The result of these developments was a way of investigating the living world by constructing hypotheses about any particular puzzling feature on two central assumptions. First, natural selection is the process that has designed living organisms. Second, time and energy are always limited so that individuals must make tradeoffs in the face of constraints. Over evolutionary time characteristics spread and persist when the individuals with those characteristics are better at contributing genes to descendant generations. The second assumption is the basis for the use of economic logic.
Tradeoffs are unavoidable.
Everything has a cost. More spent on one thing means less to something else. The theoretical foundation of behavioral ecology makes the answer to these questions in one research setting directly useful elsewhere. Not because it explains away the variation. Different cases show whether variables actually do covary in the ways expected.