DISK-WINGED BATS: Thyropteridae

PHYSICAL CHARACTERISTICS

These bats are about the size of a person's thumb, having a head and body length that ranges from 1.2 to 2.3 inches (3 to 5.7 centimeters). They weigh from 0.10 to 0.17 ounces (3 to 5 grams), about the same weight as one to two pennies.

These bats are also called New World sucker-footed bats, named after the suction cup-like feature found on their feet. These bats have circular suction cup disks with short stalks on the soles of the feet and the bottom of their thumbs. The disks on the thumb are larger than those on the feet. They also have a well-developed claw on their thumb.

Bats in this family have small eyes. There is a small wart-like projection above the nostrils, and there is no noseleaf (leaf-shaped fleshy protrusion). The tail juts out freely past the membrane (thin layer of skin), so it is visible. The ears are large and shaped like a funnel. The muzzle is long and slender. Nostrils are circular and set relatively far apart.

Species in this family have long, fluffy hair. Fur color ranges from a medium reddish brown to slightly darker. The undersides of these bats are white or brown. The ears can be either black or yellow.

GEOGRAPHIC RANGE

These bats are found in Central and South America, east of the Andes, including southern Nicaragua to the Guianas and Peru, and southern Mexico to Bolivia and southern Brazil, and Trinidad.

HABITAT

Disk-winged bats live in the moist parts of forests. They are common in many areas, and in Costa Rica there are up to four colonies (groups) for every 2.5 acres (1 hectare). They generally roost (rest or settle) in a curled leaf of some plant, such as the heliconia plant or the banana tree, before the leaf opens.

DIET

Disk-winged bats eat insects.

BEHAVIOR AND REPRODUCTION

Disk-winged bats use only their suction-like disks to grip and stick to the smooth surfaces of the curled-up leaves in which they roost. They do not use their feet or claws to touch the surface of the leaves.

These bats can support their entire weight with the suction of a single disk. Sweat glands keep the disks' undersurfaces moist, which helps provide the vacuum seal for sticking to the surface. Beneath each disk is a muscle that controls the vacuum. This muscle can create the seal and, when the bat wants to come unstuck, the muscle also undoes the seal. These bats will also lick their disks to help with the suction. Studies have found that these bats have lost the ability to roost on rough surfaces, such as trees and rocks.

Generally only one or two disk-winged bats roost in the same leaf, yet observers have found as many as eight individuals in one leaf. Roosting inside curled leaves protects them from the weather and predators. Leaves open within days, and groups must change roosts often.

SUCKER COUSINS?

One of the puzzling features of the disk-winged bats is their relationship to the Old World sucker-footed bat. Both families of bats feature suction-like disks that allow them to grip onto vertical, smooth surfaces. Yet the Old World sucker-footed bat is found primarily in the rainforests of Madagascar, far away from where New World disk-winged bats are found. The Old World bat also has suction cups on its thumbs and roosts in young, rolled leaves, but their suction cups are thought to be the result of an evolutionary convergence with the suction cups of the New World bats. This means that both groups of bats evolved the suction cups separately, not as a result of their relationship to one another. Unfortunately, there is no fossil record for the New World disk-winged bats.

Like all bats, these bats are nocturnal, meaning that they are active at night. When more than one bat roosts in a leaf, these bats spread out evenly, one above the other. In Costa Rica, a study reported that group sizes ranged from one to nine, and averaged six bats. Generally, the same group moves together from one old leaf to a new roosting site. Bats in this family have been found roosting with bats in another family, the proboscis bat.

Unlike most other bats, individuals in this family typically hang with their head upward. Disk-winged bats use echolocation (eck-oh-loh-KAY-shun) to find prey (animals hunted for food) and detect objects. Echolocation is a process for locating objects by emitting, sending out, sounds, which are reflected back to the bat by objects in the sound's path.

Females roost together in hollow logs to give birth. Males in this family are thought to be polygynous (puh-LIJ-uh-nus), meaning they mate with more than one female during the mating season.

DISK-WINGED BATS AND PEOPLE

People have caused the decline in this family's population due to disturbing and destroying their natural habitat. Because they feed on insects, these bats eat many insects that people may consider pests.

CONSERVATION STATUS

Although these bats are common in some areas, the IUCN lists Thyroptera lavali as Vulnerable. In 1999, findings observed that Thyroptera lavali was restricted to a small area in extreme northeastern Peru.

SPIX'S DISK-WINGED BAT (Thyroptera tricolor): SPECIES ACCOUNT

Physical characteristics: Fur color of Spix's disk-winged bats ranges from dark brown to reddish brown. Their undersides are a cream or yellow, and their ears are blackish. The sides of their bodies are an intermediate color, which is why they were given the name tricolor, which means to have three colors. These bats weight about 0.14 ounces (4 grams). They have a head and body length combined of 1 to 1.5 inches (2.7 to 3.8 centimeters). Females are slightly larger than males.

Geographic range: Spix's disk-winged bats are found in tropical forests from Veracruz, Mexico to southeast Brazil.

Habitat: Spix's disk-winged bats have been found in rainforests, swamps, and clearings. They have generally been found living below 2,625 feet (800 meters) and have not been recorded living above 4,265 feet (1,300 meters).

Diet: Spix's disk-winged bats feed on insects, such as small beetles and flies. Spix's bats eat about 20 percent of its weight each night.

Behavior and reproduction: This species roosts in young, partly uncurled leaves. They are found roosting in leaves of heliconia plants, recognizable by their large leaves. Roosts contain about six individual bats, composed of one or more adult males, several females and several juveniles of both sexes. Female Spix's bats have been observed taking their offspring for the evening flight in search of food.

These bats are polygynous, meaning that the bats mate with more than one female at a time. These bats breed twice annually. Gestation (pregnancy) lasts about two months. For the first month of life offspring either remain in the roosts or cling to their mothers when they go out to feed, even though young can weigh up to 46 percent of the mother's weight. Offspring can generally fly after one month.

Spix's disk-winged bats and people: Aside from eating insects some people consider pests, these bats have no known significant relationship with people.

Conservation status: These bats are not considered threatened. ∎

FOR MORE INFORMATION

Books:

Fenton, M. Brock. Bats. New York: Checkmark Press, 2001.

Fenton, M. Brock. The Bat: Wings in the Night Sky. Buffalo, NY: Firefly Books, 1998.

Nowak, Ronald M. "Disk-winged Bats, or New World Sucker-footed Bats." Walker's Mammals of the World 5.1 Online. Baltimore: Johns Hopkins University Press, 1997. http://www.press.jhu.edu/books/walkers_mammals_of_the_world/chiroptera/chiroptera.thyropteridae.thyroptera.html (accessed on July 5, 2004).

Richardson, Phil. Bats. London: Whittet Books, 1985.

Ruff, Sue and Don E. Wilson. Bats. New York: Benchmark Books, 2001.

Schober, Wilfried and Eckard Grimmberger. The Bats of Europe and North America. Neptune City, NJ: T.F.H. Publications, Inc., 1997.

Periodicals:

Reebs, Stephen. "Sticky Situation." Natural History (June 2002): 16.

Milius, Susan. "Save Our Sounds: In Some Libraries, Noise is Good." Science News (September 14, 2002): 16.

Web sites:

Clarke, Diana. "Nomads of the New World Tropics." The Sun and Your Skin. http://yourskinandsun.com/article1091.html (accessed on July 5, 2004).

"Discover the Secret World of Bats." Bat Conservation International, Inc. http://www.batcon.org (accessed on July 5, 2004).

Weinstein, B., and P. Myers. "Family Thyropteridae (Disc-Winged Bats)." Animal Diversity Web.http://animaldiversity.ummz.umich.edu/site/accounts/information/Thyropteridae.html (accessed on July 5, 2004).