They frequent the night, hidden by darkness. They are noisy, yet we never hear them. They gobble up those we dislike.
They are the microbats.
These little bats may look like small versions of the more familiar megabats, or flying foxes, but they are very different.
The megabats get their name in part due to their size. For example, the common species seen in Melbourne, the grey-headed flying fox, has a body length around 25cm, a wingspan about 1m, and weighs up to 1kg.
As you might expect from their name, the microbats are much smaller. The big ones have a wingspan up to 25cm, and weigh up to 150g, while the small ones may weigh as little as 3g. You could fit one inside a matchbox, though I don't recommend you try it.
Mosquitoes, Watch Out!
The flying foxes eat fruit, nectar and pollen. They navigate and locate their food using their night-adapted eyesight.
In contrast, the microbats eat mostly insects. If you have too many mosquitoes, perhaps encouraging more microbats in your area will help: a single microbat can eat up to half its weight in insects in one night, which works out to more than 1200 mosquitoes for the smallest of the microbats. The larger microbats can catch that number of mosquitoes in one hour!
Noisy, but Silent
To us, microbats seem to be silent. But they are actually very noisy, because they use echolocation to find their food, and to avoid crashing into things. Flying in total darkness is no problem to these little creatures.
They seem silent to us because their calls are pitched too high for us to hear.
Sleep During Daylight...
While some microbats roost in caves during the day, most of those around Melbourne roost in tree hollows, folded outdoor umbrellas, dead fronds of tree ferns, abandoned bird nests, and any other small dark place they can crawl into.
... and Winter
During winter, there are less insects around, and a microbat risks starvation. They avoid this problem by entering torpor. Torpor is like hibernation, but not as extreme.
If you find a microbat in torpor, leave it alone. If you wake it when there are no insects for it to eat, it may die.
Find the Relations
There is uncertainty about how closely the microbats are related to the megabats. It may be that the megabats are related to us. We are primates, and megabats may be closely related to primates, while the microbats evolved from a completely different branch of mammals. If this is the case, then the ability to fly has evolved independently in both megabats and microbats.
Other evidence suggests they share a common flying ancestor, with the microbats subsequently evolving the ability to echolocate.
Both types of bats have light, small skeletons which means they don't survive long after death. That means bat fossils are rare, making it more difficult to work out their evolution.
What do they really like?
We don't really know much about the behaviour and preferences of microbats in Australia. They are so small and, to us, silent, that it is difficult to study them properly.
What we do know is frequently based on a study of one species in one area. We often do not know if those conclusions apply to microbats of different species, or even those of the same species in a different area.
There are plenty of things we would like to know. Some of these we have some ideas about, but it would be nice to have some certainty.
Which species of microbats like bright lights, because they can catch the insects that are attracted to the light, and which avoid bright lights?
Which species like open areas, and which prefer trees and bushes?
Do some or all avoid flying during a full moon?
Where does which species prefer to roost? Based on research into the habits of insects and birds, it is likely different species of microbats prefer different sizes of roost space, at different heights, facing different directions.
How far do they fly in one night? Some have been recorded to fly more than 15km, but how common is this?
One way we can learn more about microbats is to capture some. They can be identified, measured and weighed and perhaps even tagged before being released.
Researchers with the appropriate permits do this using a harp trap. This is a trap with long thin wires, like fishing line. A bat flies into the wires, and slides down into a pouch-like area at the bottom, from which it cannot climb out. The pouch has a small flap the bat can hide under, so it feels safe while waiting for the researcher to extract it an hour or so later.
Trapping is obviously only for experienced and authorised researchers. However there is a way you can investigate the bats in your neighbourhood, and that is to use a bat detector.
There are different types of bat detector, that work in slightly different ways. In general, they have a special microphone that responds to the high frequencies of the bat calls. Some electronics then divides the frequency of the call, until it is within the range of human hearing. The call is then played through a normal speaker, so we humans can hear the bats as they fly past.
The cheaper detectors do only that: we can hear the bat, but we don't know what sort it is. At least we know they are present!
More expensive detectors can record the calls, and allow us to display the call on a computer screen. As different bat species have calls at different frequencies, we have an opportunity to actually identify the species.
Giving a helping hand
Consider attracting more of our little insect-eating friends to your backyard. The best way to do this is to install a bat box, so they have somewhere to roost.
If you have an electric insect zapper, there is something else you can do: turn it off! Not only does it kill off the microbats' food, it also kills off the good insects.
They may be almost invisible to us, but microbats are our allies, and need our support.