In order to study the nature of aggressive behaviour, especially the implications for human behaviour, predators are used in prey-killing experiments. The predators may be partially mutilated prior to the experiment. This can involve castration, cutting the olfactory nerve (to prevent smell), or blinding. To stimulate aggression, electric shocks may be administered through metal grids or electrodes planted in the brain. The animals most frequently used are monkeys, cats and rodents.