How an animal behaves is coordinated by nerves and hormones in different, complementary ways. Stimuli, such as the sound of a predator, cause fast behavioural responses coordinated by nerve signals. The stimuli also cause longer lasting physiological changes via hormones, which release energy sources needed for the muscle action required for escape. ‘Sensing and responding’ considers the sensory responses of bats and moths, and then explains selective sensitivity—how animals evolve to detect only what affects their survival or reproductive success. It also shows how the study of neural circuits in simple model systems, such as sea slugs, can help us understand more complicated behaviours in other animals.