All animals have the ability to perceive and respond to external cues. Cuesmay signify the presence of good things, such as food, a potential mate or shelter, or bad things, such as predators, competitors, or a hazardous environment. Typically, an organism will be exposed to a mixture of positive and negative cues, requiring the animal to weigh the inputs and subsequently determine its behavioural response based on that information. However, the response may vary: a certain stimulus may elicit different behavioural responses, depending on the context of the stimulus, previous experiences of the organism, its age or developmental stage. This variability is called plasticity, and is well known in many animal species. The ensuing flexibility is thought to enhance the chances of survival and to be essential for memory formation and for many aspects of development. It has even been proposed to be one of the driving forces of natural selection and evolution (Price et al., 2003; Agrawal 2001). However, despite the broad impact of behavioural plasticity, the cellular and molecular mechanisms are thus far poorly understood.