Optimal tempo and defence for consumers of multiple resources

Ecological models of behaviour are typically based on the assumption that decisions can be evaluated with a single resource currency. Here we present models that predict the tactics of consumers collecting two nutritionally distinct resources: fuel that is used for activity and food used for growth (F4G). Both models assume that foragers seek to maximize F4G gain subject to collecting enough fuel for activity. Our first model determines the optimal tempos of foraging for each resource. While foraging for fuel, consumers use and collect the same resource and optimal behaviour is identical to the predictions of a single resource model. However, because consumers use fuel to acquire F4G, they are predicted to work harder to acquire F4G when fuel is more available. Our second model examines how consumers should allocate their time among foraging for fuel, foraging for F4G and defence of F4G sources. Optimal investment in defence increases when fuel is available (because expenditures can be quickly recovered) and when F4G is scarce (because fewer opportunities exist for obtaining new sources of F4G). Our results suggest that behaviours will appear wasteful when foraging environments are fuel-rich and overly frugal when F4G is common but fuel is scarce.