Integrating Two Views of Foraging

What bioenergetic characteristic of foraging is most important? Most ecologists will agree that the process which ultimately should be maximized is net energy gain over some meaningfully long period of time, and this is the criterion most widely employed by current optimal foraging theory (but see Pierce and Ollason 1987). The next question to ask is, what attribute of an animal's behavior should be studied to describe its foraging? Two general answers to this question have evolved, and they have resulted in what I will refer to, for purposes of discussion, as the selection and activity approaches to foraging. Behaviorists have almost without exception used the first of these, prey selection, as the attribute to quantify, analyze, and use to test optimal foraging models (Stephens and Krebs 1986). For a large group of others, however, foraging behavior is defined less by what an animal chooses to eat and more by actual patterns of locomotor and feeding activity. Strategies of foraging behavior have been identified by this second approach (such as the sit-and-wait through active continuum), which have been associated with particular physiological and morphological characteristics (Huey and Pianka 1981, Webb 1984, Feder and Lauder 1986). Because these two seemingly different approaches are studying the same phenomenon, foraging, it should be the case that there is a direct relationship between them. My aim here is to briefly comment on how these two views differ, how they may be related, and to suggest that for most carnivores an integrated approach is needed which combines both perspectives. The foraging models of the selection approach have focused on two basic problems: which prey item to consume and the related issue of when to leave a patch (Stephens and Krebs 1986). For many carnivores, however, focusing on prey choice results in an incomplete understanding of foraging behavior. Consider, for example, the foraging behaviors of a wolf and a large feline. Since both often choose deer-like prey from their environment, describing what each animal chooses does little to increase our understanding of how each is adapted to its environment. The primary difference which characterizes these two foraging behaviors is not what prey they feed upon, but rather the pattern of energy use to obtain it. Large feline behavior is typically characterized by long periods of waiting, when very little energy is expended, followed by a brief period of extremely high power output. In contrast, the wolf behavior consists of long periods in active search of prey often followed by a long chase to exhaustion. Similar comparisons come easily to mind. It is not, for example, the selection of prey which differentiates the foraging behavior of the small sit-and-wait mudminnow (Umbra limi) from the continuous filtering paddlefish (Polyodon spathula); both will feed on large zooplankton (Barbour 1951, Chilton et al. 1984). The significant difference between the behaviors of these fish (and the wolf and the feline) is the range and, in particular, the variance of the power used during foraging.