Biology 301

Optimal Foraging

Optimal Foraging theory is an application of evolutionary thinking to the behaviors of consumers (choice of patches of food, or individual items of food).

The assumptions are that organisms maximize their fitness by maximizing their energy intake, and by minimizing their risk of exposure to enemies.

On a foraging trip, an organism spends time traveling between patches of food (travel time), and spends time in each patch of food (search time).

• The organism only gains energy when it is in the patch of food
• Eventually the patch is depleted of food, and no more energy is gained by staying in the patch.
• A graphical analysis allows one to find the optimal length of time that the organism should spend in the patch (Fig 9.17 in Ricklefs).
• One plots number of prey caught (energy gain) against time spent.
• The average rate of energy gain over a foraging trip can be represented by a line running through the origin, and tangent to the instantaneous energy gain curve.
• The optimal residence time in the patch is the time when the curves touch each other.
• Long travel times favor longer times spent in the food patch.
• Shorter travel times favor shorter times spent in the food patch.

The choice of individual food items is analyzed in a similar way

The average rate of energy gain for an animal is

E/T where E is the total energy gained and T is the total time spent foraging.

For each food item, the time spent is divided into Search Time T_s and Handling Time T_h.

An animal encounters a food item and must decide whether to eat it or not. It already has found the item, so the only further time it must spend is T_h, the handling time necessary to eat it. The animal should add the item if E/T_h for that item is greater than E/(T_s+T_h) for its preferred item. In order to get the preferred item, it must search and then handle and consume the preferred item. In order to consume the item in hand, it only has to handle it.

• New item is added to the diet if new item E/T_h > E/(T_s+T_h) for the preferred item
• If the preferred item is rare, then T_s is large, and there may be an advantage to eating the less preferred item.
• If the new item is hard to eat, so T_h is large, then the predator may ignore it.
• This is consistent with the fact that people are willing to eat food from vending machines: Preferred items are sparse, so T_s is large, and people are willing to add junk to their diets, because E/T_h of junk food is greater than E/(T_s+T_h) for real food.