Social animals that bring resources to a central place have commonly been used to test the predictions of optimal foraging models. Such animals are amenable to test because they do not themselves reproduce, and so we might expect them to be selected to maximise some measure of input of food to the colony. Several currencies have been proposed to predict behaviour, such as net rate, efficiency, and the ratio of the mortality rate to energy gain rate. Observations on social animals, especially bees, show mixed support for each currency. Here, we examine how these currencies can be united by considering the expected lifetime input of energy to the colony in a representative study of patch residence time. This currency explains partial loads because it leads to the prediction that the energy that a forager delivers to the colony over its lifetime is maximised by returning to the colony after a critical amount has been collected, even if energy is gained at a constant rate. We show that the extent to which foraging carries a greater mortality risk than travelling controls whether this currency makes similar predictions to net rate or to efficiency. We assess the evidence that bee behaviour actually maximises this currency and argue that mortality risk at resource sites is likely to be a critical determinant of foraging strategies in central-place foragers.