of a body of water and are rarely further than 4 km away from a lake or major river. The average roosting density of gray bats is 1828 bats/m2. Biology and behavior Foraging Gray bats forage over water, including streams and reservoirs, where they consume night-flying insects most of which have aquatic larval stages. and in the riparian forests nearby these water sources. M. grisescens activity tends to be concentrated over slower-moving water or quiet pools than areas of fast-moving water. Foraging usually occurs below treetop height but above 2m. Gray bats tend to fly downstream more often than upstream, suggesting a potential preference for wider sections typical of downstream sections as opposed to upstream portions (with a tendency to be narrower). M. grisescens tend to forage over extensive ranges, averaging 12.5 km but ranging from 2.5 km to 35.4 km. While gray bats have been shown to forage in small groups when prey is abundant, especially during the early hours of the night, when prey is scarce, gray bats can become territorial. Territories tend to be controlled by reproductive females. These females seem to claim the same territory year after year. Diet Gray bats consume a variety of insects including Coleoptera (beetles), Diptera (flies), Ephemeroptera (mayflies, of which gray bats consume at least six species), Lepidoptera (moths), Neuroptera (net-winged insects), Trichoptera (caddis flies), and Plecoptera (stoneflies). Juveniles have a tendency to forage more in woodlands and eat more beetles than adults, perhaps they provide a greater energy reward per unit of capture effort. For example, beetles provide 1900–2800 calories/g wet weight versus 800–1400 calories/g wet weight for mayflies. M. grisescens juveniles also eat a less diverse diet than adults, possibly because juveniles are more dependent on high concentrations of prey or swarming prey. Gray bats are believed to be part opportunists, and part selective eaters. (Outside of captivity, gray bats are limited by the sporadic emergences of potential prey. When prey emerges, there is only an abundance of a few taxa at any given time. The available taxa change based on the time of night, the month, and the time during the activity season.) In their natural habitats, gray bats appear to attack any moving target that is of appropriate size, consistent with optimal foraging theory that predicts palatable insects of an appropriate size should be eaten when encountered. In captivity, under controlled laboratory conditions, however, insectivorous bats used