On summer evenings, the insectivorous gray myotis emerges from its cave roost and flies to its feeding area, which is usually over a body of fresh water such as a stream or lake (7). Flying aquatic insects, such as mayflies, are the bats’ preferred prey. A maternity colony of around 250,000 individuals may eat as much as a tonne of insects every night (2). Gray myotis pups that fall from the cave roof are scavenged by raccoons (Procyon lotor), the Virginia opossum (Didelphis virginiana) and crayfish (Cambarus spp.). Screech owls have been observed actively predating on gray myotis leaving the roost (3).
The gray myotis uses echolocation to navigate and hunt, emitting frequency-modulated sounds from 45 to 100 Kilohertz (8). The distance flown by the gray myotis per night can vary greatly, from 15 kilometres to as far as 52 kilometres (5), and it can fly at speeds of up to 39 kilometres an hour (9).
The gray myotis reaches sexual maturity at two years, and mating occurs in autumn and early winter upon arrival at the winter caves (7) (10). The female stores sperm in the uterus (7), thereby allowing fertilisation to be delayed until after hibernation the following spring (4) (11). The female gives birth to a single offspring between May and late June (1) (2) after a gestation period of 60 to 70 days. Though the gray myotis has a potential lifespan of 17 years, survival to adulthood is around 50 percent (7).
The female gray myotis gives birth to a single pup that is born without any fur and on average weighs 2.9 grams (2) (4). The newborn pup will cling to the female for around a week and thereafter remains in the cave while the female forages. Offspring born in large colonies tend to grow more quickly than those born in smaller colonies because the warmer cave temperatures increase the growth rates of the young bats. Young from larger colonies are able to fly at just 24 days old compared to the 33 days it takes in smaller colonies (4). The distance to foraging sites also affects the development of pups, with longer distances to travel resulting in lower weight pups and reduced rates of survival (7).
During hibernation, the body temperature of the gray myotis drops almost as low as the cave temperature, allowing it to conserve body fat reserves for the six month period of hibernation. Like many other species of bat, the gray myotis will hibernate in dense clusters of over 1,800 bats per square metre in order to reduce heat loss (7).