Each year, tens of millions of straw-colored fruit bats descend on Zambia's Kasanka National Park during their roughly 2,000-mile migration, gathering in what is believed to be the largest concentration of mammals in the world. "It has to be seen to be believed, but imagine being in a winter blizzard," says Ian Salisbury, the general manager at the Bushcamp Company, a safari operator in Zambia. "Then substitute bats for snowflakes."

The bats' annual journey, which starts somewhere north of Congo and ends in Zambia, plays a crucial role in Africa's ecosystem. Each bat consumes up to two times its body weight in fruit – mangoes, wild loquats, water berries – every night, a feeding frenzy that may account for at least half of the annual seed dispersal throughout the entire continent's rain forests. "We're learning that bats might even be more important to seed distribution in the African rain forests than birds," says Heidi Richter, a bat biologist who lives in Seattle. "Their large range, wide variety of diet, and appetite all contribute to this."