Chapter 6: Optimal Foraging Theory: Constraints and Optimal foraging theory Cognitive Processes Chances are, when observing animals in the wild, you are most likely to see them foraging for food. If successful, their foraging efforts Barry Sinervo © 1997-2006 culminate in feeding. Animals search, sense, detect and feed. For humans, feeding is often associated with pleasure. Similar sensations OPTIMAL FORAGING THEORY...................................................................................may........ ..u..n..d...erl.....i.e.. ..t.h...e2 proximate drive that motivates feeding behavior of THE PREY SIZE-THRESHOLD: A DECISION RULE THAT MAXIMIZES PROFIT ........................an....i..mal......s.... ...H...o...w...ev....e6r, the ultimate reason for feeding arises from the AN OPTIMAL DECISION RULE FOR CROWS FORAGING ON CLAMS .......................................d..i..fferen..........c..e.. ..b..et....w...een.. 6 life and death. At some point in an animal’s life it SIDE BOX 6.1. THE MECHANICS OF CONSTRAINTS ON OPTIMAL FORAGING .......................may........ .ex....p...eri....en.....c..e.. 1ep0 isodes of starvation and prolonged starvation can lead A SUMMARY OF THE MODEL BUILDING PROCESS .............................................................................................. 12 to death. If animals survive and die as a function of variation in their VARIATION IN FEEDING MECHANISMS WITHIN A POPULATION ........................................................................... 13 foraging strategies then natural selection has run its course. Animals that INDIVIDUAL FORAGING CAN DETERMINED BY GENES, ENVIRONMENT, OR CULTURE ..................................................................................................................................s..u..rv....i.v...e.. .are...... ..ab....l..e1 6to contribute genes to the next generation, while the THE MARGINAL VALUE THEOREM AND OPTIMAL FORAGING ..............................................g..en.....es... ....fro.....m.... ...an... 1im8 als that die are eliminated, and along with it SIDE BOX 6.2: MARGINAL VALUE THEOREM: TRAVEL TIME AND ENERGY unsuccessful foraging behaviors. Understanding the rules that shape the GAIN.........................................................................................................................................fo....rag......i.n..g.. ...b..eh....av.....io1r9 of animals has been a central focus of behavioral CURRENCY AND LOAD SIZE IN PARENTAL STARLINGS .........................................................an....al....y..s..i.s.. .fo....r. .mo......re. 2 0th an four decades (Pyke et al. 1977). CURRENCY OF BEE COLONIES ............................................................................................................................. 22 FORAGING IN THE FACE OF A RISKY REWARD..................................................It... ..s..eems......... ...reas.......o..n. 2ab6 le to assume energy gain per unit of time might FORAGING WITH A VARIABLE REWARD AND RISK-AVERSIVE BEHAVIOR ..............................max........i.mi.....z..e.. ...t.h..e.. ...res26 ource an animal has for survival and successful FOOD LIMITATION AND RISK-AVERSE VERSUS RISK-PRONE BEHAVIOR IN reproduction. For example, the common shrew, Sorex araneus, faces SHREWS ....................................................................................................................................fo....rag......i.n..g.. ..d..eci......s.i..o.n27s that keep it only a few hours away from death. Like ADAPTIVE VALUE OF RISK-SENSITIVITY AND THE THREAT OF STARVATION.........................al...l.. .mam.........mal.......s..,. .t.h..e2 8s hrew maintains a high and constant body temperature ARE ANIMALS RISK-SENSITIVE IN NATURE? ........................................................................................................ 30 DECISION RULES AND COGNITION DURING FORAGING ................................d..u...ri..n...g.. ...act.....i.v...i.t.y... 3T3 o keep itself warm, the shrew has a very active COGNITION AND PERCEPTUAL CONSTRAINTS ON FORAGING?..............................................met.......ab....o..l..i.s..m..... ..Bec..... 3au3 se the shrew’s body is extremely small compared to RISK AVERSION AND THE COGNITIVE BUMBLE BEE .............................................................l.arg......er.... .ma......mmal..........s. 3it4s surface area is very large relative to its body mass. If SIDE BOX 6.3. MEMORY LIST LENGTHS IN BUMBLE BEES....................................................w...e.. ...co....mp......ar...e.. ...t.h..e3 7s hrew to a rat, we would see that the shrew loses THE SEARCH IMAGE ................................................................................................................co....n...s.i..d..erab........l.e.. ..h...eat. 39 to the environment. On a gram-per-gram basis the PERCEPTUAL CONSTRAINTS AND CRYPSIS ..............................................................................s..h..rew.......’s... .m....et...ab.....o..l4is0m is much higher than mammals of larger size (Peters SUMMARY: OPTIMAL FORAGING AND ADAPTATIONAL 1983; Calder 1984). Because of its high metabolism, the shrew satiates HYPOTHESES......................................................................................................................i.t..s. ..v..o...raci......o...u..s.. .ap....p4et3 ite with protein-rich insects, a high quality resource. REFERENCES FOR CHAPTER 6 ...................................................................................T...h..e.. ..s..h..r..e..w... ..mu......s.t4 6fo rage constantly, and barely has a moment to sleep because its small body size does not afford it the luxury of a thick layer of fat. The shrew has very few energy reserves onboard and only few hours without feeding (Barnard and Hurst 1987) can lead to death (Crowcroft 1957; Vogel 1976). Even if food is abundant in the environment, and the shrew does not face life-or-death foraging decisions, it must have sufficient energy to reproduce. Natural selection will favor those individuals in a population 105 that have relatively high reproductive output. Thus, survival and make decisions in the face of constraints. Temporal constraints are reproduction must also be related to the efficiency of energy acquisition couched in terms of the time it takes to find and process food. Energetic and energy storage. A reproductive female shrew has the added constraints are couched in terms of the metabolic cost of each foraging energetic outlay of nursing young. Reproductive females must maintain activity (foraging, processing, etc.) per unit time. Animals must learn a positive energy balance for themselves and acquire enough excess about the distribution of food in their environment if they are to make energy to nurse their pups with energy-rich milk. The efficiency of the the appropriate choices. How much learning is possible for an animal? Is female shrew’s foraging decisions may affect the size of her pups at there a limit to learning and memory, and do such cognitive constraints weaning. In turn, the size at weaning might impact the probability of limit the foraging efficiency of animals? their survival to maturity. Life, death, birth, and successful reproduction in the shrew are measured in terms of calories taken in on a minute-by- The first issue we must address before considering the more complex minute basis. decisions faced by economically minded, but perhaps cognitively challenged animals is the choice of currency. What are the units of Given the urgency of the “decisions” faced by shrews, the shrew may currency used by animals when conducting their day-to-day transactions not even consider every insect it encounters as a worthwhile prey item. with the environment? How do basic energy and temporal constraints Imagine that a shrew is foraging for prey. During its forays in the rich dictate the form of currency that animals use? A simple currency can be humus of the forest floor, it encounters some small, but evenly dispersed expressed in terms of the value of an item, taking into account the cost species of grub with clockwork regularity. When it encounters one grub, of acquiring the item, and the time taken to acquire the item. Natural should it eat the isolated prey? It takes some time to handle the prey and selection might shape decision rules such that animals maximize net then more time to search for another. To calculate the value of that energy gain (e.g., gross gain - costs) as a function of time: isolated prey item for the shrew we should take into account the value of Energy per prey item - Costs to acquire prey the individual prey and the time it takes to find the prey. Should the Profitability of Prey = 6.1 shrew ignore the single isolated prey item or continue searching for a Time taken to acquire prey item concentrated nest of termites that yields a much higher payoff. While the payoff from a termite nest is high, the nests are dispersed in the The Prey Size-Threshold: a Decision Rule that Maximizes Profit environment and locating them is a stroke of luck. The payoff from a large concentration of termites means the difference between making !it Prey size is one of the most conspicuous features that a predator could through the long cold night versus the sure death it faces from feeding use to discriminate prey quality. The quality of the prey expressed in on the small grubs that are evenly distributed which it encounters on a terms of energy content rises in direct proportion to mass, and regular basis. Yet these grubs