Contrast can be provided by chemical specificity of the signal (spectral contrast) and by temporal changes in concentration of the signal (dynamic temporal contrast). Spectral contrast is created by unique compounds and by unique mixtures of compounds, including ordinary ones. Temporal contrast emerges from the rate at which the concentration of a compound changes with time, including the repetition rate. Temporal changes reflect spatial patchiness and hold information for chemotactic behavior at different spatiotemporal scales. The two classical methods of camouflage, well-known in the visual signal world, may also operate in the chemical signal world, although they are virtually unstudied. To avoid detection, animals with visual predators hide and remain motionless, or they look and move like their background; animals with chemically hunting predators may build impermeable shells and store urine and feces until it is safe to release them, or they may produce metabolites that match the environment in mixture composition and temporal distribution.
Unlike wave or wave-like propagation of acoustic, visual, and other electromagnetic signals, chemical signals disperse through the environment by molecular diffusion and bulk flow. At small spatial scales—in practice below 10 µm—diffusion is a biologically useful transport mechanism and, given the constraints of viscous fluid boundary layers, often the only effective mechanism. At larger scales, flow is necessary to obtain metabolic energy (e.g., oxygen, food particles), to eliminate wastes (e.g., carbon dioxide, urine), and to send and receive chemical signals. The constraints of metabolism and sensory information are probably different, so that we could expect animals to generate separate metabolic currents and information currents. In practice, they may use the same current-generating mechanisms and then control the currents and the chemical composition of these currents to serve different functions at different times. Controlling the timing, velocity, and direction of information currents is important whether they are used to send or receive chemical signals. Animal-generated currents can be laminar at small scales (<1 cm) or turbulent at larger scales. Both include the possibility of temporal information. In this paper I will focus on temporal information in marine chemical signals and on the use of urine dispersal in chemical communication.
The marine environment is filled with sources of chemical signals in a wide range of overlapping spatial scales (1), from the metabolites of a single marine bacterium (diameter, <10-6 m) to the odor plumes left behind a traveling school of tuna (school size, >102 m) or emanating from a whale carcass (plume size, >103 m). Constrained by physics, chemistry, and biology, chemical signals have a finite lifetime. When released into the environment, they disappear below detectable levels as a result of turbulent mixing, molecular diffusion, adsorption, photolysis, and chemical transformation and through uptake and breakdown by bacteria,