Topic 9: Foraging Behavior – Study GuideOptimal Foraging Theory (OFT) sounds like a unified concept, but it actually is a series of models that explain and predict different behaviors of animals in the context of feeding. OFT is extremely important in animal behavior, particularly Behavioral Ecology, since the acquisition and ingestion of food is something that ALL animals have to perform since they are heterotrophic.Optimality models in general have 3 parts: Decisions, Currency, and Constraints.4 optimal foraging models were presented that explain and predict 4 different questions regarding feeding. Animals can be content to eat the same foods at each meal.“Prey” used in optimal foraging models can mean any food type, not just another animals consumed by carnivores.1. What food items should the forager eat?The Diet Model: In its simplest form it asks which of 2 prey items should an animal choose to eat.Variables include: encounter rate, handling time, and energy valueProfitability = Energy value (benefit) / Handing time (cost). Note that “Profitability” is an intrinsic characteristic of a piece of food and does NOT include encounter rate.This particular model assumes:1. Foragers will always try to maximize their rate of energy intake.2. Prey are encountered one at a time and in proportion to their abundance. (Foragers cannot simultaneously handle one item and search for another.)3. All food items in the environment can be ranked according to their profitability. (We can measure our prey types in some standard currency, such as calories).THE MAJOR CONCLUSION OF THIS MODEL IS:How often a predator encounters the less profitable item does NOT affect whether that item should be added to the diet or not. Rather, a critical encounter rate withthe most profitable item is calculated. If that rate is high enough, ONLY prey item 1 will be taken, if not, then prey 1 AND 2 are taken.Empirically testing with Great Tits showed that the predictions of the model were not perfect, but confirmed that the birds would select only large meal worms if the encounter rate was high enough.2. How is foraging affected when certain specific nutrients are required?Linear Programming Model: Designed to test certain optimality models, but with particular constraints.Most animals, especially herbivores, have specific nutrient needs such as salt or sodium or other vitamins and minerals which can alter the predictions of theDiet Model. Foraging decisions are often not just a matter of “maximizing profitability intake.”Moose requirements for sodium is a particularly good example of deviating from the predictions of the Diet Model to attain the needed specific nutrient. Sodium is a good candidate for a constraint study because vertebrates require large amounts, sodium is scarce in the natural environment, the hypothalamus produces a “specific hunger” for sodium just as it does for water.Moose deviate from consuming high-profitable terrestrial plants to acquire sodium from low-profitable aquatic plants, but can only do so in the summer when the aquatic plants are not frozen over.Linear programming models incorporate a lot of variables. In this particular example the variables included:1. The minimum amount of food a day that a moose needs to survive.2. How quickly a moose digests its food.3. The energy value of aquatic and terrestrial plants.4. The sodium constraint, which is about 2.57 gms of sodium per day in the summer in order to store enough to get through the winter.The prediction was that moose should spend 18% of their time feeding on high-sodium aquatic plants in the summer. This was proven by empirical observation.3. How long should a forager stay in a certain food patch before moving to another food patch?Marginal Value Theorem Model (Optimal Patch-use Model): Predicts when an animal depleting a patch should move to another patch. “Patch” is any resource that can be depleted over time. Food, available mates, and cooperation of a group can all be considered a “patch.”When an animal begins feeding on a patch, the value of the patch goes down, but the caloric intake increases. At first the animal can consume calories relatively rapidly, but as the value of the patch decreases, the caloric intake levels off until it asymptotes.Two times are measured on the X-axis: 1. Time in patch (to the right in a marginal value graph) and 2. Travel time, transit time, searching time (to the left in a marginal value graph). The Y-axis is the caloric intake of the animal.Assumptions include: Foragers attempt to maximize energy intake rateAll patches are identicalTravel time between patches is constantThe instantaneous harvest rate declines as a forager depletes a patch; the forager experiences diminishing returns in each patchDrawing a tangent line from the travel time to the caloric intake slope and then drawing a line straight down to the X-axis which is the time in patch will reveal the optimal time an animal should spend in a patch before paying the “costs” and move to another patch.Optimal time in a patch can be affected by the “costs” associated with moving to another patch: the less the cost, the sooner the animal should move.Optimal time in a patch can also be affected by the quality of the patch: the poor the quality of the patch, the longer the animal should persist in the patch- given that the cost to move is the same whether the animal is moving from a goodpatch to another good patch or from a poor patch to another poor patch.An example confirming the predictions of the marginal value theorem is thatthe lower the inter-catch interval in birds (good patch, lots of insects), the sooner they give up and move to another patch. Conversely, the poorer the patch (longer intervals between catching insects) the animals remain longer in the patch.4. How does variance in food supply affect decisions about what foods to eat?Risk-Sensitive Optimal Foraging Model: Takes variability (“risk”) into consideration when animals are choosing a patch to receive food.Given 2 patches of food in which the mean amount of food is the same, but the patches differ dramatically in variance, the hunger state of an animal will affect which patch the animal will choose to feed from.The most dramatic difference occurs when comparing satiated animals with starving animals.Satiated foragers will be risk-averse (choose 8 items with certainty)Starving foragers will be risk-prone (choose 50% 0