where the first term on the right-hand side represents the benefits ofbully-ing (a higher share in the collective harvest) and the second term represents the cost ofbullying (less collective harvest to bully for).
To find a candidate ESS solution, we set each individual's strategy equal (v = u), and then set the expression equal to 0:
= 0, which implies that u* = 0.5. With further evaluation, it can be shown that this candidate solution is an ESS. At u* = 0.5, neither forager gains from unilaterally changing its strategy (satisfying conditions for a Nash or v=u
(Box 1.4 continued)
equilibrium), and if a player has a strategy slightly away from 0.5, it will benefit from a unilateral change back toward 0.5 (this phenomenon is referred to as convergent stability; Cohen et al. 1999).
At this ESS, each forager splits its effort between procuring resources and haggling over its share of the collective harvest. Note that both foragers would be better off if they could agree to shift their strategies to values less than 0.5. In fact, a strategy of u = 0 would maximize collective gain. However, this situation would not be stable, because both players would be tempted to shift some of their effort from harvesting to bullying. Besides mimicking aspects ofa producer-scrounger game, this game also illustrates what happens when individuals can contribute to a public good (in this case by harvesting resources) but pay a private cost (inability to haggle over one's share of the harvested resources).
optimal—turned out not to be a fundamental flaw, but simply a misinterpretation of what an optimality model means. The limitations and shortfalls of the basic models have been recognized and left behind, and students of foraging have developed new ideas and techniques to conquer problems that seemed very thorny in 1986. The field has matured and expanded beyond the set of topics Stephens and Krebs considered in 1986. To paraphrase Mark Twain, reports of the death of foraging theory have been greatly exaggerated!
In the remainder of this section we give an overview of this book, placing the successive chapters in perspective. Part 1 (chapters 2, 3, and 4) deals with information, neuroethology, and cognition. Animals respond to their environment at the speed of neural transmission. Quick, coordinated movement is a hallmark of animals, and of course, animals come equipped with senses and the neural machinery that connects these senses to muscular output, with often amazing specializations and elaborations. This part ofthe book explores the connection between foraging and the information processing systems of animals at several levels.
In chapter 2, David Stephens considers the economics of information use. Starting with first principles, he asks what kinds of information should be important to a foraging animal and what constrains animal information-collecting abilities. The first model in this chapter develops the link between movement (or action) and the value of information. The model shows that the potential to direct actions is fundamentally what makes information valuable. A complication arises, however, because the world is often an ambiguous place, in which the relation between stimulus and information is not clear-cut. The theory of signal detection illustrates the interplay between economics and constraint in animal information gathering. Students interested in how sensory and neural systems can contribute to efficient foraging will want to pay close attention to this chapter.
In chapter 3, David Sherry and John Mitchell provide a gentle introduction to the "wetware" that underlies the mechanisms for the information-gathering tasks outlined in chapter 2, especially the classic psychological phenomena oflearning and memory (which are, ofcourse, fundamentally information processing phenomena). The chapter outlines thebasic properties of a simple neural system involved in foraging (the antennal lobes and mushroom bodies of the honeybee brain) and explains important new discoveries about the cellular and molecular basis of learning. Food caching and recovery (see chap. 7 in this volume) is a foraging phenomenon that has become an important model system in the neuroethology ofmemory. The chapter uses this system to introduce basic ideas about memory, including types ofmemories and current thinking about the neural structures that form and store these memories.
In chapter 4, Melissa Adams-Hunt and LuciaJacobs address a "higher" level ofmechanistic thinking, reviewing the cognitive phenomena involved in foraging. Readers without a background in this area will be surprised at their number and complexity. Even the apparently simple act ofperceiving a potential food item involves cognitive concepts unfamiliar to most behavioral ecolo-gists: sensory transduction, attention, categorization, generalization, search image, and so on. In addition to exploring perception, the chapter outlines basic ideas about memory, learning, and spatial orientation.
In the early days of foraging theory and behavioral ecology, a wall separated ultimate (or evolutionary) explanations from proximate (or mechanistic) explanations. Strong proponents of this separation held that these two approaches were different levels of analysis, each of which could be successfully pursued without knowledge of the other. But agrowing number ofbehavioral ecologists, neuroethologists, and psychologists are taking down this wall. While many questions can be asked and answered satisfactorily at one level of analysis or the other, a more complete understanding results when we combine levels ofanalysis. This part ofthe book challenges the reader to ask how mechanistic and evolutionary thinking can be profitability combined, perhaps producing an entirely new perspective on foraging behavior.
When a snake strikes and kills a kangaroo rat—a common event in desert landscapes (see chaps. 12 and 13 in this volume)—much ofthe action ofclassic foraging models ends, but in fact the snake'sjob hasjust begun. The kill begins an elaborate and time-consuming process ofconsumption and processing. The snake, as many will know, must manage to swallow its prey whole and uses several special features ofits jaw and musculature to accomplish this. Once the kangaroo rat enters the snake's digestive system, an impressive physiological up-regulation begins. The snake is not a frequent eater, and most of the time its digestive machinery is quiescent, waiting to be turned on only when it is needed. Like any other forager, the snake can use the energy and nutrients it acquires from a meal in any of several ways: it can store energy as fat or use it immediately for maintenance or reproduction. None of these "post-kill" phenomena are well integrated with conventional foraging theory, yet they are surely important to any complete understanding of foraging. Part 2 of this book (chapters 5, 6, and 7) deals with three themes that begin where conventional foraging models end.
In chapter 5, Chris Whelan and Ken Schmidt review issues of food acquisition, processing, and digestion. The chapter explains chemical reactor models of digestion and reviews evidence for the adaptive control of digestive processes. Foraging theorists have viewed digestion as ablack box with fixed properties (i.e., so much in yields so much out), but the gut is an active partner with foraging behavior. It may be adjusted seasonally, daily, or even with the mix of foods in a particular meal. Of course, this fact has consequences for the foraging models and for ecological interactions, as Whelan and Schmidt explain.
In chapter 6, Jonathan Newman presents a novel synthesis of herbivory that views the foraging problems of elephants and grasshoppers as essentially similar. Newman focuses on four issues—where to eat, what to eat, how fast to eat, and how long to eat—to highlight the special problems of herbivores. He outlines theoretical and empirical progress in studies ofthe effects ofcom-plementary nutrients, a topic that received little more than a hand-waving style comment in Stephens and Krebs (1986). In addition, the chapter reviews new models ofthe encounter process that have emerged from considering the special problems of mammalian herbivores.
Conditions can always worsen, and when they do, an animal must either reduce its energy demands (e.g., hibernate) or rely on food stored as fat or cached in the environment. Anders Brodin and Colin Clark consider this active area of modeling and empiricism in chapter 7. As they explain, a dynamic approach is essential because one is fundamentally concerned with how food collected now will affect fitness in the future. In addition, hoarded food and food stored as fat are not the same: each option has advantages and disadvantages. Fat is readily available, but may limit mobility and make the fat individual more susceptible to predation. A hoarding animal can cache large quantities of food, but cached food is susceptible to spoiling, pilfering, and retrieval costs.
Each of these themes represents an area that was in a relatively primitive state of knowledge in 1986. Now each is a growth industry in its own right.
Seven third-party text boxes convey the level of activity in this area. In chapter 5, Fred Provenza gives his perspective on how learning and taste affect the behavior of herbivores, which may in turn influence plant diversity. In chapter 7, Alasdair Houston and John McNamara provide an authoritative essay on the strengths and weaknesses of current modeling trends, while Stephen Woods and Thomas Castonguay introduce us to the neuroendocrine pathways of meal regulation.
Part 3 of this book (chapters 8, 9, and 10) offers three growing points in the direct analysis of foraging behavior. In chapter 8, Ron Ydenberg considers provisioning, the delivery of food and materials to other places (e.g., nest sites) or individuals (e.g., young). Provisioning theory derives directly from central place foraging models, but incorporates some essential differences. In particular, the theory recognizes that provisioners deliver food, but do not consume it themselves, at least not immediately. They must power this delivery with other food consumed in "self-feeding," and models must avoid mixing the energetic accounts of those who pay for the delivery (the provisioner) with those of the recipients (e.g., young in the nest). The chapter uses this approach to ask questions about the evolution of metabolic capacity and the rate of work and about the effects of demand on foraging strategies.
Chapter 9 investigates the effects of predation danger on foraging behavior. That foraging exposes an animal to predators is not a new idea; protective cover has long been a central concept in wildlife science, for example. However, the idea that foragers mitigate the danger with modifications to their foraging strategies had just begun to take hold in 1986. The idea that these modifications must be traded offagainst the foraging rate was developed once dynamic state variable models became available to help with the analysis of such trade-offs, and the implications of this notion form one of the major recent trends in foraging research. Peter Bednekofftraces these developments and surveys the current state of affairs in this chapter. Currently, the range and impact ofthese behavioral modifications continues to expand as ecologists actively investigate their effects on populations and communities.
We noted above that game theory played a limited role in early foraging research. This is mysterious, since the essential theoretical apparatus was in active use in other areas of behavioral ecology, and since topics that we now know require its use (e.g., information centers) were being actively researched. This situation has changed greatly in recent years through the integration of established paradigms, such as stable group size and the Prisoner's Dilemma, into foraging contexts. Tom Waite and Kristin Field review the current state of this area of research in chapter 10. They point out that the conditions for foragers to be considered "social"—whenever the payoff of their foraging strategy depends on the strategies used by other foragers—are probably quite general, even in cases in which foragers never interact directly. Thus, game theory is essential to the analysis of a very broad array of foraging situations.
When the young Charles Elton visited Spitzbergen in the early 1900s, his observations of who ate whom led him to imagine a community organized in terms of what he called a food chain. This early insight into the role of foraging in the organization of communities foretells the developments outlined in part 4 ofthis book (chapters 11, 12, 13, and 14), in which we consider foraging and its relation to ecological communities. An organism needs energy and materials to reproduce, grow, and stave off death. In turn, these processes influence the distribution and abundance of species (chapter 11). Population interactions and species composition and diversity form the core of community ecology. Foraging shapes the intensity, quality, and form of community interactions and consequent opportunities for species coexistence and diversity (chapter 12). Foraging often is a form ofpredation in which foragers exert mortality on their prey, compete for resources with one another, and provide opportunities for their own predators (chapter 13). Foraging behaviors, by shaping the experiences oforganisms, directly determine many of the environmental circumstances that shape the evolution and coadaptation of other, less plastic physiological and morphological traits. This, then, is the domain of foraging ecology. Foraging ecology considers the population, community, and evolutionary consequences of animals' feeding behaviors.
Feeding behaviors are central to ecological and evolutionary feedbacks between an individual and its environment. As such, they can offer behavioral indicators of a species' prospects. In chapter 14, Mike Rosenzweig considers feeding behaviors as valuable indicators for conservation. We often use population size as an indicator of conservation status, but the population itself is often the valued ecological component under protection. Unfortunately, changes in population size become a trailing indicator. By the time one notes a substantial decline in population size, it has already happened. On the other hand, flexible feeding behaviors of organisms should indicate the animals' current prospects and their perceptions of prospects in the future. A shift of feeding behavior in response to changes in the environment should provide a leading indicator of changes in population size. Furthermore, the behaviors themselves may be a valued component of the ecosystem. For example, responsiveness to predators is an integral part of being an elk, and loss of such antipredator behaviors results in significant changes in the elk's feeding behaviors and ecology (Laundre et al. 2001). Chapter 14 goes beyond the utility offoraging behavior for conservation. It proposes using the principles of foraging theory to understand human perspectives and goals. In this light, foraging theory provides a framework for incorporating human resource acquisition activities and their ecological consequences.
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Foraging and Information Processing
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