The Carlisle Mosquito Online

Friday, January 26, 2007


Biodiversity Corner Big Brown Bat

"Suspicions amongst thoughts are like bats amongst birds, they ever fly by twilight." Francis Bacon

This Big Brown Bat weighs only about half an ounce and its wing span is just under a foot. (Photo by Kay Fairweather)
Name. The Big Brown Bat is Eptesicus fuscus where Eptesicus means house flyer or flying and fuscus means brown. There are 35 species of bats in the genus Eptesicus but this is the only one found in North America. Of the 942 species of bats worldwide, only nine occur in Massachusetts. The largest of those is the Hoary Bat (Lasiurus cinereus) and the big brown is next.

When and where seen. Leslie Thomas of Estabrook Road found this bat on her deck on December 22 and brought it to me. I was delighted. I had never before seen a bat up close.

Which bat is that. No prizes for knowing, the Big Brown Bat is bigger than the Little Brown Bat but it is still a small creature. It weighs about half an ounce and with its wings folded in, will easily fit in the palm of your hand. The wing span is around one foot — this one was just a little under. The Big Brown Bat's fur looks brown (sometimes common names are reasonable) but if you have the chance to look closely, you will notice the hair is actually two-tone — almost black at the base and brown at the tips. You can identify the Big Brown Bat from a distance by its flight pattern. It flies a straight path with rather ponderous slow wing beats at a height of 20 to 30 feet above ground and you can hear it chattering as it flies. The Little Brown Bat has a more erratic flight. Leslie's bat was dead as a doornail so I identified it by body measurements and arrangement of the teeth.

I tried several identification keys. They all relied on the length of the forearm, and this bat's forearm fell right at the dividing line of 40 millimeters, which meant I needed to look at the teeth.. (By the way, did you know the fingers of the Big Brown Bat are longer than the forearm?) Once I was able to prop the mouth open, I could see no gap directly behind the upper canine and no tiny tooth there that might appear as a gap. Bingo! Big brown bat.

The teeth helped to identify this Big Brown Bat. The object in the ear is called a tragus and was also part of the identification process. (Photo by Kay Fairweather)

Hang-outs. In the summer, both the Big Brown Bat and the Little Brown Bat, given an opening, may make their home in yours, usually in the attic. Barns are also favored, and hollow trees will do. In winter, they hibernate but only the Big Brown Bat is likely to stay in a building. I would call this "hanging in." Little Brown Bats go out and find a cave. This would be "hanging out." The species sharing your home becomes important in the timing of sealing entrances to keep them out. Mass. Wildlife publishes a Homeowner's Guide to Bats which provides detailed instructions on the ins and outs of bat-proofing your house. It also has plans for building a bat house. Find it all at (search for eptesicus ). If a bat finds its way into your living area, Mass. Wildlife recommends opening a door or window and standing back while the bat finds it own way out. If you must pick it up, use thick sturdy gloves.

Homing instinct. Big Brown Bats have a strong homing instinct. Researchers have banded big browns and taken them miles away from their roosts and released them. From 20 miles away, they were back the same night; from 40 miles it took two nights, etc. Bats taken 250 miles also found their way home; some after four nights and the rest after five.

Food chain. Big Brown Bats eat insects, mainly Coleoptera (beetles) and Hymenoptera (ants, bees and wasps). Their teeth are very sharp and could easily crush beetles. Mass.Wildlife reports that Dr. Thomas Kunz of Boston University estimated the population of Big Brown Bats inside Route 128 to be around 50,000 and that in a summer they would dispose of 14 to 15 tons of insects. Big Brown Bats are prey for great horned owls, screech owls, sparrow hawks, grackles, cats and snakes. They also host a variety of blood-sucking critters. Leslie's bat had quite a population of mites.

References. Alfred J. Godin, Wild Mammals of New England; Ronald M. Nowak, John L. Paradiso, Walker's Mammals of the World 4th edition, Volume I; Gwilym S Jones, John O. Whitaker, Mammals of Eastern North America; Thomas W. French, James E. Cardoza, Gwilym S. Jones, Mass. Wildlife Homeowner's Guide to Bats.

Submissions for the Biodiversity Corner are encouraged from everyone. Send a photo, a note about a sighting, or the whole column to Kay Fairweather at 392 School St, Carlisle MA 01741 or to

2007 The Carlisle Mosquito