Central Place Foraging (CPF)
The concept of central place foraging is a special case of the marginal value
theorem. Animals that forage around a retreat site such as beavers with a
lodge, ground squirrels around the colony, or ants around a nest are faced with
movement away from a central location to the food resources. The question is,
how many items should an individual collect before returning to the retreat,
storehouse, or larder. Another question arises regarding how best to exploit the
resource around the central retreat before perhaps moving on to the next site.
The central place foraging concept is intimately related to the marginal value
theorem. In a field experiment with chipmunks, Kramer (1982) provided the
chipmunks with patches of sunflower seeds at varying distances from their
burrows. As the chipmunks stuffed their cheek pouches full, the rate of seed
collection declined in a fashion that was similar to the theoretical gain curve.
In addition, the chipmunks spent longer in each patch and took larger loads
when they were collecting at more distant patches to their burrow. While these
trends were present, their wasn't an exact match between the theoretical and
predicated values suggesting that other factors might have played a role in the
chipmunks"decision making".
Kacelnik studied central place foraging in parental starlings which have a nest
site and forage in the vicinity of the nest site for their food making trips to
collect multiple items. The question for a starling parent is how many items to
collect in its bill before returning to feed its young? Kacelnik was able to train
some birds to visit a feeder which dispensed mealworms at an ever increasing
rate to simulate the decreasing gain curve of the marginal value theorem. In
the case of parental starlings, is the parent tries to:
1. maximize its total energy gained which is given by E/time, or
2. maximize energy delivered to young which also entails the energy costs
of the parent during travel as well as the energy costs for chicks or E/
time - Ep - Ec.
By manipulating the gain curve, and altering the travel time or distance of the
feeder box, Kacelnik found support for maximization rule #2. This indicates
that the answer of what is optimal depends on what the animal might be
interested in maximizing.