Hammer-headed bat

Taxobox
name = Hammer-headed Bat
status = LR/lc
regnum = Animalia
phylum = Chordata
classis = Mammalia
ordo = Chiroptera
familia = Pteropodidae
genus = "Hypsignathus"
genus_authority = Allen, 1861
species = "H. monstrosus"
binomial = "Hypsignathus monstrosus"
binomial_authority = Allen, 1861The hammer-headed bat ("Hypsignathus monstrosus"), also known as the big-lipped bat, is widely distributed in equatorial Africa. This large bat is found in riverine forests, mangroves, swamps, and palm forests at elevations less than 1800 meters.

Taxonomy

The hammer-headed bat is a member of the family Pteropodidae. Pteropodidae is divided into two subfamilies, Macroglossinae which contains six genera, and Pteropodinae containing thirty-six genera including "Hypsignathus". The family Pteropodidae is found within the suborder, Megachiroptera. This group is commonly referred to as the mega bats or flying foxes.

There is no fossil record of "H. monstrosus", however the oldest Megachiropteran fossil dates back to the Oligocene epoch in Italy. [Disputed] (from Flying Foxes: Fruit and Blossom Bats of Australia, Hall & Richards 2000, p6), the Italian fossil evidence was destroyed in WWI and only a cast remains. Also from the same reference the first Megachiropteran fossil is from Thailand in the late Eocene.

Physical description

The hammer-headed bat is the largest bat in Africa with a wingspan of 686 to 970 millimeters. Males are significantly larger than females, averaging 400 grams while females average only 275 grams.

Pelage is grey-brown to slaty-brown with a whitish collar of fur extending from shoulder to shoulder. The flight membranes are brown and the ears are dark brown with a tuft of white fur at the base. The face is dark brown with a few long, stiff whiskers around the mouth.

The skull may be diagnosed by specific dental features. The second premolar and molars are markedly lobed. This feature is specific for this genus, and no other African fruit bats have this characteristic.

There is extreme sexual dimorphism in this species. The male possesses an enormous head for producing loud honking calls. The enlarged rostrum, larynx and lips allow these sounds to be extremely resonant. The larynx is one half the length of the vertebral column and fills out most of the thoracic cavity. It is nearly three times larger in males than females. The male also has a hairless split chin and warty rostrum with wrinkled skin around it. Females have a much more fox-like appearance similar to most fruit bats.

Ecology and behavior

Hammer-headed bats are frugivores. Figs make up much of their diet, but they may also include mangos, bananas and guavas. There are some complications inherent in a fruit diet such as insufficient protein intake. It is suggested that fruit bats compensate for this by possessing a proportionally longer intestine compared to insectivorous species. This enhances their ability to absorb protein. They also have very rapid digestive systems allowing these bats to assimilate high amounts of fruit to ensure that adequate protein is absorbed. It is also suggested that by eating a wide variety of fruits with varying protein contents, fruit bats are able to maintain an entirely frugivorous diet. Generally fruit is picked and taken to a nearby tree where it is chewed, the juice squeezed out and the pulp discarded. Since they often do not consume the pulp, these bats are not considered to be good seed distributors.

Males may forage long distances (up to 10 km) to locate the highest quality food. Females rely on established feeding routes that offer a constant supply of lower quality food. This may reflect different metabolic requirements based on body size differences.

Large bats often experience difficulties with overheating during flight. The limited thermoregulatory capabilities of flying bats appears to be one factor closely associated with why flight activity primarily occurs during cooler nocturnal temperatures. It has been found that hammer-headed bats are able to tolerate higher ambient temperatures during flight than other bats. This ability is associated with this bat’s high thermal conductance (C_f) which is defined as the total heat loss less the heat loss due to evaporation divided by body temperature less the ambient temperature (C_f = [H₁ – H_e] / [T_b – T_a] ). However, they are especially sensitive to ambient temperatures below 11ºC and a decrease in flight coordination is seen. Due to the large surface area of the wing, convective heat loss to cool air may be significant enough to chill flight muscles preventing the precise coordination essential for flight.

These bats are nocturnal, roosting during the day in the forest canopy. They rely on camouflage to hide them from predators. Specific species of trees are not selected for roosting, however some roosts may be used for long periods of time. Roosts are generally 20–30 meters from the ground.

The main predators of this species are humans and nocturnal and diurnal birds of prey. However, infection by parasites is often the most significant problem for the hammer-headed bat. Adults are often infected with mites and the hepatoparasite, "Hepatocystis carpenteri".

Reproduction and mortality

Little is known about reproduction in hammer-headed bats. In some populations breeding is thought to take place semi-annually during the dry seasons. The timing of the dry season varies depending on the locality, but in general there are two breeding seasons, one from June to August and the other from December to January. However, in other populations, breeding is not restricted to dry seasons and occurs during all months of the year.

This species is often cited as a classic model of lek mating. In this type of mating system, males cluster in dense groups at specific locations known as arenas. In some populations of hammer-headed bats, males gather along rivers at night and display by rapid wing flapping accompanied by loud vocalizations. An arena may contain from 25 to 132 males. Females fly through the arena assessing the males. Once the female’s choice is made, the female lands on the branch and sits beside the male. Once chosen, the male emits a buzzing call and copulation ensues.

However, some populations of hammer-headed bats do not use the lek mating strategy. Males actively display but are not found clustering in groups.

This species is highly polygamous. Some estimates suggest that as few as six percent of the males in a population account for up to seventy-nine percent of the matings.

Females generally produce one offspring at a time. Neither gestation nor time until weaning have been reported for this species. Females mature more quickly than males and are sexually mature after six months. They continue to grow and reach adult size at nine months. Males do not reach sexual maturity until approximately eighteen months of age and they do not obtain their unique facial morphology until twelve months.

This bat is rather long-lived. The average life expectancy in the wild is thirty years.

Conservation

"H. monstrosus" has no special conservation status. In 2004, the IUCN Red List of Threatened Species listed this species as LC (least concern) on the basis of its widespread distribution and lack of threats to its habitat.

Interactions with humans

Due to this bat’s diet of fruit, it may be considered a crop pest. It has also been observed to attack live chickens. This observation was reported once and cannot be considered a regular occurrence. Humans hunt this large bat and consume it as bushmeat.

The hammer-headed bat is one of three species of African fruit bat found to be asymptomatically infected with the Ebola virus. It is not known whether these species are incidental hosts or a reservoir of "Ebolavirus" infection for humans and other terrestrial mammals.

Images

Hammer-headed Bat: http://www.worldwildlife.org/wildworld/profiles/photos/at/at0129aS.html

Male facial morphology: http://noorderlicht.vpro.nl/artikelen/25193751/

Skull (Male & Female): http://www.biolib.cz/cz/image/id7193 http://www.biolib.cz/cz/image/id7191

References

1. Boland, J. 2003. "Hypsignathus monstrosus" (On-line), Animal Diversity Web. Accessed December 03, 2006 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Hypsignathus_monstrosus.html

2. Carpenter R E. 1986. Flight physiology of intermediate-sized fruit bats (Pteropodidae). Journal of Experimental Biology. 120:79-103.

3. Courts S E. 1998. Dietary strategies of Old World fruit bats (Megachiroptera, Pteropodidae): how do they obtain sufficient protein? Mammal Rev. 28(4):185-194.

4. Deutsch J C. 1994. Lekking by default: Female habitat preferences and male strategies in Uganda Kob. The Journal of Animal Ecology. 63(1):101-115.

5. Eisenberg J F and Wilson D E. 1978. Relative brain size and feeding strategies in the Chiroptera. Evolution. 32(4):740-751.

6. Langevin P and Barclay R M R. 1990. "Hypsignathus monstrosus". Mammalian Species. The American Society of Mammalogists. 357: 1-4.

7. Mickleburgh S, Hutson A, Bergmans W and Fahr J. 2004. "Hypsignathus monstrosus". In: IUCN 2006. 2006 IUCN Red List of Threatened Species. http://www.iucnredlist.org/. Downloaded on 17 September 2006.

8. Miltgen F, Landau I and Bradbury J. 1980. Hepatocystis of "Hypsignathus monstrosus" (Pteropinae) in Gabon. II. Description of "Hepatocystis carpenteri" n. sp. Ann Parasitol Hum Comp. 55(5):485-90.

9. Truxton G T. 2001. The calling behavior and mating system of a non-lekking population of "Hypsignathus monstrosus". Ph.D. dissertation. State University of New York at Stony Brook. 240 pages.