Batsš¦ are easily overlooked as most bat species are nocturnalš. It is easy to miss them flying swiftly several metres over your head in the nightās darknessā¦ unless you actively search for them and then you realise how common they can be. Bats belong to the order Chiroptera (meaning "hand-wing" in Latin); like all mammalsš , they possess four limbs. Through evolution, however, their front limbs have changed into wing-like structures. More precisely, bats' wings are made up of flaps of skin between the bones š¦“ of the fingers and arm, making their wings more dissimilar from bird wingsā, i.e. bird wings consist of feathers extending along the arm. This unique trait makes them the only mammal capable of flight. And for those of you already thinking of flying squirrels and other colugos, those are āonlyā able to glide between treesš²ā”ļøš², not fly, despite possessing other impressive morphological adaptations. But for today, letās focus on bats and let us introduce šš©š¢š¦š³š¦š±š©š°šÆ š±ššŖš¤š¢šµš¶š“ (Buchannan, 1800), the wrinkle-lipped free-tailed bat, a species widely distributed across Southeast Asiaš³, with its range reaching India, Sri Lanka or Hainan Island in China.
The wrinkle-lipped free-tailed bat is a rather small size bat species (12 to 16 g) with a body length of 65-75 mm and a 300-330 mm wingspan. They can be identified by their wrinkled upper lip with protruded nostrilsšš½that open sidewards. Their ears are connected by a fleshy membrane-like structure across the forehead, and each ear has a blunt tip and six fleshy transverse ridges on the front side. Like many other bats, the wrinkle-lipped free-tailed bat possesses the ability of echolocationš” to navigate and find prey in the dark. When the high-frequency echolocation callšis returned by bouncing back on an object, their big earsšš½finely tune the sounds to allow them to recognise their own unique calls. Just like our voices define individuals, every bat has its special call distinct from other individualsā calls, and as you will see in our story, this is important in this species' lifestyle.
The wrinkle-lipped free-tailed bats are insectivores, feeding mainly on beetles, planthoppers and other hemipteransš¦. Thanks to their diet, many farmersšØš½āš¾perceive those little bats as great allies to keep insect outbreaks in check and protect agricultural activities. But how is such a small bat able to do such a featš¤?
Imagine it is just before sunsetš, and dark small cloudsā¬ļø form in the sky; those clouds are animated by sudden movements and changes of direction. What you are observing is a flock of wrinkle-lipped free-tailed bats going on the huntš½ļø after leaving their roosting sites. In some areas, like in northern Borneo, some colonies are estimated to contain nearly 280,000 individualsš±, with the bats leaving their cave at an incredible frequency of 14,000 bats per minute! Obviously, those hungry bats are not here for the show but to satisfy their appetiteš¤¤, as bats are easily prone to starvation if they donāt eat for just a few daysā ļø. A colony of wrinkle-lipped free-tailed bats like the one mentioned above can thus eat about 930 tons of insects a year, or 9 kg of insects per 1000 bats ingested every night.
Fun fact, bats are seldom seen bumping into each other though they flyš¦ and forageš“in humongous colonies. Thanks to their precise sonarš”, they will emit sound waves and locate one another, i.e. a leader and non-leaders. The leader in a flock will echolocate the surrounding landscapes and find out the best pathš§ , and itāll signal others by unique callsš£ļø. Non-leaders will then follow the leaderās callš§. By mimicking the turns and dives of a leader bat and locating other bats in the flock, their movements can stay synchronised and prevent collisionsā ļø. Interestingly, a leader bat is not one dedicated individual; as studies found that the leadership position switches between bats during a single flightš; and since echolocation allows bats to sense the environment from 360 degrees, the leader is not always at the front. When it is about time for the bats to return to their roosting sites, bats will emit social calls to signal one another that they should be heading back towards their roosting sites and end their hard night of labourš„±. And thus, before the sun rises the next dayš , the bats will return to their roosting sites one after the otherš“. The wrinkle-lipped free-tailed bat forms an incredible social network in which they forage and rest together. With this enormous population and the high energy intake each bat requires, you can easily imagine a large amount of faecesš© excreted daily containing mainly insect exoskeletonsš¦¾ composed of chitin. This represents a real treasureš° full of numerous nutrients needed for plant growth, such as nitrogen, potassium and phosphorus, and many people collect the bat's excrement as fertilisers known as "guano". That is why bats are very welcomed by farmers not only as predators of insects but also for their recycling propertiesš„°.
Next time you walk past limestone cavesš¦, rock crevices or even abandoned buildings and templesšļø in rural areas at duskšļø, be sure to watch out; the large flock of vigilantes of the night may emerge. To learn more about this marvellous creature and other Chiropterans, be sure to sign up for our guided tours at the Hong Kong Biodiversity Museum nowš¤©!
Text: Ryan Tse
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