Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and by sensory cells of the antennae of invertebrates.
The importance and sensitivity of smell varies among different organisms. Most mammals have a good sense of smell, whereas most birds do not. Among mammals, it is well-developed in the carnivores and ungulates, who must always be aware of each other, and in those that smell for their food, like moles.
It is estimated that dogs have an olfactory sense approximately a hundred thousand to a million times more acute than a human’s. This does not mean they are overwhelmed by smells our noses can detect, rather, it means they can discern a molecular presence when it is in much greater dilution in the air.
Bears, such as the Silvertip Grizzly found in parts of North America, have a sense of smell seven times stronger than a dog, essential for locating food underground. Using their elongated claws, bears dig deep trenches in search of burrowing animals and nests as well as roots, bulbs, and insects. Bears can detect the scent of food from up to 18 miles away.
Fish also have a well-developed sense of smell, even though they inhabit an aquatic environment. Salmon utilize their sense of smell to identify and return to their home stream waters. Catfish use their sense of smell to identify other individual catfish and to maintain a social hierarchy.
Insects use their antennae for olfaction. Sensory neurons in the antenna generate electrical signals called spikes in response to odor. The antennae have sensory neurons in the sensilla with axons terminating in the antennal lobes where they synapse with other neurons in semidelineations called glomeruli.