Information Is Where You Find It

A basic problem of foraging is how to obtain information about the location and quality of food. Relevant information is obtained from immediate stimulation, from previous sensory experience stored in memory, and through the genes. Apparently, even the simplest organisms can use information from all these sources. A good example comes from infective juveniles of the plant-parasitic nematode Meloidogyne incognita, which only feed after they locate the root of a suitable host plant. Available evidence indicates that carbon dioxide from host roots attracts the juveniles but that it would also be useful for the juveniles to move to an optimal soil depth in which to search for chemical gradients. Although there are few cues to soil depth, M. incognita is able to solve this problem in a surprising way. Juveniles can orient to extremely shallow temperature gradients that are nearly always present in the environment. The juveniles also have the peculiar property of moving toward a preferred temperature that is set several degrees above the temperature to which they are acclimated. Computer modeling of this behavior demonstrates that it provides a mechanism for moving to a particular soil depth. Although average temperature is the same at all depths, the amplitude of daily temperature variation declines with depth and provides the required information, which is extracted by interactions among environmental temperature, preferred temperature, rate of acclimation, rate of locomotion, and temperature range of locomotion. Pseudoplasmodia of the slime mold Dictyostelium have similar capabilities. This behavior resembles true navigation in that the stimulus used for guidance has no connection to the goal and suggests that even simple organisms can extract useful information from surprisingly complex stimulus patterns.