals even days after the crime (4, 5). These, however, are far from the most difficult tasks that our four-legged friends can solve. Experiments have shown that dogs are just as good as specialised medical devices (and often even better) at detecting various diseases in humans such as tuberculosis, diabetes, cirrhosis, and even cancer (4-7). In addition, they can 'predict' epileptic seizures (8).
Inside, a dog’s nose is a complex labyrinth containing more than 200 million olfactory neurons – special cells that can 'smell' odours (for comparison, humans have about 10 million such cells ) (5,9). Olfactory neurons are natural chemosensors, highly specialised cells that have a particular type of receptors on the surface, capable of recognising various molecules. When molecules bind to the receptors, an electrical impulse is generated that travels to higher parts of the nervous system. This is how we can smell (1-3). In 2004, the American scientists Linda Buck and Richard Axel received a Nobel Prize for research into olfactory receptors and organs.
Dogs can identify molecules by smell at an incredibly low concentration – up to 0.001 parts per trillion (ppt) i.e. 1 ml of odourant per 1015 cm3 of air (4,6,10). This is about the same as adding a drop of ink to a body of water twenty times the volume of an Olympic swimming pool. There are, however, other 'record breakers'. Rodents such as mice and rats have also earned a reputation for being extremely sensitive to smells. It has recently been discovered that they can smell the aromatic aldehyde bourgeonal at a concentration of 0.0001 ppt! (11). It is interesting to note that bourgeonal, which smells like lily of the valley, is a chemical attractant for human sperm – in other words, they can also smell! It is believed to help sperm find the egg. In addition, bourgeonal is likely the only odour to which men have a higher sensitivity than women (12).
The ability of dogs and other animals to react to concentrations of odourants too low for most people to detect has led to the belief that humans have a weak sense of smell. However, this is not quite true. According to large-scale studies conducted on various groups of mammals, sensitivity of smell depends on which substance we are sensing (4,10,13). Thus, it turns out that humans and primates are superior to dogs, rats, and pigs in the ability to recognise many odour groups (10,13). For example, humans are more sensitive than rodents in detecting the smell of urine or mammalian blood. It also emerged that we are more sensitive than dogs to many plant scents (10). In general, there are quite a few odourants that people can smell better than animals (11,13).
However, as noted by scientists studying the sensitivity of smell, it is difficult and far from accurate to compare individual species using this indicator. After all, if a person can say whether they can smell something or not, then an animal must first be taught to give some kind of signal. Findings can vary greatly depending on how the training and experiments are conducted. In addition, different species are specialised in recognising different odours that are vital to them. Therefore, there is little sense in comparing the sensitivity of different animals to the same smell (4,10,13).
An 'electronic nose' is a device that can smell. It helps to evaluate the quality of food and is also capable of detecting poisons. Source:
Maybe in this case we should try using a different approach to compare animals' sense of smell? For example, counting the genes responsible for olfactory receptors? The more genes there are, the more different types of olfactory receptors there can be. A group of scientists from the University of Tokyo compared the number of 'olfactory' genes in 13 different species of mammals (including elephants, horses, cows, rabbits, guinea pigs, rodents, primates, and humans). Oddly enough, the record holder in this case was not rats or dogs, but African elephants – with around 2000 working olfactory receptor genes. Rats took second place, with 1200 genes. In dogs, the number of these genes turned out to be two and a half times less than in elephants (about 800), and in humans and other primates it was about half that of dogs (396 in humans, 380 in chimpanzees, 309 in macaques) (14). Scientists emphasise that the results do not imply acuteness of smell or how vital a certain smell is to one species or another. Despite the fact that humans do not utilise around half of the olfactory genes (395 have a use compared to 425 pseudogenes) (14), our sense of smell is well developed and strongly affects emotional, reproductive, and social behaviour. The olfactory system affects communication between people. It has been shown that we can distinguish not only the emotional state of other people by smell, but also genetic information, kinship, or even more genetically suitable sexual partners (13, 15-17).
If we think about the absolute sensitivity of natural biosensors, it is logical to assume that the ability to detect a signal can be limited only by the physical characteristics of the stimulus. Thus, sensitive inner ear cells can detect displacements of their hairs over atom-sized distances. Visual photoreceptors can even pick out a single photon. One odorant molecule should probably be considered a quantitative unit of the olfactory stimulus (18). We can therefore assume that those living organisms that are able to respond to just one molecule of the substance will have the best olfactory acuity. It has been possible to register such sensitivity in insects – more specifically – in some species of butterflies (18-20). A butterfly's 'nose' is a special antenna on the head containing thousands of unusually sensitive olfactory cells. Just one pheromone molecule (pheromones are special substances for 'communication' between individuals) secreted by a female silkworm is enough to trigger a nerve impulse in the olfactory neuron of the male antenna. With this mechanism, the male can sense the female even at 5 kilometres (18-20).
Nevertheless, the answer to the question of the best chemosensor is quite controversial. Perhaps dogs can better recognise scent-marks on a fence than people, and people can smell good wine better than dogs (13). Even simply organised plants can possess a 'sense of smell' (21) and, at the same time, in highly organised animals such as dolphins, all the 'olfactory' genes are inactive (1). What complicates the picture further is the fact that many factors affect sensitivity to odours: age, individual experience, various diseases, emotional and reproductive status (10,13), and even how full the stomach is (22). Although there is still much unexplored in the field of chemical reception, it is safe to say that our sense of smell is much more important than we think.
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