Bats are the only mammals that have wings and are capable of true flight. Many are grotesque-looking animals with huge ears and strange face. They produce high-pitched sounds which bounce off nearby objects. This echo is picked up by the bats large ears and the bat uses it to locate its prey and avoid obstacles. This is called echolocation. Almost a quarter of all mammal species alive in the world today are bats. Nearly all are nocturnal (active only at night). Most eat insects, but some feed on larger creatures such as mice, or other bats. A few catch fish, and three species take blood from large birds or mammals. Some bats feed on nectar and are important in carrying pollen from flower to flower, while others are fruit eaters. Most bats fly more slowly than birds, but have more control. They can turn, twist and fly without hesitation, through small gaps that would defeat the majority of birds. Flight uses a lot of energy. Most bats conserve their resources by letting their temperatures drop and remaining still when they are not flying. In cool parts of the world bats hibernate, but a few species migrate to where there are better supplies of food. There are still huge populations of many sorts of bats, but others are becoming rare, as a result of pollution and changes in the environment.