Long and Short Term Regulation of Food Intake in the American Cockroach, Periplaneta americana (L.) (Orthoptera: Blattidae)

Abstract

Cockroaches apparently select balanced diets using simple. but robust rules (Geissler & Rollo 1987). These rules are general enough to deal with a complex habitat yet sufficiently specific to contribute a balanced diet, thus leading to the successful maintenance of fitness. Both internal and external stimuli are integrated to elicit the appropriate responses. Feedback information concerning nutritional status may be short-term (e.g. immediate physiological state or sensory input) or long-term (e.g. storage reserves. hormones or learning) so that both coarse and fine-tuning mechanisms are involved in the choice of nutritious and non-toxic foods. The insects' ability to evaluate palatability (scent and taste cues) and nutritional value are essential aspects of the decision-making process. This study investigated the general mechanisms governing changes in preferences with respect to tradeoffs between short and long-term stimuli. On a short-term scale, animals should prefer foods that present immediate positive stimuli according to their innate disposition. However, if a food does not meet long-term physiological needs. the animal should develop an aversion and learning may override a decision based upon a short-term stimulus, or alter innate preferences entirely. The experimental protocol involved measuring amounts and time spent eating. Where appropriate, these values were related to female reproductive productivity to show the link between nutrition and reproductive capacity (fitness). Investigations demonstrated the ability of cockroaches to use coarse controls to compensate for long term general starvation as well as specifically depleted nutrients such as carbohydrates and protein. Cockroaches also showed discrimination between the quality of carbohydrate diets. The ranking of palatability among foods was not a fixed attribute but varied relative to the items available. With large nutritional imbalances that required time to correct, dietary management was less discriminatory with respect to food quality and was largely concerned with increased quantity. Relatively well nourished animals showed much greater selectivity with regard to quality. Thus, the coarse and fine-tuned control systems stressed different strategic tactics. Reproductive success was linked to responses to malnourishment. Reproductive output was increased even over control levels when animals compensated for energy depletion by switching to alternate resources. This contradicted the prediction of maximization of reproductive output (fitness) by maximization of energy intake from optimality theory. The results were interpreted with respect to optimal foraging theory, the currently accepted paradigm for ecological feeding relations. The results not only conflicted with the predictions of optimal foraging theory in most respects, they suggest a completely different paradigm that is incompatible with a hypothesis of maximization of net energy gain. Instead. cockroaches appear to regulate feeding with respect to evolutionarily determined set points related to innate growth and production rates. Moreover, the criteria for decisions are a multiplicity of nutritional requirements, not simply energy, as proposed by optimal foraging theory. Some of the decisions made with respect to nutritional balancing result from direct response to external or internal stimuli, and others are a learned association between long-term benefits and correlated sensory cues.