THE GEOMETRY OF COMPENSATORY FEEDING IN THE LOCUST

A framework is presented for the study of feeding behaviour in the context of the multiple nutrient requirements of animals. Each nutrient required is represented by a single axis in a multi-dimensional plot. At any time the tissues will require a particular quantity and mix of nutrients. This ideal point is termed the 'nutritional target'. Meeting it involves the regulation of feeding behaviour and processing of ingested food. The ideal mix and quantity of nutrients that needs to be ingested to achieve the nutritional target is termed the 'intake target'. When fed a single food with a fixed proportion of nutrients, an animal is effectively restricted to a 'rail' in the nutritional space. Depending on this rail's slope, the intake target may not be achievable, in which case the animal should reach some optimal point of compromise on the rail. Considered together, the points of best compromise for a range of different rails will form an array on the plane, the shape and position of which, relative to the intake target, reveal the functional rule expressed by the physiological controlling mechanisms. Such concepts were investigated experimentally using locust, Locusta migratoria L., nymphs. Insects were given one of 25 artificial foods, containing one of five levels each of protein and digestible carbohydrate, and intake and growth were recorded across the fifth stadium. Two-dimensional plots of nutrient intake and growth were used to estimate the position of the intake and nutritional targets. On the basis of the array of intakes and estimates of the position of targets it was possible to test hypotheses about the functional rules employed by locusts to regulate their intake of nutrients. Both protein and carbohydrate intake were regulated equally efficiently. Locusts appeared to eat a given food such that they reached the geometrically closest point on their nutritional rail to the intake target ('closest distance optimization'). They 'jumped rails' by differentially utilizing ingested food and were thereby able to reach the growth component of the nutritional target (the 'growth target'). Only on extremely unbalanced foods did insects have a longer instar. The proximate mechanisms controlling the patterns are discussed. A simple, model animal is generated which is capable of making these sophisticated behavioural decisions without requiring a complex, central command centre. © 1993 Academic Press, Inc.