Bats are typically categorized into two main groups; megachiroptera (Old World Bats) and microchiroptera (Old and New World). The silver-haired bat, scientifically known as the Lasionycteris noctivagons, is a member of the microchiroptera group. Further classification places the silver-haired bat into the animalia kingdom (animals), the phylum chordate (vertebrates), the mammalian class (mammals), the order chiroptera (bats), and the Vespertilionidae family (vespertilionid bats) (Silver).
The silver-haired bat is a small bat that is recognized by the unique ³silvery² highlights that are found in the hair on the bats back. Despite there being over 900 different species of bats within the microchiroptera group, the silver-haired bat has become the focus of much research in recent years as it has been found to carry a unique strain of rabies that has been determined to be the cause of numerous deaths over the last few decades. The silver-haired bat is a medium-sized bat that when fully grown can range in length from two and ¾ inches to four and ¼ inches and the bat can range in weight from four grams to twelve grams (ttu.edu). The silver-haired bat is one of the more common species of bats and has been found to live in suitable areas in Alaska, southern portions of Canada, the northern tip of Mexico and all but the southern most states in the United States (unm.edu). Unlike most other species of bats which tend to hibernate during the colder months when flying insects are unavailable, the silver-haired bat is one of the few species which migrates during the colder months. During the spring and summer the silver-haired bat has been found to be distributed quite evenly throughout the region described above, however, during the late fall and winter months, the silver-haired bat is found to be much more abundant in the southern United States as well as the northern tip of Mexico only returning North once Spring returns (Silver).
One unique characteristic that bats have versus other members of the mammalian class is their method of reproduction. Bats mate using a very unique process referred to as delayed fertilization. Through this process, mating typically occurs from late August through October during the time of swarming at cave entrances (As most bats begin to hibernate for the winter months). The spermatozoa are then stored inside of the females reproductive tracts during the winter and ovulation does not occur until the spring following the bats return from a state of hibernation (Vespert). The silver-haired bat appears to follow this same pattern with peak sperm formation in late August and ovulation typically occurring in late April to early May. Silver-Haired bats, like other bats, typically produce one to two offspring with each reproductive effort and new-born bats are typically able to fly after three to four weeks and reach their full sexual maturity after their first summer (Nowak). The average life-span of the silver-haired bat has been found to be relatively low (Under 12 years) when compared to other temperate zone vespertilionids which have been known to live up to 20-30 years (Schmidt).
Another unique characteristic of the silver-haired bat are its roosting habits which also act as a limiting factor to potential growth in abundance of the species. The silver-haired bat has been found to be very specific with regard to areas in which it chooses to roost and it has been found that the bat almost exclusively roosts in areas of Old Growth within mixed coniferous and deciduous forest (batcon.org). Because silver-haired bats are dependant upon roosts in these areas of Old Growth, managing forests for diverse age structure and maintaining forested corridors are very important to these bats‹³Increasing pressure to use resources from forests, especially those of older stands, has increased the urgency for identifying characteristics of trees and surrounding habitat that are important in determining suitable roosts for bats² commented Burr J. Betts (Betts(1)). Studies have shown that the silver-haired bat typically require snag densities of at least 21 per hectare and many forest management practices have fallen well short of this figure (batcon.org). As for population densities, this figure is not well known for the silver-haired part as it typically tends to roost alone or in very small numbers in hollow trees, beneath the bark of trees, and even beneath rocks which makes estimates of density very difficult (Campbell). Lastly, the silver-haired bat is also said to have a very different, distinguishable call structure from most other bats with the only similarity being to that of the big brown bat (Betts(2)).
The silver-haired bat is an insectivore and feeds off primarily off of small, soft-bodied insects. Studies have shown silver-haired bats to have a diet that varies widely but includes flies, midges, leafhoppers, moths, mosquito¹s, beetles, crane flies, lacewings, caddis flies, ants, crickets, and occasional spiders (batcon.org). It is capable of catching these insects in one of two ways. The first method used by the silver-haired bat to catch prey is the most obvious, the bat uses its sharp teeth and take it into its mouth. The second method that the silver-haired bat uses to catch its prey is through the use of its tail which it is able to turn into a pouch-like compartment catching insects inside (Silver). As for predator natural predators, there is not much information available, however, there have been known cases of mortality amongst silver-haired bats resulting from predation by a skunk, a great-horned owl, and a rabid Lasiurus cinereus (Commonly known as the hoary bat).
As stated above, one reason why silver-haired bats have become so widely studied over the past few years is the unusually high number of rabies cases that are being linked to the unique strain of rabies that is found in the silver-haired bat. In December of 1998 it was concluded that a prisoner in a Virginia prison had died from rabies (Grady). There was no known contact between the prisoner and any rabid animal, however, other prison inmates reported seeing bats occasionally flying outside during the summer. Upon further investigation it was determined that the strain of rabies that killed the prisoner was one found only in two types of bats, the silver-haired bat and the eastern pipistrelle. Despite no formal knowledge of the prisoner having come in contact with a bat, it has been determined that due to the exceedingly sharp teeth and the very tiny claws of these bats, that one could be ³punctured² by either the claw or the teeth and not even realize it. This was the case with a Connecticut woman who felt something brush up against her leg in her bathroom and upon turning on the lights she noticed a bat on the ceiling. Despite the woman telling her doctor that the bat merely brushed against her, upon closer look of the area through a magnifying glass, two small punctures were found about a fifth of an inch apart which were said to have probably come from a silver-haired bat (Messenger). This proof defied earlier belief that a bat could not bite you without you knowing it. Since 1981 there have been twenty-five cases in which people have contracted rabies in the United States. Twenty-two of those twenty-five cases contained strains of the rabies virus that could only come from bats and of those twenty-two cases, sixteen had the silver-haired bat strain which killed the Virginia man. In only one of those cases did the patient actually report being bitten by a bat, in most cases family members or patients themselves merely recalled that a bat had gotten into their house or workplace but that they had no never came into actual contact with the bat (Grady). The first well documented human death due to rabies associated with an insectivorous bat took place in 1958 and was also the first case reported to involve a silver-haired bat (Messenger). To date, all rabies in humans that have been associated with the strain of rabies found in the silver-haired bat have taken place in the northwestern and north-regions of the country. Experts have attributed the isolation of cases in these regions due to the fact that the silver-haired bat is more abundant in the northern regions during the summer months in which rabies is typically more prominent as well as to the fact that most rabid young bats would most likely not survive the migration South and as a result there were be a lower probability of cases occurring South (Messenger). The large number of rabies cases that have been attributed to the strain of rabies found in silver-haired bats (Referred to herein as the Ln strain) has intrigued scientists and prompted several recent studies and the Center for Disease Control (CDC) formally recognized silver-haired bats as a leading cause of rabies on December 1st, 2003 (Bats). In several of these recent studies it has been hypothesized that the Ln strain has ³evolved genetic changes that may allow a higher likelihood of infection after superficial contact² (Messenger). After conducting a series of experiments comparing the strains of rabies virus found in silver-haired bats, domestic dogs, and coyotes it was found that the rabies virus from the silver-haired bat grew to higher titers (Concentration of a substance in a solution) in epithelial and muscle tissue and they also discovered changes in the molecular structure of the glycoprotein which may be linked to increased infectivity.
The unique roosting habits of silver-haired bats as well as the unusually large number of rabies cases attributed to them have led to numerous recent studies, however, these studies are sure to be followed by countless others as more information is discovered that could lead to explanations of the Œstrange¹ connection between these bats and the deadly rabies virus.
Bats & Rabies. 1 Dec. 2003. Centers for Disease Control and Prevention. 21 Feb. 2004 <http://www.cdc.gov>.
Bat Species: U.S. Bats: Lasionycteris noctivagons. Bat Conservation International. 21 Feb. 2004 <http://www.batcon.org>.
Betts, Burr J. "Effects of Interindividual Variation In Echolocation Calls On Identification Of Big Brown And Silver-Haired Bats." Journal of Wildlife Management 62 (1998): 1003-1009.
Betts, Burr J. "Roosts Used By Maternity Colonies Of Silver-Haired Bats In Northeastern Oregon." Journal of Mammalogy 79 (1998): 643-650.
Campbell, Lori A., James G. Hallett, and Margaret A. O'Connell. "Conservation of Bats in Managed Forests: Use of Roosts by Lasionycteris Noctivagans." Journal of Mammalogy 77 (1996): 976-983.
Conservation Assessment for the Silver-Haired Bat in the Black Hills National Forest, South Dakota and Wyoming. Comp. Cheryl A. Schmidt. Apr. 2003. USDA Forest Service. 21 Feb. 2004 <http://www.fs.fed.us>.
Grady, Denise. "A Bat's Swift Bat, Unfelt, That Could Bring Rabies." New York Times 19 Mar. 1999, Late ed., sec. F: 6+.LexisNexis. 21 Feb. 2004 <http://web.lexis-nexis.com>.
Messenger, Sharon L., Charles E. Rupprecht, and Jean S. Smith. "Emerging Epidemiology of Bat-Associated Cryptic Cases of Rabies in Humans in the United States." Clinical Infectious Diseases 35.6 (2002): 738-748. 21 Feb. 2004 <http://www.lib.umd.edu>.
Nowak, Ronald M. Walker's Bats of the World. Baltimore: Johns Hopkins UP, 1994. 191-192.
Silver-haired Bat - Lasionycteris noctivagans . 26 Jan. 1998. The University of New Mexico . 21 Feb. 2004 <http://sevilleta.unm.edu/>.
Silver-haired Bat. Discover Life In America. 21 Feb. 2004 <http://www.dlia.org>.
Silver-Haired Bat. National Science Research Laboratory. 21 Feb. 2004 <http://www.nsrl.ttu.edu>.
Vespertilionidae - Bats. Discover Life In America. 21 Feb. 2004 <http://www.dlia.org>.