Our fascination with minutiae is associated with the search for distinctions.Nowhere is this better exemplified than by the recently discovered genetics ofscent reception. Scent-related genes make up the largest family in the mammaliangenome. Visual and auditory stimuli might seem complex, but future research islikely to reveal them as abbreviated and clumsy compared with the subtleemission and reception of odors.

The canid nose is finely tuned to scent reception. However logical it seems thatair-borne compounds in the glandular secretions, urine, and feces of wolves anddogs contain important (and presumably) intraspecific signals, the currentevidence is overwhelmingly observational. Simply watching how a dog behavesafter sniffing urine or feces, for example, does not establish a causal link.Conclusions based solely on behavior are inferential until validatedphysiologically. Between signal and response must be evidence of specificchemical compounds produced by senders that subsequently bind to identifiedreceptors in neurons of receivers.

Odor and pheromone reception

Reception at a molecular level involves olfaction (the detectionof odorant molecules), possible detection of pheromones, and taste (gustatoryreception). Odorants taken in from the external environmentconsist mainly of volatile compounds. Pheromones, which areexcreted into the environment in fluids like urine or retained on the emitter’sbody (e.g. in sweat or saliva), are discrete compounds or blends of compoundsserving as chemical signals among conspecifics to elicit sexual and socialchanges in behavior and physiology. Pheromones are mainly nonvolatile andrequire direct contact with sensory cells to stimulate detection. In otherwords, pheromones are both excreted and taken up as fluids, notaerosols. Their uptake by rodents after arousal is mediated bypumping action of the vomeronasal organ (see below). Because each sensory neuronexpresses just one allele on a single receptor gene, the connection betweenemission and reception becomes fused only when the appropriate molecules bind totheir specific receptors. The result is a signal sent to the central nervoussystem culminating in a sensation of smell (odorants) or some behavioral orphysiological change (pheromones). Olfaction and taste are not easily separable,odorants and pheromones even less so.