Ghost bat
The ghost bat is a species of bat found in northern Australia. It is the only Australian bat that preys on large vertebrates – birds, reptiles and other mammals – which they detect using acute sight and hearing, combined with echolocation, while waiting in ambush at a perch. The wing membrane and bare skin is pale in colour, their fur is light or dark grey over the back and paler at the front. The species has a prominent and simple nose-leaf, with large elongated ears that are joined at the lower half, sharp teeth for attacking prey and large dark eyes.
The first description of the species was published in 1880, since when its recorded range has significantly contracted.
Taxonomy
A species of Macroderma, one of several genera in the family Megadermatidae. The family all have large eyes, a nose-leaf and tragus, long ears joined at the base, and are also found in southern Asia and central Africa. The description was published in 1880 by George Dobson, emerging from an examination of specimens held by the Göttingen Museum. The author compared his specimen to taxa named as species of Megaderma, identifying species Megaderma spasma as the closest in resemblance. A revision of bat genera by Gerrit Smith Miller Jr. reassigned the species to a new genus Macroderma, creating the current generic combination. The name Macroderma gigas combines the Greek words macros and derma, due to the large size of their partially conjoined ears. The epithet gigas denotes it as the largest species in the family. The species M. gigas is placed with the Megadermatidae. An arrangement separating a "Pilbara district" population as subspecies Macroderma gigas saturata is noted as synonymous with this species concept.The specimen had previously been described by Gerard Krefft in a communication to the Zoological Society of London, and had been forwarded to the Göttingen museum by Dr. Schuette. The description and illustration were presented to the society in 1879, accompanied by the suggestion that it was an unnamed genus of Phyllostomatidae ; a member of the society, Edward Richard Alston, proposed instead it was a species of Megaderma which were unknown in Australia. The type locality is at the Wilson River near Mt Margaret in Queensland, where the collector, also named Wilson, obtained the bat. An earlier observation had been noted by Robert Austin in 1854 at Mount Kenneth while surveying the inland regions of Western Australia.
Studies of brain structures indicate that Macroderma gigas is an intermediate and divergent species of the insectivorous microchiropterans and the carnivorous species from South America.
Common names that refer to Macroderma gigas have included ghost bat, false vampire, false vampire bat, and Australian false vampire bat. The name ghost bat derives from its distinct colouring, the predominant colour of its fur may be near white or pale grey.
Description
A larger species of microchiropterans and the largest in Australia, the size is comparable to the megabat species. The fur colour is grey, ranging in tone from mid, sometimes dark, to very pale grey at the back and whitish at the ventral side and head. The colour of populations is a geographic cline, becoming darker toward the coastal regions. A ring at the eye is dark and colour of the wing membranes and other bare skin is pale and brownish. The interfemoral membrane extends the wing surface between the length of the legs. The great length of the ear, from notch at head to tip, is 44 to 56 millimetres and they are fluted in form; the inner margins of the ears are fused for half their length.The length of the prominent ears is twice that of the head, a seam at the inner margins stiffens these to keep them upright while in flight.
Other measurements of the species are a range for the forearm length, for the combined length of the head and body, and a weight range of.
The eyes are comparatively large and well developed for nocturnal vision, a feature shared with the long-eared genera Nyctophilus. The nose-leaf is also large and prominent, and simple in form. The nasal appendage is presumably similar in usage to the elaborate forms of the horse-shoe and leaf-nosed bat genera, manipulating, directing and receiving echolocation signals to detect prey. The lack of a tail, effectively absent, is a characteristic of the species. The membrane at the rear is, however, supported by a calcar at the ankle. The teeth and short robust jaw allow the consumption of a wide variety of animals, either the flesh and bones of other vertebrate species or hard shells of larger invertebrates. Ghost bats are known for their unique hunting behavior, which includes ambushing prey from perches and using their powerful jaws to capture and consume various vertebrates, including small mammals, birds, reptiles, and amphibians. A pregnant bat possesses two sets of teats, one pair beneath the armpits provide milk from the mammary glands and another pair at the pubic area. The pubic teats do not have a lactational function, rather they act as purchase points for the new-born to be carried in flight by the mother.
The voice is audible to humans, one sound resembles the avian species Petrochelidon ariel and transliterated as 'dirrup dirrup'. This twittering call is given when the bat is excited or before leaving the roost to feed. A chirp uttered by the species resembles crickets. The species is generally quiet, but some vocalisation is known in captivity when a squealing sound accompanies a squabble over food. The twittering sound was reported in captivity as hunger, and young will continually emit a chirp when the mother is absent.
Behaviour and diet
Although Macroderma gigas is inactive during daylight hours, they do not hibernate. The colony size reduces in the austral winter, increasing when they gather to breed or females form maternity groups.They leave the roost several hours after sunset, alone, in pairs, or as small groups. Hunting occurs via a 'sit and wait' technique while suspended from a tree or as low surveys over vegetation. The large ears allow the bat to hear prey moving on the ground. Field observations at Pine Creek using nightscope equipment reported seeing M. gigas suspended from a tree and dropping to catch large locusts detected moving through grass at ten to twenty metres distance. They are also able to detect the echolocation signals of the microbats on which they prey. The bat has good vision for a microchiropteran, and echolocation is also utilised to directly locate approaching prey. They are able to visually locate birds roosting in trees, and budgerigars are detected by their silhouettes against the evening light. The budgie is a favoured food of the bat, which they detect by the flock's chatter while retiring for the night, and these are taken to a perch to be consumed head first, with the feet and wing parts eventually being discarded.
Prey may be taken at or to the ground, where it is enveloped with the wings and killed with bites to the neck. The sharp teeth and strong jaws are able to subdue animals as large as the bar-shouldered dove, species Geopelia humeralis, that may weigh as much as 150 grams, although most prey is smaller. Bird and mammal remains are most frequently recorded in their scats. Once located, an animal is held down via the thumb claws and killed by a single bite to the neck. The prey is killed at the ground or in flight, and taken to a perch to be consumed, at an overhanging rock or smaller cave for this purpose.
The family Megadermatidae is carnivorous, feeding on vertebrate species including arthropod, mammalian, amphibian and reptilian prey. Large insects, small mice, other bats, other small mammals, small birds, frogs, Pygopodidae, geckos and snakes are also taken. Macroderma gigas is formally referred to as a specialised carnivore, but they have been known to feed on insects if prey is scarce. Vertebrate prey is eaten much more frequently and is usually consumed at the site of capture. Other species of microbats are taken in flight, including species of Taphozous, Saccolaimus, horseshoe bats and the little cave eptesicus. The microbat species consumed by M. gigas feed at a variety of habitat and altitudes, and includes those known to cohabit with this species at cave roosts.
A study of the avian prey of the bat revealed that over fifty species of birds are targeted, in a range of sizes but a preference for those weighing less than 35 grams. Birds that roost in flocks make up a large part of the diet, and a quarter of the species are non-passerines. One nocturnal species of bird is recorded at their middens, the Australian owlet-nightjar Aegotheles cristatus. The examination of the remains of their middens has given support to interpretation of fossil depositions, that have similar assemblages of discarded remains, at the Riversleigh formations where this and other species of Macroderma are exceptionally well represented.
Field workers report that the species is remarkably passive when handled, whilst other workers have recorded and confirmed reports of Macroderma gigas preying on rodents caught in their pitfall traps.
Distribution and habitat
The ghost bat is endemic to Australia. Three population centres are identified, the Northern Pilbara and Kimberley in Western Australia, the Top End of the continent and in Queensland. The distribution of colonies is non-contiguous, and they usually occur in small isolated pockets within each region. Analysis of fossil remains shows that distribution patterns within Australia changed, in waves of both expansion and contraction, and the probable cause was ecological changes resulting from the increasing aridity of the continent's climate. Remains of the species have been found with other mammals at the Devils Lair archaeological site in Southwest Australia, including other predators such as Thylacinus cynocephalus and Sarcophilus harrisii that inhabited the region after the first peoples had arrived. Little is known of the genetics of the ghost bat, although the colonies shows a high degree of genetic distinctiveness at a local and regional level.The southernmost record is Austin's 1854 note at Mt Kenneth. A limestone cave site favoured by M. gigas is in the region of the Mitchell and Palmer rivers at Cape York. They are occur around Shoalwater Bay at the east coast.
The species is well represented at Litchfield National Park, which provides important caves and habitat for a number of bat species in the northern regions near Darwin.
The largest recorded breeding colony of M. gigas is at a gold mine named Kohinoor at the Top End. The mine was dug in the late nineteenth century.
Another well known breeding site is found at Nourlangie Rock in Kakadu National Park, a region that is protected by law. It is also recorded in national parks at Mount Etna Caves NP and at Tunnel Creek where they cohabit with other bat species. Small colonies have been recorded along the Victoria River and at Camooweal Caves NP. The range extends in association with rocky cliffs, gorges, or outcrops along watercourses in the Kimberley region of northwest Australia.
Built environments may be used as feeding grounds, but the ghost bat selects daytime roosts in caves, sheltered rock crevices, boulder piles or disused mines; occupation of abandoned buildings is only occasionally reported. A preference is given to sites with a complex of shafts or cavities and several openings to the outside. Macroderma gigas favours these caves with multiple entrance ways as they are large enough to accommodate the greater wingspan of the species and allow an alternative exit when sensing a threat. They require several suitable sites for rest, feeding and reproduction, and change locations seasonally. The species is especially sensitive to disturbance by humans, and this contributes to the selection or abandonment of a roost site.
The bat and the caves they occupied were well known to peoples of Australia, often informing field workers of their locations in central Australia; some sites were part of 'men's business' that imparted a story of the being to young initiates.
Macroderma gigas is present in the fossil record of Australia and found at the Riversleigh World Heritage Area. The species was sympatric with others of the genus at Riversleigh. such as Macroderma malugara, and is the modern representative of a lineage that extends to at least the early Miocene epoch.