An Ecological Theory on Foraging Time and Energetics and Choice of Optimal Food-Searching Method

Abstract

In the past few years there has been much theoretical work on optimal foraging behaviour in animals. It relates to the assumption that animals tend to optimize their feeding activities. Reviews appear in Schoener (1971), MacArthur (1972), Emlen (1973) and Pianka (1974). Interest has been paid particularly to optimal choice of diet for predators with a choice among different prey types (Charnov 1976a; Emlen 1966, 1968, 1975; MacArthur 1972; MacArthur & Pianka 1966; Marten 1973; Pulliam 1974; Rapport 1971; Schoener 1969a, 1969b, 1971, 1974), to strategies of movement and to exploitation of different habitat patches (Charnov 1976b; Emlen 1973; Krebs, Ryan & Charnov 1974; MacArthur & Pianka 1966; Royama 1970; Schoener 1974; Smith 1974a, b; Smith & Dawkins 1971; Tullock 1970). Interest has also been focused on relations between various features of morphology, energetics of foraging, and feeding strategies (e.g. Feinsinger & Chaplin 1975; Gill & Wolf 1975; Hainsworth & Wolf 1972a, b, 1975; Heinrich & Raven 1972; Wolf, Hainsworth & Stiles 1972; Wolf, Stiles & Hainsworth 1976). Various components of the time expenditure for foraging were considered by Holling (1965, 1968) in constructing models on functional response of predators to density of their prey (see also the recent reviews by Hassell, Lawton & Beddington (1976) and Beddington, Hassell & Lawton (1976)). The present approach differs from those of earlier papers in that particular emphasis is laid here on the energy cost of locomotion during foraging and its effect on the foraging time. One possible goal is to construct a general mathematical model of foraging time and energetics. The moment an animal starts to move about in search of food to cover its energy needs, it suffers an extra energy drainage that must also be replaced. Thus, for example, the energy cost of locomotion for foraging of some birds may readily come up to 5-10 times the basal metabolic rate. Luckily, the energetic cost of foraging is usually easily outweighed by the energy gains. However, this is not always necessarily so. When food density decreases, an animal has to spend progressively more time searching for food, and it may at times have difficulty finding enough time to cover its energy budget. If it were to continue with the same foraging technique it would eventually find it difficult even to make foraging gains catch up with energy losses due to foraging activities, let alone provide also the energy required for non-foraging periods. In this paper, I want to assess the consequences to an animal of the energy and time expenditures for foraging. Further, I will explore whether or not an animal may gain a selective advantage by employing different search methods in different seasons in a fluctuating environment. More precisely, I look for answers to the following main questions.