Rats assess costs and benefits according to an internal standard.

Variation in effort to obtain rewards is a fact of mammalian everyday life. In this study, we assess how rats scale variable costs and benefits. Different groups of rats were trained in a T-maze to discriminate a high (three or five sugar pellets) from a low reward (one sugar pellet) arm. Subsequently barriers were introduced at the high and low reward side such that the overall long-term pay-off of the high reward arm finally became lower than that of the low reward arm. The data show that under different regimes of costs (climbing barriers) and benefits (number of rewards) of the two arms rats appear to shift their behaviour towards the better side according to a constant relative cost-benefit ratio between the arms. Such a ratio allows them to deal with variation in the (physical appearance of) costs and benefits and choose the best long-term option.