GREGORY DIMIJIAN / SCIENCE PHOTO LIBRARY GREGORY DIMIJIAN / SCIENCE PHOTO LIBRARY
Cloud of Mexican Free-tailed Bats (Tadarida brasiliensis), after emerging from Bracken Cave, near San Antonio, Texas, Aug. The 20 to 30 million bats in the cave may be the largest colony of mammals on Earth. The bats are emerging at dusk, flying in great clouds towards the horizon to feed on flying insects.
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