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Bats Northwest

"helping bats in Washington State"

The Little Brown Bat - (Myotis lucifugus)

Also known as Little Brown Myotis

The Little Brown Myotis (Myotis lucifugus) is probably the most numerous as well as the most studied bat in America. It has adapted so well to humans it routinely forsakes its natural maternity roosts in tree cavities for hot attics and building walls. The heat actually speeds the development of their babies, both during gestation and after birth. And Attics are usually safer from small carnivores, snakes, and other bat predators. Given the room in an attic, it's not surprising that some of these colonies number over a thousand! There is some thought that Little Brown Myotis populations have increased because of their successful exploitation of human structures.

This is not necessarily a bad thing. This bat is an extremely efficient hunter and therefore very useful to us for insect control. It prefers to hunt low over water. (It is a capable swimmer if it accidentally ends up in the drink!) But it also hunts in trees and over lawns. And it eats a wide variety of insects including flies and mosquitoes, moths, beetles, caddisflies, mayflies, termites, leafhoppers and midges. Since many of our human, home and garden pests are on this menu, these bats in large numbers can be a good thing!

This is the bat often reported "capable of eating 500 (or 700 or 1000) mosquitoes an hour." This sounds really good to the backyard barbequer, but unfortunately this factoid has lost a lot of accuracy in its multiple incarnations. First of all, substitute "mosquito-sized insect" for mosquito. It hunts insects from 3-10 mm in size, but certainly is not a mosquito specialist, turning its nose up at other prey!

Secondly, research shows that it can fill its stomach in about 15 minutes. Although it processes food quickly (in less than an hour), the bat still needs time to rest and digest. An adult male may feed several times a night. That "500 (or 700, etc.)… an hour" is only if the bat could feed continually.

Now a lactating female must feed more often and may consume over 100% of her own body weight in a night. Of course that's only around 6-8 grams. But hey, along with hundreds of her sisters, that's still a lot of insects. "Lucys", as researchers affectionately call them, are still amazing creatures without exaggerating the facts!

Little Brown Myotis are gregarious with large social groups forming at the maternity roost and the hibernation roost. At the maternity site it is all females. The males stay away, leaving the warm roosts and optimal feeding grounds to them. Sometimes males join the colony in mid-summer. At that time all the adults will leave the most productive foraging areas to the new juveniles who are just perfecting their skills.

By September, first the adult males, then the moms and juveniles start checking out the hibernaculum. They fly in and out of the cave or mine in a behavior called "swarming". But they do not stay more than a few nights. This swarming is thought to be prenuptial behavior as well as serving to introduce the new young to the hibernaculum. The bats then forage for several more weeks before returning to the site to begin hibernation. By October they are all together, settling in "for a long winter's nap".

Bats as a whole are very long-lived mammals for their size, and Myotis lucifugus has been recorded living 34 years in the wild.