Smell, Taste and Touch
Olfactory Equipment
The ability of mammals to smell depends on a relay system made up of receptor cells, nerves and the olfactory lobe, a structure found in the anterior portion of the brain that is responsible for the collection and analysis of scent molecules prior to the journey to the neocortex and various brain centers.
The receptor cells are concentrated in a pair of olfactory clefts located just above the air passages in each nasal chamber of the nose. The clefts are lined with a complex network of small scroll-like bones, called turbinates, and a membrane of yellowish epithelium that is composed of millions of receptor cells. These cells pick up strong scents as the mammal breathes. Weaker scents are purposefully drawn into the nose and over the receptors by sniffing.
The larger the surface area of the nose's epithelium — and therefore the more scent receptors — the better a mammal's sense of smell will be. Dogs, for example, have more than 250 million receptors in an area of membrane about 50 times bigger than that found in humans, who are served by about 5 million receptors.
The olfactory system is most highly developed, of course, in mammals that rely on it to stay alive, such as nocturnal insectivores, carnivores and rodents. Humans and other higher primates have more poorly developed olfactory systems. Essentially diurnal, they depend on their superior vision to find food and to detect both prey and predators. Most cetaceans have very few olfactory receptors and appear to have lost their sense of smell. Baleen whales, however, have more olfactory receptors than toothed whales and may "sniff" the air with their blowhole when they surface to smell for plankton-rich waters.
Apart from its value in seeking the food they need to survive, mammals use their sense of smell to communicate. They manufacture volatile chemical signals, called pheromones, and excrete them along with feces and urine or emit them from scent glands located in various places on their body. These personal messages are left sometimes deliberately, sometimes unknowingly. Often one sniff is all the receiver needs to determine the age, gender, health, and sexual and social status of the sender.
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