Disk-winged bats

(Thyropteridae)

Class Mammalia

Order Chiroptera

Suborder Microchiroptera

Family Thyropteridae

Thumbnail description
Tiny dark-backed bats with moist fleshy "suctioncups" at the base of the thumb and heel; the feet are tiny, and have the toes fused together; the tail membrane is large and obvious and the face long and delicately pointed

Size
Head and body length 1.2–2.3 in (3.0–5.7 cm); tail 0.9–1.3 in (2.4–3.3 cm); forearm 1.2–1.4 in (3.1–3.5 cm); weight 0.10–0.17 oz (3–5 g)

Number of genera, species
1 genus; 3 species

Habitat
Lowland rainforest, often by water

Conservation status
Vulnerable: 1 species

Distribution
Central America and northern South America

Evolution and systematics

There are no known fossil thyropterids. In a remarkable example of convergent evolution, bats have evolved foot suck-ers on several occasions. They may be found in Madagascar's endemic bat family, the Myzopodidae, and some vespertilionid bats (e.g., Eudiscopus denticulatus from Southeast Asia; the club-footed bats, Tylonycteris; and the thick-thumbed pipistrelles, Glishropus). None of these foot suckers, however, are as well developed as in Thyroptera. A larger third species of Thyroptera, T. laveli, is recognized by some authorities. Much darker furred than the other two species, it is known only from the type series of four individuals, collected near Loreto in the Peruvian Amazon. Apparently highly restricted in distribution, this little-known species is considered Vulnerable by the IUCN. Unlike other thyropterids, it may roost in canopy vegetation.

Physical characteristics

These small delicate bats have long fluffy fur and a long triangular muzzle, like the tip of an ice cream cone. There is no nose leaf, though there is a small wart-like projection between and above the nostrils. The ears are large and funnel-like. The front edge of the ears reaches forward, though not sufficiently to conceal the eyes as in the related families Natalidae and Furipteridae. The outer edge of the ear attaches to the head near the mouth. The eyes are small. The tail membrane is longer than the legs and comes to a point; the tail is longer than the membrane and pokes out beyond it. The disks are stalked and those on the thumbs are larger than those on the feet. Loosely translated, the family name means "disk foot" from the Greek thureos, a door-shaped shield, and "wing" from the Greek pteron.

Distribution

From Chiapas, Mexico, south through Central America across northern South America to the edge of Amazonian Brazil and Peru, and in Atlantic coastal forest, stopping at the Auricaria zone. Though widely spread in Central and South America, the genus does not occur west of the Andes.

Habitat

Evergreen forest and tall secondary growth. Absent from areas with a prolonged or pronounced dry season. On Costa Rica's Osa Peninsula, Thyroptera occur at densities of up to four colonies per 2.5 acres (1 ha).

Behavior

Disk-winged bats use their suction cups to cling to near-vertical smooth surfaces of the rolled-up leaves inside which they roost. While moving around a roost, neither the feet nor claws touch the smooth leaf surfaces, with the cups alone being responsible for grip. Situated on short stalks, the cups generate sufficient suction to allow captive Thyroptera to move easily and without slippage across a clean pane of glass. The suction is not generated passively; modified sweat glands in the disks produce a sticky secretion and there is also a tendon leading from a cartilaginous plate in the disk to muscles out-side it; this helps keep the shape appropriate. A bat will also lick its disks to aid adhesion. Using this combination of suction and wet adhesion, a single disk can support the bat's entire weight. Suction requires constant muscular expenditure to keep the cup in the right shape. This could be energetically demanding over a night's roosting. So, the wet adhesion may be an energy-saving device. Experiments have shown that, in specializing to roosting on smooth surfaces, Thyroptera has lost the ability to roost on rough surfaces (rock, bark) as most bats can and do. The roosting sites protect the tiny bats from the rain and from predators. In captivity, disk-winged bats with no suitable substrate will hang head-up by their thumb-claws, rather than try to suspend themselves from their tiny weak feet. Within the order Chiroptera, the suction cups of Thyroptera are the most specialized organs of their kind. Some African bats (Myotis bocagei and Glishropus nanus) also roost in rolled-up banana leaves. The latter has thickened pads at the wrist. These are reported to allow adhesion to the leaf surface. M. bocagei uses its fine sharp claws for attachment to the leaf.

Feeding ecology and diet

Disk-winged bats are insectivorous. They emit feeding echolocation calls that are low in intensity with broadband, multi-harmonic, and narrowband components.

Reproductive biology

A female moves to a maternity roost in a hollow log to give birth and wean her offspring. This species is most likely polygynous.

Conservation status

Thyroptera laveli is listed as Vulnerable by the IUCN; the other two species are not threatened.

Significance to humans

None known, beyond natural insect control.

Species accounts

List of Species

Spix's disk-winged bat

Thyroptera tricolor

taxonomy

Thyroptera tricolor Spix, 1823, Amazon River, Brazil. Three subspecies are recognized.

other common names

German: Haftscheiben-Fledermaus; Spanish: Murcielago de ventosas, murcielago tricolor con mamantones.

physical characteristics

Back (and sometimes throat) dark brown to reddish brown. Belly white or yellowish; flanks are frequently an intermediate color; ears blackish. Calcar has one cartilaginous bump. Tail long, extending (0.19–0.31 in [5–8 mm]) beyond the uropatagium. Females are slightly larger than males.

distribution

From Veracruz, Mexico, to southeast Brazil. Apparently absent from El Salvador and Nicaragua.

habitat

Individuals captured in a number of different habitats within rainforest, including primary forest, swamp, and man-made clearings. Not recorded above 4,265 ft (1,300 m), and usually below 2,625 ft (800 m).

behavior

All known roosts have been in foliage. In French Guiana, most roosts were found in unrolled new leaves of Heliconia plants or, to a lesser extent, of Phenakospermum. A smaller number of roosts were found in old, dead, scrolled leaves of Phenakospermum. Elsewhere, also recorded in rolled-up arrowroot (Calathea, Marantaceae) leaves. Most roosts are near water and none are out of direct sunlight. Preferred leaves form vertical tubes 1.9–3.9 in (50–100 mm) in diameter and do not touch any other vegetation (so reducing the danger of predation by snakes). Such roosts are ephemeral, because the leaves generally unroll within 24 hours. Consequently, the bats must find a new roost once every few days. A stable, socially cohesive group will sequentially occupy all favorable roosts in an area as they become available and defend their patch against other groups. Roosts are generally occupied by one to nine individuals; it is rare for there to be more than one adult male in a roost. Within the rolled-up leaf, individuals roost with the head pointing upward toward the opening, making for a swifter escape if danger threatens. When roosting together, individuals are aligned one above the other. A group's home range may average some 32,290 ft² (3,000 m²). Groups are clearly social.

feeding ecology and diet

Slow, fluttering, agile flight and a tendency to fly low indicate a diet of insects caught close to the ground. Small beetles and flies may be important diet components. Each individual consumes 0.03 oz (1 g) of insects a night (one-quarter of its body weight).

reproductive biology

Polygynous. Births probably occur during the peak of the rainy season. Gestation is about two months. Once born, young cannot fly for a month and the female flies and forages with them

conservation status

Not threatened.

significance to humans

None; roosting occurs in banana plants but does not endanger any economic interests.

Peters's disk-winged bat

Thyroptera discifera

taxonomy

Thyroptera discifera (Lichtenstein and Peter, 1855), Puerto Cabello, Carabobo, Venezuela. Two subspecies are generally recognized. Abdita means "hidden" and refers to the fact that the subspecies was described 84 years after the specimens were collected.

other common names

Spanish: Murcielago con mamantones de Peter.

physical characteristics

Dorsal fur brown or reddish, belly gray-brown or yellowish. Tail membrane (uropatigium) hairy for first half of its length; tail extends to 0.15 in (4 mm) beyond trailing edge of uropatagium. The calcar (the ankle spur that stiffens the uropatagium's trailing edge) is also characteristic, having one anterior nodule. The ears are yellowish.

distribution

Central America (few records), and northern South America (southeastern Brazil and northern Peru).

habitat

Evergreen forest and banana plantations.

behavior

Roosts underneath leaves, rather than within rolled-up ones. Uses bananas, heliconias, and palms.

feeding ecology and diet

Nothing known.

reproductive biology

Little known, but most likely polygynous. Disks and claws are not well developed in newborns. Consequently, females have broad strap-like nipples to which the young cling with their teeth. One youngster was observed to cling to its mother for 20 minutes without using its claws.

conservation status

Not threatened.

significance to humans

None known.

Resources

Books

Eisenberg, J. F., and K. H. Redford. Mammals of the Neotropics. Vol. 3, The Central Tropics: Ecuador, Peru, Bolivia, Brazil. Chicago: University of Chicago Press, 1999.

Reid, F. A. A Fieldguide to the Mammals of Central America and Southeast Mexico. Oxford: Oxford University Press, 1997.

Periodicals

Findlay, J. S., and D. E. Wilson. "Observations on the Neotropical Disk-winged Bat, Thyroptera tricolor Spix." Mammalia 55 (1974): 562–571.

Pine, R. H. "A New Species of Thyroptera Spix (Mammalia: Chiroptera: Thyropteridae) from the Amazon Basin of Northeastern Peru." Mammalia 57 (1993): 213–225.

Riskin, D. K., and M. B. Fenton. "Sticking Ability in the Spix's Disk-winged Bat Thryoptera tricolor (Microchiroptera: Thyropteridae)." Canadian Journal of Zoology 79 (2001): 2261–2267.

Organizations

Bat Conservation International. P.O. Box 162603, Austin, TX 78716 USA. Phone: (512) 327-9721. Fax: (512) 327-9724. E-mail: [email protected] Web site: <http://www.batcon.org>

Adrian A. Barnett, PhD