Animals use the sense of smell to locate food, mark their territory, identify their own offspring and the presence and sexual condition of a potential mate. The organ of smell (olfactory organ) is located in the nose and responds to chemicals in the air. It consists of modified nerve cells that have several tiny hairs on the surface. These emerge from the epithelium on the roof of the nose cavity into the mucus that lines it. As the animal breathes, chemicals in the air dissolve in the mucus. When the sense cell responds to a particular molecule, it fires an impulse that travels along the olfactory nerve to the brain where it is interpreted as an odour (see diagram 15.2).
Diagram 15.2 - The olfactory organ - the sense of smell
The olfactory sense in humans is rudimentary compared to that of many animals. Carnivores that hunt have a very highly developed sensitivity to scents. For example a polar bear can smell out a dead seal 20 km away and a bloodhound can distinguish between the trails of different people although it may sometimes be confused by the criss-crossing trail of identical twins.
Snakes and lizards detect odours by means of Jacobson’s organ. This is situated on the roof of the mouth and consists of pits containing sensory cells. When snakes flick out their forked tongues they are smelling the air by carrying the molecules in it to the Jacobson’s organ.