Gray myotis (Myotis grisescens)

Also known as: grey bat
GenusMyotis (1)
SizeMale head-body length: 49.4 mm (2)
Female head-body length: 50.3 mm (2)
Male weight: 7.9 - 9.1 g (2)

The gray myotis is classified as Near Threatened (NT) on the IUCN Red List (1).

The gray myotis (Myotis grisescens) is one of the largest species of the genus Myotis in North America. The genus name is derived from Greek and means ‘mouse-ear’, while the species name grisescens comes from the Latin word ‘griseus’, referring to the grey fur colour (2).

In general, the fur of the gray myotis is uniformly grey from root to tip, although some individuals may be whitish underneath. During the summer months, ammonia fumes produced by large colonies may cause the grey colour to fade, sometimes causing the gray myotis to appear a dull russet colour, or brownish above and a whitish or buff below (2). A characteristic feature of the gray myotis is that the wing membrane attaches to the side of the foot rather than at the base of the toes as is found in most other species of Myotis. The ears extend to the nostrils or slightly beyond when folded forward and the tragus has a bluntly pointed tip (2). 

The range of the gray myotis extends from Kansas and Oklahoma in the United States, east to western Virginia and western North Carolina, and south to Alabama and northwestern Florida (1).

The gray myotis is one of the few bats to roost in caves throughout both the summer and winter (2). Particular in its choice of caves, over the summer months the gray myotis will select several caves of a certain temperature range which are also close to rivers and reservoirs. Summer colonies are usually located in caves less than one kilometre from a major river or lake (3). These specific roosting requirements are necessary to provide the ideal conditions for breeding and feeding (2). Optimal foraging habitat is riparian forest where the bat can fly over bodies of water under protection of the forest canopy (2).

In the winter, the gray myotis will migrate to a cave suitable for hibernation (4). For hibernation, the gray myotis will select colder caves than other species in its genus. Deep, vertical caves with large domed halls (1) (5) and low temperatures (6 to 11 degrees Celsius) are preferable sites (6). Due to its very specific cave requirements, the gray myotis tends to gather in large numbers in very few hibernating caves, with almost 95 percent of the population spending the winter together in the same nine caves (1). The gray myotis maintains the same summer and winter ranges year after year unless disturbed (7).

Maternity caves typically have a stream flowing through them and are separate from the summer caves used by the males (1). These caves often also have certain heat retention properties such as pockets and small chambers which allow the young gray myotis to cluster and conserve heat. Warmer maternity roosts promote the rapid development of young (2).

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).

Between the 1960s and early 1980s, the gray myotis population declined by at least 50 percent (1). The biggest threat to the gray myotis is the disturbance of its cave habitat by humans, particularly as it is highly sensitive to disturbance and will leave a cave in response to humans with flashlights (1). Many of the bats’ most favoured caves have been commercialised by the tourism and recreation activities of humans, causing the bats to leave. As the cave conditions the gray myotis require are so specific, not many caves are suitable, and bats can forced to move to less suitable caves (2).

Large-scale habitat destruction has also caused a decline in the gray myotis as deforestation of foraging areas reduces the quantity of food available (6). Use of insecticides and crop pesticides in areas where the gray myotis forages may also reduce the amount of prey, impacting on reproduction and survival rates (1). 

Protection of roosting sites from human disturbance is vital to conserving the gray myotis. Methods to protect caves used by the gray myotis include gating cave entrances to minimise disturbance, and acquisition of caves by agencies to prohibit access, allow restoration and prevent commercialisation. The most important summer roosting cave used by the gray myotis has been acquired by the U.S. Fish and Wildlife Service (2).

These efforts have been partially successful, and by 1991 there were indications that the population of gray myotis was not only stable but potentially growing (1). Since 1981, the maternity colony population in Oklahoma has tripled from around 50,000 to 150,000 (13). While the majority of the large caves are being protected, managed and monitored, many of the small caves are still threatened (1).

Further suggestions made by the Alabama Forestry Commission include introducing a buffer of undisturbed vegetation, preserving woodland from roosting caves to foraging locations, and controlling and monitoring pesticide use in close proximity to gray myotis colonies (1).

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  1. IUCN Red List (November, 2011)
  2. Decher, J. and Choate, J.R. (1995) Myotis grisescens. Mammalian Species, 510: 1-7. Available at:
  3. Tuttle, M.D. (1976) Population ecology of the gray bat (Myotis grisescens): factors influencing growth and survival of newly volant young. Ecology, 57: 587-595.
  4. Tuttle, M.D. (1975) Population ecology of the gray bat (Myotis grisescens): factors influencing early growth and development. Occasional Papers of the Museum of Natural History, 36: 1-24.
  5. Tuttle, M.D. (1976) Population ecology of the gray bat (Myotis grisescens): philopatry, timing and patterns of movement, weight loss during migration, and seasonal adaptive strategies. Occasional Papers of the Museum of Natural History, University of Kansas, 54: 1-38.
  6. Tuttle, M.D. (1979) Status, causes of decline and management of endangered gray bats. Journal of Wildlife Management, 43: 1-17.
  7. Mitchell, W.A. and Martin, C.O. (2002) Cave- and Crevice-dwelling Bats on USACE Projects: Gray Bat (Myotis grisescens). EMRRP Technical Notes Collection (ERDC TN-EMRRP-SI-25). U.S. Army Engineer Research and Development Center, Vicksburg, M.S. Available at:
  8. Shimozawa, T., Suga, N., Hendler, P. and Schuetze, S. (1974) Directional sensitivity of echolocation system in bats producing frequency-modulated signals. Journal of Experimental Biology, 60: 53-69.
  9. LaVal, R.K., Clawson, R.L., LaVal, M.L. and Caire, W. (1977) Foraging behaviour and nocturnal activity patterns of missouri bats, with emphasis on the endangered species Myotis grisescens and Myotis sodalis. Journal of Mammalogy, 58: 592-599.
  10. McNab, B.K. (1974) The behaviour of temperate cave bats in a subtropical environment. Ecology, 55: 943-958.
  11. Guthrie, M.J. (1933) The reproductive cycles of some cave bats. Journal of Mammalogy, 14: 199-216.
  12. Rice, D.W. (1955) Status of Myotis grisescens in Florida. Journal of Mammalogy, 36: 384-388.
  13. Hensley, S. (2003). The treasures of the Ozark plateau. Endangered Species Bulletin, 28(1): 32-33.