Land animals exploit the odorsphere, the world of vapors around them. In any given locale, they move in an odorscape, a landscape of volatiles. Even in fish we speak of odors because neurophysiologically the olfactory system is involved, even though water-soluble stimulants are not necessarily volatile. We expect vertebrates to have taken advantage evolutionarily of the physicochemical characteristics of their environment first to select and then to optimize chemical communication. The chemical communication system of a cold-water fish differs vastly from that of a tropical bat. Despite similar biological functions, each system has been shaped by, and is adapted to, a distinct set of environmental circumstances.

In air, temperature, relative humidity, barometric pressure, and air currents not only modulate the movement of molecules from the source but also affect odor reception once the molecules have arrived near the receptors. The evaporation of an odor from a surface such as animal skin, a scent mark, or vegetation is regulated by air temperature, relative humidity, the porosity of the surface, and other compounds present (Regnier and Goodwin, 1977; Figs. 1.1 and 1.2).

The evolution of chemical communication was probably influenced by such additional factors as adsorption of aerial pheromones to vegetation, or waterborne pheromones to suspended clay. The influence of these environmental features has very likely selected for both the choice of chemical constituents of the signals and the appropriate signal-emission behaviors (Gleeson, 1978).