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

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

Odor and pheromone reception

Reception at a molecular level involves olfaction (the detection of odorant molecules), possible detection of pheromones, and taste (gustatory reception). Odorants taken in from the external environment consist mainly of volatile compounds. Pheromones, which are excreted into the environment in fluids like urine or retained on the emitter’s body (e.g. in sweat or saliva), are discrete compounds or blends of compounds serving as chemical signals among conspecifics to elicit sexual and social changes in behavior and physiology. Pheromones are mainly nonvolatile and require direct contact with sensory cells to stimulate detection. In other words, pheromones are both excreted and taken up as fluids, not aerosols. Their uptake by rodents after arousal is mediated by pumping action of the vomeronasal organ (see below). Because each sensory neuron expresses just one allele on a single receptor gene, the connection between emission and reception becomes fused only when the appropriate molecules bind to their specific receptors. The result is a signal sent to the central nervous system culminating in a sensation of smell (odorants) or some behavioral or physiological change (pheromones). Olfaction and taste are not easily separable, odorants and pheromones even less so.