There is no consensus among researchers regarding what qualifies a substance as a pheromone. While most agree on a basic definition of pheromones as chemicals released by one individual of a species which, when detected by another individual of the same species, elicit a specific behavioral or physiological response, some researchers also specify that the response to pheromones must be unconscious. In addition, the distinction between pheromones and odorants - chemicals that are consciously detected as odors-can be blurry, and some researchers classify pheromones as a type of odorant. Evidence that pheromone responses may not involve conscious odor perception comes from the finding that in many species, pheromones are processed by the vomeronasal (or accessory olfactory) system, which uses a special structure in the nose, the vomeronasal organ (VNO), to receive chemical signals. The neural connections between the VNO and the brain are separate from those of the main olfactory system, whose processing of odorants triggers sensations of smell. But while the VNO does process many animal pheromone signals, not all animal pheromones work through the VNO. Conversely, not all chemical signals transmitted via the VNO quality as pheromones. For example, garter snakes detect a chemical signal from earthworms-one of their favorite foods-via the VNO, and they use this signal to track their prey.

It can be inferred from the passage that in classifying pheromones as a type of odorant, the researchers referred to in the highlighted text posit that

pheromones are perceived consciously

most pheromones are processed by the VNO

most chemical signals processed by the VNO are pheromones

pheromone perception does not occur exclusively between members of the same species

pheromones do not always elicit a specific behavioral or physiological response


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