Missed Opportunity Costs Research Articles

Size-Specific Differences in Tail Loss and Escape Behavior in Liolaemus nigromaculatus

Facultative tail loss (autotomy) is a common and widespread antipredation adaptation in lizards (Arnold, 1988; Zani, 1996). Although offering an escape from predators, tail loss has been demonstrated to influence many life-history traits and to have fitnessrelated ecological and behavioral correlates (e.g., Martin and Avery, 1998; Wilson and Booth, 1998; Martin and Lopez, 2000, and references therein). Animals with autotomized tails also may have reduced abilities to escape predators (Dial and Fitzpatrick, 1984). Costs and benefits of tail autotomy have been discussed for many taxa, but these studies encompass only a small percentage of species employing this tactic, and further studies are critical to evaluate the generality of these observations. Theoretical models of escape behavior predict that animals will adjust the distance at which they flee from an approaching predator (fleeing distance) according to the relative costs of staying put versus the costs of fleeing (Ydenberg and Dill, 1986). When the former cost exceeds the latter, over any distance, the animal should seek refuge. A number of factors are known to influence the decision to stay put, including both intrinsic (e.g., age) and extrinsic factors such as predator characteristics, ambient temperature, distance and quality of available refuges, etc. (e.g., Rocha and Bergallo, 1990; Martin and L6pez, 1995, 2000; Cooper, 1997). Costs associated with fleeing are predominantly missed opportunity costs associated with abandoning a food resource, but for ectotherms may also include metabolic costs such as reduced time spent basking in the sun. Although several authors have evaluated the role of tail autotomy on escape behavior (e.g., Vitt and Cooper, 1986; Formanowicz et al.,

PATCH USE UNDER PREDATION HAZARD: EFFECT OF THE RED IMPORTED FIRE ANT ON DEER MOUSE FORAGING BEHAVIOR

We examined patch use of deer mice foraging in the presence vs. the absence of red imported fire ants in a laboratory experiment. We tested the H = P + C + MOC patch use model (H, harvest rate; P, predation risk; C, metabolic costs; MOC, missed opportunity costs), an extension of the marginal-value theorem, by comparing in-patch harvest rates and giving-up densities (GUDs) in the presence and the absence of fire ants: The model predicts that foragers should seek patches that yield higher feeding rates in the presence of increased costs. GUDs also should increase, since they should correlate directly with quitting-harvest rates, as long as the forager experiences diminishing returns on its rate of resource harvest. We identified behavioral trade-offs made by deer mice when foraging in the presence of fire ants. In the presence of fire ants, mice harvested a greater proportion of seeds from, spent more time in, and made more visits to rich patches than to poor, whereas in the absence of fire ants, these variables did not differ between rich and poor patches. Mice also increased their in-patch harvest rate in the presence of fire ants, as predicted by the H = P + C + MOC model, but harvested patches to a lower GUD. Mice left patches at lower final seed densities, but were able to maintain a higher mean in-patch harvest rate, suggesting that mice foraged more efficiently in the presence of fire ants. When fire ants were present, deer mice spent more time on out-of-patch handling (fire ants were confined to patches). Despite increased costs or lost time due to taking seeds away from the ants for consumption, mice were able to harvest the same number of seeds in the same total time, at the same mean harvest rate. By biasing effort toward more energetically profitable rich patches and by increasing their in-patch harvest rate, deer mice were able to obtain the same net yield in the same time, despite increased costs.

Applying Patch Use to Assess Aspects of Foraging Behavior in Nubian Ibex

Consideration of optimal patch use by animals that may face danger from predators and that allocate time among several activities is an extension of the marginal value theorem (Brown 1988). According to the theorem, an animal should leave a food patch when its harvest rate of food equals the sum of its energetic and predation costs arising from foraging and its missed opportunity costs arising from foregoing other activities. A technique derived from this theory involves presenting foragers with depletable food patches and measuring food density remaining after foraging. The food density remaining is a measurement of an animal's foraging efficiency and can be used to measure various foraging inputs

Foraging Under Predation - a Comparison of Energetic and Predation Costs in Rodent Communities of the Negev and Sonoran Deserts

We used patch-use theory, giving-up densities in experimental food patches, and harvest-rate measurements within these patches to determine the relative contributions of predation risk and energy to foraging costs in four species of rodents from communities in the Sonoran and Negev deserts. To partition costs into components of energy and predation, we converted field measurements of giving-up densities into harvest rates (J min(-1)), used these harvest rates as an estimate of total foraging costs, estimated energetic foraging costs from published physiological measurements of activity and thermoregulatory costs, and assumed that missed opportunity costs were either zero or negative. Our results showed that predation costs predominate. Energetic costs represented only 24%, 19%, 16% and 13% of the foraging costs for Merriam's kangaroo rat (Dipodornys merriami; Sonoran), the round-tailed ground squirrel (Spermophilus tereticaudus; Sonoran), the greater Egyptian sand gerbil (Gerbillus pyramidum; Negev), and Allenby's gerbil (G. allenbyi; Negev), respectively. Equally important were predation-risk differences between bush and open microhabitats; the microhabitat differences in predation cost were often 2-4 times larger than the animals' energetic costs. Seasonal patterns in foraging costs also were predominantly influenced by predation rather than energetic costs. Predation costs appear to be greater in the Negev Desert, but rodents of the Sonoran desert experience greater seasonal and microhabitat variability in predation costs. As a result, predation risk may contribute more towards species coexistence in the community of the Sonoran Desert than that of the Negev Desert.

The Effect of Competition on Foraging Activity in Desert Rodents: Theory and Experiments

We studied, both theoretically and empirically, the effect of intra— and interspecific competition on the foraging effort of individuals. We considered two models, one for a time—minimizer satisfying an energy requirement, the other for an animal maximizing fitness as a function of multiple inputs subject to a time constraint. The goal of satisfying an energy constraint predicts that foraging effort should increase with increased competition. The goal of maximizing fitness subject to a time constraint on multiple inputs may also predict that foraging effort should increase with increased competition because of the missed opportunity cost that results when different inputs are complementary. However, if the fitness—maximizer with multiple inputs incurs an energy cost of foraging (in addition to missed opportunity costs), then it should often reduce foraging effort in response to an increase in competition. We experimentally tested the foraging response to increased competition by two species of gerbils, Gerbillus allenbyi and G. pyramidum, over a range of manipulated population densities in field enclosures located in the Negev Desert of Israel. Our results support the cost—benefit model when the additional energy cost of foraging is important. Per capita activity (as measured by spoor) declined as a function of intraspecific density for each species and as a function of interspecific density for G. allenbyi. We detected no interspecific effect, however, on G. pyramidum.

Density-Dependent Harvest Rates by Optimal Foragers

We develop and test a theory of an optimal functional response, a function that maps resource density into the number of resources harvested over a period of time which includes foraging, dormancy, and alternative (non-foraging) activities. We characterize fitness as a function of a vector of inputs which may be complementary. The inputs depend upon the allocation of time among activities. We use the technique of Lagrange multipliers to characterize the allocation of time at a constrained optimum. The time constraint causes a foraging animal to incur a missed opportunity cost, which, counter to intuition, implies that the marginal cost, as well as the marginal benefit, of foraging time may increase with resource density. Consequently, the relationship between the proportion of resource consumed and resource density is indeterminate unless further assumptions are made. We consider two assumptions which ensure that, over a range of resource densities, the optimal functional response will be stabilizing in the sense that the per capita mortality rate of the resource increases with resource density. These assumptions are: (1) there is a variable energetic cost of foraging, and (2) resource depletion occurs within the foraging period. We experimentally tested predictions of the theory by offering to free-living desert rodents (Dipodomys merriama) millet seeds which had been mixed with dirt and placed in aluminum trays. The results support the theory's predictions. The proportion of seeds eaten increased with seed density at relatively low seed densities. The rodents' optimal functional response indicated a threshold density of seeds below which foraging time was unprofitable. Above this threshold, the slope of the optimal functional response decreased from an initial value of one, indicating that missed opportunity costs increased with an increase in initial seed density.

Patch use as an indicator of habitat preference, predation risk, and competition

A technique for using patch giving up densities to investigate habitat preferences, predation risk, and interspecific competitive relationships is theoretically analyzed and empirically investigated. Giving up densities, the density of resources within a patch at which an individual ceases foraging, provide considerably more information than simply the amount of resources harvested. The giving up density of a forager, which is behaving optimally, should correspond to a harvest rate that just balances the metabolic costs of foraging, the predation cost of foraging, and the missed opportunity cost of not engaging in alternative activities. In addition, changes in giving up densities in response to climatic factors, predation risk, and missed opportunities can be used to test the model and to examine the consistency of the foragers' behavior. The technique was applied to a community of four Arizonan granivorous rodents (Perognathus amplus, Dipodomys merriami, Ammospermophilus harrisii, and Spermophilus tereticaudus). Aluminum trays filled with 3 grams of millet seeds mixed into 3 liters of sifted soil provided resource patches. The seeds remaining following a night or day of foraging were used to determine the giving up density, and footprints in the sifted sand indicated the identity of the forager. Giving up densities consistently differed in response to forager species, microhabitat (bush versus open), data, and station. The data also provide useful information regarding the relative foraging efficiencies and microhabitat preferences of the coexisting rodent species.