This is the largest family of bats: it includes 35 genera and 318 species! With this many species there are exceptions to almost every generalization about this family.
To make the family more manageable to taxonomists, some authors split it into subfamilies. Unfortunately, there seems to be little agreement about the composition of these taxa. One scheme (Wilson and Reeder, 1993) employs the following subfamilies: Kerivoulinae (genus Kerivoula), Murininae ( Murina and Harpiocephalus, both with tube-like nostrils), Miniopterinae (genus Miniopterus, these have very long 3rd fingers), Tomopeatinae ( Tomopeas, with thick, leathery ears like the molossids), Vespertilioninae (all the other genera).
Vespertilionids, or evening bats, have small eyes, no noseleaf (rudimentary in Nyctophilus and Pharotis), and ears with both a tragus (fleshy ear outgrowth) and an anterior basal lobe (except Tomopeas). Their tails are relatively long and extend to the edge of the tail membrane or beyond. This large family includes a wide range of sizes. Some vesper bats weigh only 4 grams as adults, whereas others weigh up to 50 grams. Most of these bats are black or brown colored, but some are orangish or have other markings.
Morphologists identify vespertilionid bat skulls by the unfused premaxilla, the trend towards short jaws, and the absence of any postorbital process. The number of teeth in this family varies from 28 to 38 depending on the species. The dental formula is 1-2/2-3, 1/1, 1-3/2-3, 3/3. The incisors are small and separated right from left, and the premolars and molars are dilambdadont. These bats are primarily insectivorous, and most hawk insects in flight, often using their wings like tennis rackets and swatting the insects into the tail membrane. Some species eat fish ( Pizonyx, Myotis spp.), and most species defend a feeding territory.
Many vespertilionids live in caves, but these bats can also be found in mine shafts, tunnels, tree roosts, rock crevices, buildings, etc. Some species contaminate human habitations with feces and noise, but this annoyance is more than offset by the bats' consumption of huge quantities of insects. Some species roost in large colonies, but others are solitary or live in small groups or pairs. Males and females tend to roost apart most of the year, and some species have maternity colonies.
The mating system varies widely within the vespertilionids. Many species show resource defense polygyny, the most common mating system in bats. Other vespertilionid mating systems include promiscuity in Myotis lucifugus and lekking in some Miniopterus species.
This group of bats has a worldwide distribution. Vesper bats live in tropical forests, deserts, and temperate zones -- only polar regions and some remote islands are vespertilionid-free. The vespertilionid genus Myotis has the widest distribution of all bat genera. Temperate zone bats are faced with a dilemma when cold weather drives insects away. Some species ( Lasiurus, Lasionycteris, Nyctalus, some Pipistrellus) migrate to areas where there is more food, while many other species of vespertilionids hibernate.
Hibernating bats lower their body temperatures and remain inactive for several days or even months at a time. They often hibernate in caves where the ambient temperature does not fall below freezing. These bats then drop their body temperature to as low as 2 C. Since maintaining a high body temperature requires using up many calories, these bats save substantial energy by lowering their body temperature in hibernation. These bats survive the winter by living off fat stores and making occasional foraging trips during warmer weather. Hibernation is known in at least some species of the following genera: Antrozous, Eptesicus, Miniopterus, Myotis, Nyctalus, Pipistrellus, and Plecotus.
Many hibernating species and a few non-hibernators delay giving birth until spring or summer, when food is plentiful. Vespertilionid bats typically mate in the fall. They then postpone birthing by three different methods. Females from many species of vespertilionids store sperm in the uterus from fall matings throughout the winter. The females ovulate in the spring and give birth in late spring or early summer. Miniopterus females do not store sperm; instead, they delay implantation of the blastocyst, which remains free in the female reproductive tract until environmental conditions improve. Finally, in some Eptesicus, Miniopterus and Myotis females, the blastocyst implants in the uterus, but further development is delayed until spring.
The reproductive and physiological adaptations described above allow vespertilionid bats to succeed in temperate as well as tropical zones. This family shows extraordinary variability in mating, reproductive, and seasonal strategies. Vespertilionids also show a wide range of wing shapes, roosting behavior, and foraging strategies. These bats have been successful world wide.
Michigan is home to eight species of bats, shown here in dorsal, ventral, and lateral views. An additional species (the evening bat, Nycticeius humeralis) has been captured in the state, but it is at the limit of its northern distribution in the Great Lakes region and may not reside here regularly.
Literature and references cited
Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. xii+563pp.
Fenton, M. B., P. Racey, and J.M. V. Rayner (eds.), 1987. Recent Advances in the Study of Bats . Cambridge University Press, Cambridge.
Hill, J. E., and J. D. Smith. 1984. Bats, a Natural History. University of Texas Press, Austin. 243 pp.
Koopman, K.F. 1984. Bats. Pp. 145-186 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.
Nowak, Ronald M., 1994. Walker's Bats of the World . Johns Hopkins University Press, Baltimore.
Ransome, Roger, 1990. The Natural History of Hibernating Bats . Christopher Helm, London.
Vaughan, T. A., J. M. Ryan, N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia. vii+565pp.
Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.
Laurel Hester (author), Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
uses smells or other chemicals to communicate
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
having the capacity to move from one place to another.
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate