Complex, context-dependent decision strategies of archerfish (Toxotes chatareus).

Operant conditioning is fundamental to many animal behaviour experiments, including the alternative
forced-choice (AFC) task, a powerful and popular paradigm for establishing learning and discrimination
abilities across a wide array of species. One frequently overlooked aspect of this paradigm is the relative
importance of the positive (Sþ or ‘target’) and negative stimulus (S or ‘distractor’). It is often assumed
that subjects learn to associate Sþ with a reward. However, it is equally possible that they learn to
associate S with the absence of a reward. The rule learned may have repercussions for the design of
experiments, and may also provide information about the decision strategies employed by a particular
animal or species. Archerfish are becoming increasingly popular as a model for testing cognition in lower
vertebrates. We conducted three experiments to explore how archerfish learn to complete a common
type of forced-choice test. In the first experiment, we showed that archerfish were able to discriminate a
range of shapes in an AFC task. In the second experiment, we found that replacing S with novel stimuli
caused a greater disruption in performance than replacing Sþ. In the final experiment we showed that
archerfish: (1) were probably using complex decision rules; (2) made context-dependent choices; and (3)
examined some or all of the stimuli before making a decision when faced with multiple stimuli. Our
results show that archerfish have a flexible learning strategy which may facilitate exploitation of novel
food sources. The discovery that archerfish avoid negative stimuli fits with their generalist feeding
ecology in which they may learn to avoid distasteful/toxic prey. At the same time the fish demonstrate a
willingness to try new prey, which have the potential to offer a greater reward than familiar target prey.