With more than 1,000 different species living on all continents except Antarctica, bats make up about one-quarter of all terrestrial mammal species. Bats come in all sizes, with wingspans ranging from 15 centimeters to 1.8 meters (6 inches to 6 feet). They make their homes in caves, tree canopies, and abandoned buildings, using these shelters to roost during the day. While some bats live as solitary individuals, others gather in colonies that can number more than a million. What makes bats unique among mammals is their ability to fly. However, unlike certain gliding species of squirrels or lemurs, bats evolved a more complex form of aerial locomotion: flapping flight.

Bats are divided into two suborders, which reflect not only their size, but also their diets and the manner by which they sense and obtain food. It's hard to overstate the impact bat diets have on the environment. Microchiropterans, or "small bats," eat insects and small animals, including lizards and frogs. They use a special adaptation—a navigation system called echolocation—to find prey in darkness. Echolocation is a bat's ability to emit ultra-high-frequency sounds in flight that bounce off objects around them. Depending on how quickly the sound waves echo and return to its ears, a bat can tell the distance and size of an object, and whether it is moving or stationary. Echolocation is highly effective, as bats can catch a hundred or more insects per hour. Insect control on this scale is important, especially for humans, because insects can cause significant crop and forest damage and spread disease. With bats patrolling the skies by night—and birds patrolling them by day—the need for toxic pesticides can be minimized.

Unlike microchiropterans, which rely primarily on hearing, megachiropterans, or "large bats," rely on a keen sense of smell and sight to find their preferred food sources: fruit and nectar. These bats also perform a critical role in the regions they inhabit—aiding plants in reproduction. In turn, the plants produce fruits and seeds that numerous other animals rely on for nutrition. Fruit-eating bats, which are primarily found in the tropics, deposit seeds in their droppings in places new plants can grow. Nectar-feeders pollinate plants as they move from flower to flower. This function is especially important in the desert, where some cacti open their flowers only at night when bats, not birds, are out.

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