Introduction

One of the most important, intriguing aspects of animal behaviour is that it continually changes. Some of the changes are parts of the processes of development and maturation, whereas others allow the animal to learn about alterations in its environment. Learning enables an animal to make and modify predictions based on experience, for example that a particular action will be followed by a rewarding or an aversive event. The ability to change is a fundamental property of many nerve cells and their interconnections, and much recent research has focused on events at the molecular and cellular level that could underlie learning or events during the development of a nervous system. A major challenge in neuroethology is to relate these changes in cellular properties to alterations in animal behaviour.

One type of trigger for changes in behaviour is provided by hormones, which can ensure that events are initiated at particular times. Steroid hormones act by regulating gene expression and can produce modifications in the morphology of nervous systems that are correlated with changes in behaviour (Breedlove, 1992; Weeks & Levine, 1995). Polypeptide hormones, on the other hand, often act to trigger particular behaviour patterns by exciting particular target neurons. Another type of mechanism triggers learning, in which an animal forms a new association between a sensory stimulus and a motor program.