In the optimal foraging graph, what do T and T′ represent?
In marginal value models of patch use, travel time T is the interval spent moving between resource patches, whereas patch residence time T′ is the period devoted to exploiting the current patch. A forager should leave when its instantaneous gain rate falls to the average rate available in the habitat, including travel costs. Longer travel generally favors remaining longer in each patch because departure would impose a larger unproductive interval. Search and handling times are components used in prey-choice models and may occur within a patch, but they are not the paired quantities identified by these symbols in the keyed graph. The distinction links movement costs to optimal patch departure. The alternatives can be separated by asking what changes for each participant and which process causes that change. That approach is more reliable than treating familiar examples as fixed labels, because many interactions shift with environmental context. From an evolutionary perspective, traits persist when their net effects improve inclusive or direct fitness under prevailing conditions. The ecological terminology therefore summarizes both an immediate mechanism and its likely consequences across generations.
Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 10