Foraging Currency

To evaluate the decision-making process of animals when foraging, it appears relevant to define a variable that can measure the efficiency of foraging strategies or options. This variable is defined as a "currency", usually with a specific unit, and a specific formula.

Currencies can vary from very simple to elaborate, depending on the model studied. Here are three examples of simple currencies:

1) Net Rate of Energy Gain = Gross Rate of Energy Gain - Rate of Energy Expenditure, in Watts, which are Joules per second. This is purely the absolute difference between the energy harvested from foraging and the energy spent on the activity. A better foraging strategy will show a higher net gain.

2) Efficiency = Gross Rate of Gain / Gross Rate of Expenditure, which is dimensionless (Joules divided by Joules) The Efficiency measures how many Joules of energy are returned per Joule spent foraging. Quite clearly, a more efficient foraging strategy returns a higher amount of energy for a fixed spent amount.

3) Gain-to-Mortality ratio: this slightly more complex currency considers foraging within the objective of generating offspring rather than just direct survival. It links the growth rate of a forager to its mortality rate and its fertility, based on the principle that a larger size means a lower amount of natural predators. It would thus be in the interest of a forager to grow quickly at the early stages of the life cycle. On the other hand, this currency also makes use of the birth rate, considering if there is a better use of energy to produce a large amount of offspring to increase survival rate instead.

An example of a much more elaborate currency including the presence of predation, the varying fitness of the animal and its probability to mate and reproduce, could look something like:

with γ(c) the rate of increase of reserves from foraging, δV/δx the rate of increase in future reproductive success with increasing fat reserves, K(c) the rate at which a restraining condition (for example the building of lactic acid in muscles from continuous hunting) decreases and function of the metabolic expenditure, δV/δy the rate of increase in future reproductive success with increasing restraining condition, M(c) the rate of mortality due to predation, and finally V the animal's future lifetime reproductive success.

Without necessarily being this complex all the time, it is a regular practice to define a currency more adapted to the behavior of the surveyed animal and to the study objectives when observing the foraging pattern of a species. For example, due to its community-based life, the foraging currency of the honeybee is trickier to consider: the honeybee does not solely consume the nectar but brings it back to a central place (the hive) to be shared. It might be then necessary to define a currency to evaluate how much energy the entire colony is gaining, rather than the individual bee. It could take into account for example how far the bees could fly before carrying a load of nectar to the hive becomes unprofitable, which strategy is viable short-term or long-term for the entire community, etc.

In general, most of the general factors included in a foraging currency can be linked to:

• Predation
• Fitness
• The solitary or gregarious social habits (feeding a community or feeding shared offspring)
• The maximal amount of energy an animal can spend per unit of time
• Energy reserves (such as stored as fat)
• The densities of prey in the area
• The distance or time between foraging patches

Foraging currencies can be used for example with the Daily Energy Budget rule to study the behavior of a forager and its associated risk sensitivity.