Which animal species comprises one fifth of all the mammals on earth?

Bats are mammals of the order ‘Chiroptera’, and are the only mammals capable of true sustained flight. They can be found on every continent except Antarctica. There are an estimated 1,400 species of bats in the world, and they make up one-fifth of Earth's total mammalian population, the second largest order after rodents. That number may be even larger, due to undiscovered species.

Most of the world’s bat population is found in tropical areas, with one-third of Earth’s bats found in South and Central America. Asia has its fair share also; the island country of Indonesia is home to some 175 species, the most of any single country.

Naturalists generally divide bats into two categories; megabats and microbats, with size being the main difference. The smallest known is Kitti’s hog-nosed bat, weighing in at 2 grams (0.07 oz) and the largest is the giant golden-crowned flying fox, which reaches a weight of 1.6 kg (3.75 lbs).

Although they are viewed with suspicion in some cultures, bats do provide humans with benefits. Bat dung (guano) can be mined from caves and used as fertilizer. As important members of their ecosystems, bats pollinate flowers and disperse seeds; many tropical plants depend entirely on bats for these services. In addition, some 70% of the world’s bat population feeds on insects, which reduces the need for pesticides.

Unfortunately, due to their physiology, bats can be a natural reservoir for pathogens such as rabies, and thus potentially dangerous to humans.