Behavioral Indicators and Behavioral Titrations

Building community models in which species interactions emerge from the foraging decisions of individuals requires an understanding of how behavior influences fitness. Testing such models requires methods that lead animals to reveal aspects of their fitness through their behavior. Such methods are based on the costs and benefits of foraging when the forager experiences diminishing returns.

For example, Kotler and Blaustein (1995) examined microhabitat selection and patch use in the gerbils of the prologue, Allenby's gerbil and the greater sand gerbil (Gerbillus andersoni allenbyi and G. pyramidum, respectively). They asked how much richer open and dangerous microhabitats had to be for gerbils to value them equally with safer microhabitats under bushes. Kotler and Blaustein conducted their experiment in a large aviary where gerbils could forage on artificial patches (trays filled with seeds mixed into sand) placed in bush and open microhabitats. The gerbils experience diminishing returns while foraging in these trays, so the density of seeds left in a tray after anight of foraging, thegiving-up density (GUD; Brown 1988; see box 13.2), reflects the forager's harvest rate when it leaves the patch. A forager exploits the patch until the harvest rate falls to a value equal to the cost of foraging (see chap. 13). A higher giving-up density signifies higher costs.

The experiment used barn owls (Tyto alba) to manipulate the danger level. In response to the owls' presence, the gerbils showed higher giving-up densities in the open than under bushes, revealing that owls pose a greater threat in the open. Then Kotler and Blaustein added seeds to the open trays until the gerbils were harvesting the same amount of seed from open and bush trays. G.pyramidum needed 4 times and G. a. allenbyi needed 8 times as much initial seed in the open trays to make the open microhabitats of equal value to the bush microhabitats (fig. 12.1).

A similar experiment studied guppies (Poecilia reticulata) foraging in the presence of predaceous cichlids (Cichlasoma sp.) and gouramids (Trichogaster leeri) (Abrahams and Dill 1989). The study was based on the idea that foragers should distribute themselves according to an ideal free distribution (see box 10.1). The experiment offered guppies a choice between two patches differing in danger (one side of the aquarium contained a predator). Most guppies avoided the dangerous side in favor of the safe side, leaving those fish willing to take the risk with higher feeding rates. The resource supply rate in the dangerous habitat was then increased to the level required to equalize the number ofguppies on each side.

We call studies like these "behavioral titrations" (Kotler and Blaustein 1995). Foraging theory tells us that a forager should perform an activity (feeding, hiding) so long as the marginal benefit it derives from this activity exceeds its marginal cost. A forager should continue with the activity until the marginal benefit falls to equal the marginal cost. When choosing which activities to perform, a forager should allocate more time to activities with

Figure 12.1. Behavioral titration. Total amounts of seed harvested from bush versus open microhabitats for (A) Gerbillus andersoni allenbyi and (B) G. pyramidum. Resource trays in the bush microhabitat contained a constant amount of seed from night to night, but trays in the open microhabitat varied. Bars of equal height for bush and open habitats indicate that gerbils place the same value on the two microhabitats. (After Kotler and Blaustein 1995.)

Figure 12.1. Behavioral titration. Total amounts of seed harvested from bush versus open microhabitats for (A) Gerbillus andersoni allenbyi and (B) G. pyramidum. Resource trays in the bush microhabitat contained a constant amount of seed from night to night, but trays in the open microhabitat varied. Bars of equal height for bush and open habitats indicate that gerbils place the same value on the two microhabitats. (After Kotler and Blaustein 1995.)

higher net marginal values and reduce time spent on activities with lower net marginal values. Hence, a forager's optimal allocation of time among activities should equilibrate the marginal values of the activities. Behavioral titration experiments provide a window into this equilibration. Researchers can take advantage of the animal's natural tendency to perform fitness titrations by conducting titrations of their own involving total value, total effort, and so on. In titration experiments, we use a quantifiable dimension of quality, such as food abundance, to measure the fitness value of another, more difficult to quantify dimension, such as predation risk. Titrations carried out in this manner form the basis for behavioral indicators that reveal a forager's perception of costs and benefits. Titrations can be used to test models of species interactions that involve foraging behaviors.

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