meaning that on any particular trial, reward could be available regardless of which of the two actions the monkey chooses, only if it makes either a lift response or a turn response, or may not be forthcoming no matter what it selects (Figure 9a). If the animal chooses a response which has been allocated a reward, naturally it will obtain the reward. However, as it is only possible to select one response on each trial, once allocated to either a lift or turn, the reward will remain available until that action is selected. This means that an animal will not harvest the maximum amount of available food simply by working out which is the richest option of the two and choosing it every time as the cumulative probability of the less profitable alternative increases the more trials on which it is ignored until its likelihood of offering reward is actually greater than the more profitable response. Instead, to optimise foraging efficiency, the animals need to sample both options to develop a sense of the yield of each alternative and to learn when and how often it is advantageous to switch away the more profitable option to the one that normally
