Sunday, January 17, 2016

Little Brown Bat

Myotis lucifugus. Photo from Animal Diversity Web
Over the last week, the Eatles have completely devoured the soft tissue remains of a little brown bat (Myotis lucifugus), with a great big story. Well, maybe neither "big" nor "great"... but certainly a "good" story. In Summer 2010 we attended the birthday party of a family friend's toddler. The party was thrown at the birthday boy's grandparents' farm located in Sullivan, Missouri, about 65 miles SW of St. Louis.

As a comparative anatomist by training, I have been fortunate to learn so much intricate, detailed anatomy through dissection. However, with that fortune sometimes has come the misfortune of using specimens that were less than fresh - ripe, even. When we arrived at the birthday, I noticed the very faint, but unmistakable, stench of decomposition wafting from under the picnic table where the birthday cake was set. I looked under the table and found a recently deceased little brown bat. I discreetly informed the party host of my find, and as any self-respecting comparative anatomist would do, I asked if I could take it home! So my friend the little brown bat from Sullivan stayed in my lab chest freezer for the next five and a half years awaiting his fate at the hands of The Eatles.

Bats belong to the mammalian order Chiroptera, which is the second most speciose group of mammals after rodents (Order Rodentia). Chiroptera is divided further into two groups of bats: the mega bats - the large Old World fruit bats; and the micro bats - the small, generally insectivorous bats. The little brown bat belongs to the microchiropteran family Vespertilionidae, which includes over 300 species of common bat.

Little brown bat feast
Bats are fascinating creatures. Most vespertilionids navigate by utilizing a combination of eye sight and echolocation. In order to echolocate, bats create a series of high-pitched sound pulses at frequencies that their sense of hearing is optimized to detect (usually well above the range of human hearing). These sound pulses originate in the animal's larynx (vocal apparatus) and resonate through the nose and mouth. The sound pulses reflect off of objects in the distance, and the echo that returns indicates how far away the object is, such as food or trees or another bat. These echoes can also help the animal determine how large the object is, and whether or not it is moving, which is helpful for finding potential mates and prey, and for avoiding predators. You can read more about echolocation in bats at the Bat Conservation Trust.

Little brown bats are small in size, averaging somewhere between 5 and 14 g in weight and between 60 and 100 mm in length (head to tail), with a wingspan that is 222 to 269 mm. They live all over North America except in the forested high mountains of Mexico, and their diets consist largely of insects such as moths, wasps, beetles (oh no... please NOT BEETLES!), mosquitos and mayflies. Unlike many bats, little brown bats do not migrate long distances to warmer climates during the winter. Instead, they move their roost sites to places where outside temperatures can be modulated in colder temperatures, such as inside abandoned mine shafts or caves, usually within 100 miles of their summer roosting sites.

Upper Limb Skeletal Homology, Arizona State University.
But probably the most fascinating thing about bats is that they are the only living mammals that have evolved anatomical adaptations for sustained flight. The basic elements of the mammalian upper limb musculoskeletal system are present in bats, although they have been significantly modified to support a wing membrane (patagium), and the sizes of many of the bones and muscles differ from those of nonflying animals. For example, the distal elements of the upper limb - the metacarapal bones of the hand and the phalanges of the fingers - have been elongated in order to provide support for the patagium and to control its movements. The patagium, which is a double-layered membrane of skin derived from the abdomen, encloses a series of nerves, blood vessels, and muscles, and it extends from a bat's fingers to the torso and abdomen and to the lower limb. In order to support the patagium with the lower limb, a bat's hip joint is rotated 90° so the knee faces backward. This altered hip joint prevents normal walking in bats, but also allows the bat to hang upside down in its roost. Upside down hanging is also helped by a tendon configuration in the toes that locks their toes in place so they can continue to hang without conscious effort during sleeping.
Contributed by: Jason Organ, PhD 

Read more about bat science in these references:

Dzal YA, & Brigham RM (2013). The tradeoff between torpor use and reproduction in little brown bats (Myotis lucifugus). Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology, 183 (2), 279-88 PMID: 22972361 

Fenton, M., & Barclay, R. (1980). Myotis lucifugus Mammalian Species (142) DOI: 10.2307/3503792

Veselka, N., McGuire, L., Dzal, Y., Hooton, L., & Fenton, M. (2013). Spatial variation in the echolocation calls of the little brown bat ( ) Canadian Journal of Zoology, 91 (11), 795-801 DOI: 10.1139/cjz-2013-0094