| Western pipistrelles sometimes leave their roosts before sundown, and can be mistaken for late-flying butterflies, because they are so tiny and fly slowly and erratically, with much fluttering of their wings. Most common at low elevations in desert scrub and arid grassland habitats, they are also found in adjacent woodlands. Although they range over the arid West, western pipistrelles require a ready source of water—a lake, stream, or even a swimming pool. They—and some shrews—are the smallest mammals in North America, with weights ranging from 2—6 g. In spite of their tiny size, western pipistrelles usually give birth to twins, which are born and raised in small maternity colonies. The largest colony yet found comprised just four female bats and their eight young.
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Females are larger than males.
Allen, H., 1864. Smithsonian Miscellaneous Collection, 7:43.
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