ABSTRACT
Olfaction, the ability to detect and identify a very large panel of molecules, mainly of small molecular size, either dissolved in water for the aquatic species or carried in air for terrestrial animals, has been developed to a great degree of sophistication during evolution. Thus, olfaction, as one of the five main senses, is essential for all animals but particularly for those living in the wild to survive. It allows them to find food and escape predators and dangers, but also to look for sexual partners and to reproduce. This sense is obviously not as important for domesticated animals, which usually do not have to care about finding food, protect themselves against predators, or choose the most appealing sexual partners. In that respect, it may be wondered whether this function has evolved differently or as successfully in the domesticated descendants of a species compared to its wild ancestors. Though perhaps in many cases, there has been a decline in the olfactory acuity of domesticated groups, dogs are probably an exception. Many breeds, from early in domestication, have been selected for hunting purposes, for which olfaction is of primary importance. Olfaction as a whole depends on several aspects:
1. Perception, the ability to detect the presence of a volatile compound in the air or water 2. Discrimination, the ability to distinguish different signals in a chemically complex
environment 3. Identification, the ability to recognize a signal and compare it with previously memorized
information
The first step of olfaction occurs in the nasal cavity. There, the odorant molecules are captured by specialized receptors expressed by specialized olfactory neurons. The role of these neurons is dual. Each of them must bind specifically to a small set of defined compounds, and through a complex downstream pathway (Figure 4.1), they must convert a chemical signal into an electrical signal sent to several brain areas. Ultimately, this last signal is converted into information that will be stored for further comparison. In the dog, the study of the genetics of olfaction has been mainly focused on the study of the olfactory receptor (OR) gene repertoire.
