Cave Dwelling Bats of East Gippsland
Bats are one of the most fascinating mammals in the world. They are divided into two groups. Megabats (megachiroptera) are the flying foxes or fruit bats. They eat fruit, nectar, blossoms and the like, and use sight to search for food and to navigate. Microbats (microchiroptera), of which almost all species eat insects, mainly use echo location to navigate. Although the subject of some debate, these two groups appear to have developed the ability to fly by independent evolutionary paths. The microbats evolved many millions of years ago and have filled their ecological niche so neatly that they have barely changed since.
Of the 14 species of microbats found in East Gippsland, three are trogloxenes – they spend the daylight hours in caves. The remaining 11 species are forest dwelling bats which roost in hollows inside trees during the daylight hours.
Distribution of cave dwelling bats in Victoria
The cave dwelling bats in East Gippsland are quite small – any of the three species would easily fit into a child’s cupped hand. Despite their diminutive size these bats have a major effect on insect populations because of their voracious appetites and their large numbers.
Contrary to popular belief, bats are not blind. They have perfectly good eyesight which they use for broader scale navigation. In common with other microbats, the cave dwelling bats found in East Gippsland use a specialised sense called echo location to navigate within the total darkness of caves and to catch insects at night. Echo location, as its name suggests, involves the bat sending out an ultrasonic call, from 15 kHz to 150 kHz, depending on species. (The human ear cannot hear above about 18 kHz.) The bat interprets the echoes it receives through its well developed ears, to determine the size, nature and location of objects around it. The Eastern Horseshoe Bat has a specially developed nose-leaf (which looks not unlike a horseshoe from the front), which helps concentrate the sonar signals into a narrow beam.
Cave dwelling bats in East Gippsland go through distinct phases each year, during which they require caves and roost sites with different characteristics. In general, mating takes place in late autumn but insect populations are reduced in winter, so the development of the foetus is suspended for several months while the bats enter a period of torpor. During this time their internal temperature is only a few degrees above the cave temperature and they remain inactive for days at a time. The torpid bats are vulnerable during this phase because, if they are disturbed in their overwintering caves, they will use up vital energy reserves and may not survive.
In spring, insect populations grow and the bats recommence hunting. Once the pregnant females have built up sufficient energy reserves for foetus development to proceed, they migrate to their maternity caves, of which there are only a few in the whole of Victoria. There they give birth and suckle the entirely helpless baby bats. The pink hairless young bats huddle together, hanging upside down in the highest, and warmest, part of the cave. Disturbance at this time can cause many young bats to die.
The three species of cave dwelling bats in East Gippsland are:
- Common Bentwing Bat (Miniopterus schreibersii)
- Eastern Horseshoe Bat (Rhinolophus megaphyllus)
- Large Footed Bat (Myotis macropus)
As their name suggests, these are the most widespread cave dwelling bats in the world but in East Gippsland they are near their most southerly limit. The young are born in early December and suckled until February-March when they disperse to the various cave areas throughout Gippsland – sometimes travelling many hundreds of kilometres.
When hunting, Bentwing Bats skim the tops of trees at about 40 kph using a loud (but inaudible to humans), frequency modulated sonar operating between 50 and 100 kHz.
These bats have very long wings. The farthest joint in the wing tip hinges back along itself to allow the bat to fold the wings for roosting on the cave roof (hence the name). Bentwing Bats cluster together for warmth in the cool caves, and in the summer months can be seen in tight groups of up to thousands.
The Horseshoe Bat has a specially adapted nose-leaf to allow it to focus the echo location pulses. The focussing mechanism means that the bat can emit a constant frequency (70 kHz) low intensity beam while still being able to listen for echoes. This suits its hunting technique, which is to fly very quietly, close to the ground and lower foliage, gleaning perched insects off vegetation. These bats are quite slow fliers but are very acrobatic – even capable of hovering!
Eastern Horseshoe Bats use a single maternity cave from which they disperse no more than 100 kilometres. They are more solitary than Common Bentwing Bats and may be seen in caves in only small groups or even ones or twos. Eastern Horseshoe Bats may be identified by their distinctive nose-leaf, large ears, shaggy brown appearance and habit of roosting in ones and twos.
Least known of all the East Gippsland cave dwelling bats, this bat is known to breed in only one cave in East Gippsland. They almost always roost within sight of water. The Large Footed Bat uses its sonar to detect insects and small aquatic creatures at or near the water surface. It catches its prey by skimming the water surface and grasping whatever food it detects, using its large feet.
A male bat collects a harem of females during the breeding months. These bats may use hollows in cliffs or trees to roost – they are not solely dependant on caves. They may be identified by their large feet and shaggy dark grey or brown appearance.
The populations of cave dwelling bats in East Gippsland may be threatened by:
- Human disturbance
- Ill-advised closure of caves
- Over collecting in the past
All bats are protected under the Wildlife Act, 1975, and some are further protected under the Flora and Fauna Guarantee Act, 1988. This means bats cannot be collected or handled without a permit.
If you are interested in learning more about the cave dwelling bats of East Gippsland, you may like to contact:
- Peter Ackroyd, Friends of Buchan Caves Inc. Buchan Vic 3885. Phone (03) 9347 8058.
- Jim Reside, Department of Natural Resources and Environment, 7 Service Street, Bairnsdale Vic 3875 Phone (03) 5152 6211.
The Australian Museum Complete Book of Australian Mammals, edited by Ronald Strahan, 530 pages, published in Sydney by Angus & Robertson, 1983.
This electronic version was taken from a pamphlet written by Peter Ackroyd. The original pamphlet acknowledged the assistance of Noel Speechley of Adelaide, SA, Jim Reside, Graham Parkes and Michelle Costigan of the Department of Conservation and Natural Resources (now Natural Resources and Environment), Bairnsdale Region, Ian Temby of the Arthur Rylah Institute, Melbourne, Elery Hamilton-Smith, Geoff Hammond and Jenny Smith.