Molossidae

(vertebrate zoology) The free-tailed bats, a family of tropical and subtropical insectivorous mammals in the order Chiroptera.

(Molossidae)

Class: Mammalia

Order: Chiroptera

Suborder: Microchiroptera

Family: Molossidae

Thumbnail description
Free-tailed and mastiff bats, with a thick tail extending well beyond the tail membrane and long, narrow wings adapted for rapid flight in open airspace; a broad snout, often with wrinkles, which projects well over the lower lip; ears usually somewhat flattened and stiff, lying low over the head, and tilted forward; body covered with short, dense fur

Size
Small- to large-sized with forearms ranging 1.1–3.4 in (2.7–8.5 cm) in length and weighing 0.2–3.8 oz (5–167 g)

Number of genera, species
12 genera; 90 species

Habitat
Tropical, subtropical, and warmer temperate regions; arid as well as moist, forested habitats

Conservation status
Critically Endangered: 3 species; Endangered: 1 species; Vulnerable: 15 species; Lower Risk/Near Threatened: 21 species; Data Deficient: 4 species

Distribution
Throughout the warmer parts of the world, on all continents except Antarctica, on the Malay Archipelago and southern Pacific islands east to Fiji, and throughout much of the Caribbean

Evolution and systematics

Fossils of the family date from the late Eocene in Europe, late Oligocene or early Miocene in South America, Miocene in Africa, and Pleistocene in Asia, Australia, North America, and the East and West Indies. Morphological and molecular data place free-tailed and mastiff bats in the superfamily Vepertilionoidea allied with the vespertilionid bats (Vespertilionidae) and the funnel-eared bats (Natalidae). Other authors place them in the superfamily Molossoidea.

Physical characteristics

Free-tailed and mastiff bats (molossids) are small- to large-sized bats, distinctly characterized by a thick tail that protrudes well beyond the tail membrane. The tail is not so long as in the mouse-tailed bats (Rhinopomatidae). Most species have a broad face, with a wrinkled snout and lips, and the snout projects well over the lower lip. Their eyes are small. Their umbrella-like ears vary in size, but typically the ears lie low over the head, are stiff, rounded, and project forward, often with a lateral orientation. The ears are often connected over the forehead. These bats have long, narrow wings, and the wing and tail membranes are tough and leathery. The hind legs are short and strong and the feet are broad and fringed with long bristles. Some species are well endowed with glands on their chins, throats, and chest regions, and they often have a distinctive odor. Most free-tailed and mastiff bats have short brown fur, but in some, the fur is gray or black. Males of some species of the genus Chaerephon develop head crests of erectile hair during the mating season, while others have tufts of hair associated with chest glands. Members of the genus Cheiromeles (one or two species) are almost completely hairless. These "naked" bats also have wing pouches, or flaps of skin along the sides of the body, into which they place their folded wings. A few crevice-roosting species in the genera Molossops and Mormopterus have distinctly flattened skulls.

Distribution

Except for bats of the family Vespertilionidae, molossids have the widest distribution of any family of bats. They are found throughout the warmer parts of the world, including southern Europe, much of Africa, southern Asia, Malaysia, Australia, the Australasian region east to Fiji, in central and southern North America, Central America, the Caribbean Islands, and all except the southern-most portion of South America.

Habitat

They occur in a wide range of habitats and are common in natural, rural, and urban areas. They reach their greatest abundance in arid and semi-arid habitats. Natural roosting sites include caves, rock crevices, tree cavities, bark, rotting logs, foliage, and holes in the ground (Cheiromeles). These bats also commonly roost in human-made structures, including buildings, mines, tunnels, culverts, under bridges, and in bat houses. They are commonly found under corrugated steel roofs, roof tiles, or in attics of tropical houses, and they tolerate high temperatures that can exceed 130°F (55°C).

Behavior

Molossids tend to be active throughout the year. Populations of Brazilian free-tailed bats (Tadarida brasiliensis) in temperate, seasonal habitats are known to engage in long-distance, annual migrations that exceed 800 mi (1,300 km). However, other populations of the same species are known to remain in place or to engage only in short-distance seasonal movements and to utilize torpor to survive cold temperatures during relatively mild winters. Most molossids are colonial, with colony sizes typically reported as a few tens to a few hundreds of individuals. There are some reports of solitary bats, and numerous accounts of colonies into the thousands of bats. Brazilian free-tailed bats in the southwestern United States and northern Mexico form cave colonies of tens of millions of bats, which are the largest known aggregations of mammals. The behavior of this large family of bats is characterized by diversity and plasticity.

Feeding ecology and diet

These strong-flying bats typically pursue insects in open, uncluttered airspace above the canopy and they can fly to high altitudes. Studies in Africa, Australia, and North America document foraging by molossids at altitudes of several hundred feet (meters) above ground level. Radar shows that Brazilian free-tailed bats fly to altitudes of up to 2 mi (3.2 km) over central Texas, and research has confirmed that large numbers of these bats are actively feeding on insects at altitudes of at least 4,000 ft (1,219 m) above the ground. Molossids detect and pursue insects using relatively low-frequency echolocation calls (typically <30kHz, but <10kHz in some species) that travel long distances in open airspace. Recent studies suggest remarkable diversity in their echolocation calls.

Molossids are known to forage in groups and to exploit patches of insects such as emerging swarms of termites, winged ants, and large migratory populations of moths. They also forage around streetlights that attract concentrations of insects. These bats prey on a great variety of insects. Recent studies of Brazilian free-tailed bats document that their insect prey consists of at least 12 orders and 35 families of insects. Moths (Lepidoptera) and beetles (Coleoptera) provide the bulk of their prey, but all evidence indicates that Brazilian free-tails, in particular, and molossids, in general, are highly opportunistic feeders that exploit a diverse diet of insects.

Reproductive biology

Little is known regarding mating behavior of most molossids. Chaerephon pumila is reported to roost and mate in stable harem groups of about 20 females attended by a single male. Evidence suggests that Tadarida brasiliensis mates promiscuously during a brief period in spring when males and females assemble at specific sites. Many reports of the use by molossids of low-frequency vocal communication, the abundance of scent glands, and the existence of obvious structures for social displays such as head crests all suggest that molossids engage in a diversity of social interactions and mating systems that are, as yet, unstudied.

Females of most species appear to give birth to a single young annually. However, some species are reported to be polyestrus, giving birth twice (Molossus ater and M. molossus) or three times (Chaerephon pumila) annually, in parts of their geographic ranges. Cheiromeles is additionally unique among mollosids in giving birth to twins during a single annual reproductive period. Where known, gestation is usually two to three months in length, and the period from birth to weaning typically lasts five to six weeks. Studies on milk composition and reproductive energetics in Brazilian free-tailed bats demonstrate an extremely high-fat content in the milk of females, allowing for the rapid growth of their young. During the period of peak lactation, it is estimated a lactating female Brazilian free-tailed bat has energy demands of 106 kj/day to meet her needs and those of her growing young, requiring that the female consume approximately 70% of her body weight in insects each night.

During pregnancy and lactation, females typically roost in maternity colonies, separated from adult males. But, even in the largest maternity colonies that contain tens of millions of individuals, females relocate and selectively nurse their own young. The mating system is not known for all species, but most are thought to be polygynous.

Conservation status

Extremely fragmented or limited distributions, coupled with near-term concern for the integrity of critical habitat, are responsible for the listing of Chaerephon gallagheri, Mops niangarae (both African), and Otomops wroughtoni (Asian) as Critically Endangered, and for the listing of Mormopterus phrudus (South American) as Endangered. All 15 species listed as Vulnerable are considered to be at risk because of projected or suspected loss and degradation of habitat, and/or because populations are small and restricted in their area of occupancy. Thus, the loss of habitat, coupled with restricted distributions, contributes the most concerns for the conservation of these bats. Although it still forms the largest aggregations of mammals in existence, Tadarida brasiliensis is the only bat listed on Appendix I (Endangered) of the Bonn Convention of Migratory Animals. This listing results from the extreme vulnerability of these bats to so many individuals being aggregated at only a limited number of sites, and the fact that many formerly huge cave colonies in the United States and Mexico either no longer exist or have suffered severe reductions in the populations. Habitat destruction, disturbance, vandalism, and poisoning from pesticides are the major risks to these bats.

Significance to humans

Where they are abundant, molossid bats can provide important service to humans by consuming huge numbers of insects that are agricultural pests. The 100 million Brazilian free-tailed bats that occupy Texas each summer consume an estimated 1,000 tons of insects each night, with many of these insects known to be adult cotton bollworms, fall armyworms, and other moths that are major crop pests. The guano of molossid bats that live in large colonies is harvested commercially by local farmers as a rich source of nitrogen for fertilizer.

As with all mammals, bats can contract and transmit rabies virus. The rabies virus associated with Brazilian free-tailed bats has been implicated in the deaths of approximately 12 people in North and South America over the last three decades. Other human health concerns involve Histoplasma capsulatum, a fungus that commonly grows in bat (and bird) guano that can infect humans and cause histoplasmosis, typically of the human respiratory system via inhalation. The habits of molossids of roosting in houses and other buildings may result in human contacts and their risks of exposure to rabies or histoplasmosis.

Species accounts

Resources

Altringham, John D. Bats: Biology and Behaviour. Oxford: Oxford University Press, 1996.

Barbour, R. W., and W. H. Davis. Bats of America. Lexington: University of Kentucky Press, 1969.

Churchill, Sue. Australian Bats. Sydney: New Holland Publishers, 1998.

Crichton, E. G., and P. H. Krutzsch, eds. Reproductive Biology of Bats. London: Academic Press, 2000.

Hill, J. E., and J. D. Smith. Bats: A Natural History. Austin: University of Texas Press, 1984.

Hutson, A. M., et al. Microchiropteran Bats: Global Status Survey and Action Plan. Cambridge, UK: International Union for the Conservation of Nature and Natural Resources, 2001.

Neuweiler, Gerhard. The Biology of Bats. Oxford: Oxford University Press, 1998.

Nowack, R. M. Walker's Bats of the World. Baltimore: Johns Hopkins University Press, 1991

Wilson, Don E., and DeeAnn M. Reeder, eds. Mammal Species of the World: A Taxonomic and Geographic Reference. 2nd edition. Washington, DC: Smithsonian Institution Press, 1993.

Jones, Kate E., et al. "A Phylogenetic Supertree of Bats (Mammalia: Chiroptera)." Biological Reviews 77 (2002): 223–259.

Bat Conservation International. P.O. Box 162603, Austin, TX 78716 USA. Phone: (512) 327-9721. Fax: (512) 327-9724. E-mail: batinfo@batcon.org Web site:

The Bat Conservation Trust. 15 Cloisters House, 8 Battersea Park Rd., London, SW8 4BG UK. Phone: 020 7627 2629. Fax: 020 7627 2628. E-mail: enquiries@bats.org.uk Web site:

IUCN Species Survival Commission, Chiroptera Specialist Group. Web site:

[Article by: Gary F. McCracken, PhD]