These include several very advanced methods that prevent the successful reproduction of pest species. They include the sterilisation of males (tsetse fly, mosquito), sex distortion or replacement of genes. All of these methods are expensive and often complex to monitor. Other biological methods involve introducing or encouraging predators of the vector species. For example, small fish can be used to feed on larvae of mosquitoes. Reptiles, birds and frogs feed on adult insects and cats will prey on rats.