The squatter on the roof

Credit: HKU biology dept/Porcupine

If you were assigned the task of spotting one wild mammal in the next 24 hours your best bet in Hong Kong would be to find a bat. A quiet footpath at dusk will do, just a bit away from a main street or highway, near lampposts. Bats will soon appear as winged silhouettes against the fading light in the sky. They flit in and out of view in jerky aerobatics as they twist, dive and turn in a feeding frenzy on insects. If you’re lucky you might see one zigzag across the sky for a good few seconds before it disappears into a dark shadow.

     And there lies the paradox of bat watching. They are easy to find, but difficult to see. You would have to be an expert to even have a chance of identifying on the wing one of the 26 species of the territory. And much of that would probably be based on circumstantial evidence such as knowledge of known roosts and habits. Bat spotting is a frustrating activity, all the more so for these tantalising flickers they routinely offer. 

     Most people know bats through this constant peripheral, almost subliminal awareness, but only the most dedicated get to know them well. Thanks to their painstaking research, we know some surprising things about them. 

     Bats are the only mammals capable of true flight. There are gliders and jumpers, but no other aviators capable of powered flight. Some find in this a stumbling block for Darwin’s theory of evolution, which hinges on the idea that animals evolve when tiny changes give individuals an advantage. The theory rules out the possibility that shrew-like furry mammals one day sprouted wings whole and kept them. They must have gradually evolved. But what good is a shrew with two percent of a wing? The detractors asked, and still do today.

     The Latin name for bats gives a clue to how the wing is formed: chiroptera, meaning hand wing. This makes sense when you look at the bone structure, showing that most of the wing comes from skin that joins up disproportionately long fingers. When creationists argue that there are no “missing-link” species, the more coherent ones mean that there is no progressive fossil record showing bat ancestors with gradually elongating fingers. The solution to this “problem” has always been the same and simple, the fossil record is incomplete. Richard Dawkins points out that we are lucky to have any fossils at all, but the theory of evolution doesn’t hinge on the tiny percentage of extinct animals that have been preserved in rock. All fossils found necessarily support the theory, but there are of course huge gaps where millions of animals died in locations that lacked the precise coincidences required to preserve animal bones.  It is amazing to me that creationists fixate on this fossil gap, and ignore the fascinating phenomenon of homologous skeletons across species that allows us to call the bat chiroptera, or hand wing. 

Berkley.edu

     The fact that we can even talk about finger bones forming the structure of bat wings comes from the uniformity of skeleton design that cross vertebrate species and even classes, from reptiles, to birds and mammals. You can find upper arms, lower arms, wrists and fingers in lizards, eagles, monkeys, and bats. The same goes for skulls, rib cages and pelvises, because our skeletons are homologus, meaning that every major human bone has an equivalent bat bone. We carry the evidence of evolution in our spine and rib-cage, as bats do in their ‘fingers.’

Paradise tree snake in flight. Photo: Jake Socha/Nat. Geo

     The study of flying snakes offers a clue how to answer the two-percent question. The paradise tree snake in Malaysia has the disconcerting strategy of escaping tree climbing predators by throwing themselves off the higher branches of the jungle canopy. Curious observers noted that they manage to gain up to 100 metres horizontal distance before landing in a lower level of the forest. Researchers using high-definition cameras looked at what was going on at the physiological level. They came to the conclusion that the snakes are more than gliders, they actively fly forwards. They captured images of the airborne body changing shape into a flattened ‘wing,’ moving with a swimming motion to control flight. It is easy to see how such rudimentary aviation would tangibly improve with every two percent gain in efficiency.

     Whatever stages bats took to gain their wings, they became artful aviators. Since their emergence around 50 million years ago, they have spread right across the globe, forming twenty percent of all mammal species, and becoming the only mammals that made it to New Zealand without the aid of humans.

     Despite the success of the model, we’ve been strangely disdainful of the species, finding them creepy, and even suggestive of evil. Bat flight doesn’t fit into to satisfying human rhythms. They make erratic turns and jerky altitude changes. They don’t inspire us like soaring birds do. They look impossible to catch, but sometimes they come close.

     One touched my hair on an evening jog, it came from nowhere, a split second apparition from a parallel universe, may be hell. I assume it was swooping for a mosquito, either that or it came to anoint me with the seal of Beelzebub. 

     Another time I scared a bat, nearly slicing it with a Stanley knife. I was dismantling a roof canopy that had ripped to shreds in a storm months earlier. I cut through a roll of canvass rapped around a pole, to expose a cosy sleep-pod for a lone bat. The startled animal flapped away in panic, giving me the best broad-daylight bat viewing I’ve had chance to see. It did a few clumsy laps of my rooftop then landed on the back of a neighbour’s air-conditioning unit. 

     Bats airborne may not be smooth, but when they land they are even more awkward. Their wings are like cumbersome crutches made from broken umbrellas that refuse to close. Somehow the little beast clung to the back of the unit, quivering like a half-drowned pup, and forced itself into the narrow gap between the air-conditioner and the ledge that supported it.

     

Japanese pipistrelle: AFCD

It may have been a common Japanese pipistrelle, or perhaps a rare bamboo bat, like the species newly discovered to Hong Kong in 2005. Certainly its chosen tubular roost resembled a section of bamboo better than the cave dwellings of many species. But really there was no way I could tell what it was. The only identity I’m confident of is that it was one of the 24 diminutive microchiroptera species, rather than the larger megachiroptera fruit-bats, of which we have two kinds in Hong Kong.

     The short-nosed fruit bat, for instance is more likely to be found in an urban park, nesting under Chinese fan palms. Hong Kong’s only nest-building bats create shelters by chewing through the veins of upward pointing palms, and collapsing them down to form a ‘tent.’ The males do the work, and they use their creations to attract females for a harem. 

     The other fruit bat, the Leschenault’s Rousette, is the largest in the territory, with a wingspan of 40 cm. It plays a role in pollinating trees and dispersing seeds for Hong Kong’s forests. The species is a cave-dweller, which is unusual for fruit bats more commonly found to roost in trees. 

     I don't think it was a Rickette's big-footed bat either. I might have seen prominent hooked claws had it been, and these are also cave inhabitants, rather than solitary sleepers in tight tubes. They are known to roost in disused mines and their sharp, forward pointing toes give a clue to their unusual diet. 

    “The fish feeding behaviour of Rickett’s big-footed bat was first discovered during my thesis work at HKU in the early 1990s,” Dr Gary Ades of Kadoorie farm told me. “Bat droppings collected at Lin Ma Hang mines contained many fish scales which was quite a surprise at first.” 

     He said that they trawl water surfaces with their pointed toes to spear fish. They are often seen cruising freshwater reservoirs and slow-moving streams. Unfortunately the reliance of this species on freshwater sources is endangering its survival in mainland China, according to the IUCN which declared it ‘near-threatened’ on the 2008 Red List. It has a wide distribution from the far northeast to Hainan Island, but severe water pollution throughout the mainland is wiping out much of their hunting ground. The IUCN estimates a projected 30 percent decline of these piscivores over the next 15 years.

     I doubt if I was lucky enough that day to accidentally disturb the unidentified bat that became a contender for ‘new species’ status. In a 2005 AFCD study in Plover Cove country park, CT Shek and his team discovered a bat that didn’t exactly match any of the animals already recorded in the territory. A wider search offered no definite answers, but a similar rare Vietnamese species was found. Shek told me that they were hoping to name it pipistrellus hongkonggensis, but that is yet to be confirmed.

    One way of telling the smaller bats apart is to look at their hideous faces. In Hong Kong it could be a tiny whiskered myotis with its beady eyes and squashed up pig-face. Or a brown noctule that looks like it has a permanently sore face. Admittedly the pipistrelles tend to have relatively normal mammalian features, albeit on the ugly side, but it is the leaf-nosed and the horseshoe varieties that are the true shockers. 

     The Pomona leaf-nose with comedy giant ears, looks as if its real nose was bitten off in a fight outside a bar. The Chinese horseshoe really looks like it was branded on the face with a horseshoe, leaving a clear imprint, and a fleshy mess all around. But the gurning champion of the bat world has to go to the Himalayan leaf-nose whose face is a grotesque mess of wobbly grooves and pointy ends. The effect of looking at its face is disorientating and disturbing.

     There is of course a very good reason for the troughs and spikes of a bat face. What you are seeing is in fact a finely honed acoustic device. The leaf-nosed bats have dismantled their mammalian nose and reconstructed it into a highly specialised speaker that precisely targets powerful ultrasonic echolocation pulses. Once you understand the function, the hideous bat-face appears more forgivable, even admirable.

     Before WWII we didn’t know how bats hunted tiny insects in the dark, because before then we didn’t have radar. Radar taught physicists the principles of echolocation, and with that knowledge biologists discovered bats had developed a similar system millions of years earlier. 

     Radar uses radio waves, bats rely on sound, or sonar, but the principles are the same. Echolocation works by analysing data that bounce back from objects in the path of an outgoing signal. The bat emits a loud screech, everything in its path bounces sound waves, some of which make it back to the source. The bat learns where the food is, which way it is moving, how fast it is going, every thing it needs to make the kill. 

     That is the basic principle, but the details are breathtaking, as described brilliantly by Richard Dawkins in ‘The blind watchmaker.’ 

     Most bat echolocation calls are inaudible to human ears because they are in the ultrasound frequency. That’s just as well because if we could hear them we would be driven mad by the cacophony. They are blasted at extremely high volume. The emissions need to be loud because, as radar physicists discovered, out-going waves decay quickly as they spread increasingly wider, and so do the incoming waves. Which also means that the receiving or listening part of the system needs to be very sensitive. This explains the super-size lugs of the Pomona leaf-nose and other similar species. 

     But radar technicians discovered that this posed a tricky problem, that the powerful transmitter could damage the sensitive receiver. Or to pose the equivalent in the world of bats, the bat would deafen itself with outgoing shrieks.

     The communications specialists got round this by wiring up the receiving antennae to be momentarily switched off each time a signal went out. Biologists discovered that bats were doing the same with muscle contractions that dampened vibrations on sound carrying ear bones, as the outgoing shriek was projected.

     There is convergence too in other design solutions that refine echolocation. The ‘Chirp radar’ was introduced to put a marker on the outgoing signal, by using a frequency that shifts as it is transmitted. The frequency of the returning signal indicates which part of the outgoing found an object in its path. Bats use the same principle to sift through the data, emitting a swoop that can drop an octave. This helps the bat judge the distance to its target. Is the incoming sound at the C-sharp that emitted when the bat passed the third branch of the banyan tree? Or is it the B-flat it had dropped to by the time it had reached the lamppost? 

     Similarly the Doppler effect is exploited by both the manmade system and its biological equivalent. This is also known as the ambulance effect where an approaching vehicle has a higher siren tone because its sound waves are squeezed together. As soon as the ambulance passes the tone drops because the waves stretch out, as the source moves further away. Police radar traps are based on the Doppler effect, calculating the speed of targets by measuring the shift in the frequency of incoming radio waves. Astronomers also exploit the Doppler shift to find out if stars are moving away from us or towards us. Thus we learn about the expanding universe by utilising the same analytical tools bats use to hunt moths.

     Studies on horseshoe bats have shown up a complex set of 'calculations' dealing with a double Doppler shift from the movement of  both target and hunting bat. Not only that but rather than analysing the tone of the returning sound, these bats alter the pitch of their outgoing screech to keep the incoming pitch constant, at the best frequency for hearing. 

     All these split-second adjustments to focus bat sonar would be mind-blowing to human mathematicians trying to keep track without the aid of computers. But it would be a mistake to confuse our understanding of the physics with how bats experience the world.

     Bats don't do the sums dealing with frequency, velocity, distance and so on. They process the information instantly, as we do when we focus our eyes without doing the maths on the data involved. We convert light-waves into colour, while bats use sonar pulses that speed up as they close in on a target. Their focussed view of the world is at least as detailed as the world that humans see. Dawkins compares bat transmissions at 200 pulses per second on the hunt with mains electricity cycles at less than half the speed. Our perception of visual continuity inside a brightly lit room is an illusion half as informed as the world a bat 'sees.' 

     Whether information comes in sound or light, it is the brain that has to make sense of it. We see light, as bats hear sound, and our respective brains construct a three-dimensional model of space as detailed as necessary. Whichever form the data comes in, they are processed for the same ends.

     Human echolocation has been studied in recent years, including the development of practical systems to aid blind people. Accounts of visually impaired people riding mountain bikes or playing basketball have shown that echolocation can work for us. The systems on offer do not rely on superhuman perception, they are based on clicks of the tongue and training to interpret the incoming signals. 

     To a certain extent we all do it, for instance when we hear echoes in the mountains, we can detect the presence and direction of cliffs, and we can get an idea of how far away they are. I wonder how much more we do it subconsciously when we weave through a crowded street in Hong Kong for example? But a more refined system that can judge the distance of a moving ball, or allow firemen to find an exit in a smoke filled room, takes practice. 

     Tests suggest that echolocators don't necessarily have better than average hearing. fMRI studies have shown that the part of the brain most active during echolocation is the segment normally stimulated by visual cues, while audio areas remain dormant. 

Thaler, L. Arnott, S.R. & Goodale, M.A. (2011)

     The possibilities become even more intriguing by the suggestion that tongue clicking could be used as an x-ray device. An echolocator not only has the ability to tell that there is a soft bag in the vicinity, the bouncing sounds can give clues to the contents of the bag. Surely the next James Bond should have that written into the script.

     But back in the real world, as soon as the military use of radar became the norm, so did efforts to evade it. It should come as no surprise that pretty much everything we thought of to hide from, and foil, enemy radar already existed in nature. 

     The most expensive military plane in history, the B2 stealth bomber, looks like a moth. Its flat triangular morphology is designed to deflect probing radar pulses, its skin is sheathed in an absorbent material that dampens incoming beams. Its makers couldn’t have picked a more apt model than the moth.

Moths at first glance look like soft targets for night hunting bats. But the evolutionary arms race has not left them helpless and the outcome of an attack is not a given. Their powdery wings form a triangular shield over their abdomens. They are covered in uneven scales that dampen bat sonar. In bat ‘vision’ a moth must have a somewhat blurred outline.

     Some moths have developed hearing attuned to bat frequencies. With just two to four vibrating cells attached to the eardrum, they monitor the searching calls of their enemy. They combine this with sophisticated aerobatics, such as split-second twists, and sudden dives to evade the predator. This could explain the eratic flight path of bats at dusk.

     Co-evolution might be a more interesting way of looking at the bat-moth relationship than an arms race. Bats have had to adapt to moths as much as moths have to bats. For example some have dropped the frequency of their calls to foil the ultrasound detectors developed by moths. Others have dropped decibels to quietly sneak up to their vigilant prey. The European barbastelle bat is thought to have taken this strategy. Its oversized ears designed to detect feint echoes are a testimony to the form altering power of natural selection. 

European barbastelle: bio.bris.ac.uk

     Passively listening isn’t enough for all moth species threatened by sonar fitted destroyers. Some have developed clicks audible to bats. This can startle a hunting bat, buying a moth a crucial split-second. Studies have shown that the moth click can advertise toxicity, as colours do in other species. But the most sophisticated strategy is radar jamming. Tiger moths emit precisely tuned clicks at moments in the bat swoop calculated for maximum disruption of their sonar. Slow motion footage shows the effect of this split-second blip that throws the bat into a wrong turn.

     Moths are not the only creatures with an aversion to bats. Many human societies have had an irrational fear of these nocturnal apparitions. Though the old European association with vampires seems unfair to say the least. There are blood-sucking species, but only three, and they all live in central and South America. 

     In China however things often look different and bats are seen as a good omen, so good that people eat them for good health. In fact bat eaters have taken exotic cuisine to new levels by including faeces on the menu. Asthma, kidney ailments and general malaise form a familiarly vague list of complaints said to be cured by traditional medicine.

     This tradition led to tragedy in 2003 when the SARS virus jumped from horseshoe bats and infected humans, killing nearly 800 people in a matter of months. Civet cats were the first to be blamed, and thousands of them were killed for it. Later virologists pointed the finger at bats, often found jammed into cages near civets at southern Chinese markets. 

     Much as outsiders may be revolted by the idea of eating bat droppings, it should be noted that there was no evidence of direct human to bat infection. Feasting on their meat, or feces, was irrelevant to the spread of SARS. Virologists often have more to worry about from the transport and marketing of live animals, than how they are consumed after they are cooked.

     Bats are the reservoir species for several other viruses that have killed humans. Five different genotypes of the rabies virus for example have been found living exclusively inside bats. This may trigger alarm bells to anyone regularly seeing bats near their home, but it is unlikely there is any cause for concern. It should be noted that there are no known cases of bats infecting people with rabies in post-war Hong Kong, although there are rare cases elsewhere. By far the most dangerous animal when it comes to rabies, is ‘man’s best friend,’the dog.

     The last record of a person infected with rabies in Hong Kong was in 1981, the last animal in 1987. Ninety-five percent of animal cases since 1949 have been found in dogs, three percent in cats, and one each in pigs and cattle. There is no sensible argument to be made that bats are a rabies threat here.

     But on the surface the track record for bats doesn’t look that great. Hendra, nipah and ebola are three other terror-inducing viruses that have emerged from bats and killed people in recent years. Ebola is the most notorious, with the latest west African outbreak carrying a 53 percent death rate. The attack of the virus is truly horrific, taking over the immune system and spreading throughout the body to destroy every organ. It forms blood clots that deplete coagulants so that victims bleed both internally and externally, leaking blood from every orifice, mucus membrane and wound. Fruit bats of the pteropodidae family are considered to be the reservoir species for this nightmare microbe.  About another 60 less well-known viruses have also been isolated from bat species.

      So what should we do? Kill bats? Napalm their caves? Drone-strikes on their leaders?

      SARS, rabies, hendra, nipah and ebola have coexisted with their bat hosts for hundreds of thousands of years. They continue to do so even as the WHO monitors for outbreaks that put the fear of God into people. Most of the time these viruses stay in their reservoir hosts, and do not cause any trouble, though there are good reasons for virus specialists to be vigilant. 

     The relationship between viruses, their hosts, and other species is a relatively young topic that is expected to open up a huge area of scientific knowledge in the coming years. A recent study found that eight percent of the human genome is considered to have viral origins and we are only just beginning to get to grips with what that means. It could have huge implications for medicine and the study of evolution, and our understanding of who – or what – we really are. 

     That some of the most prominent viral outbreaks of recent years may have originated in bats is a point of interest, but it doesn’t hold any answers to how we can prevent disease. It certainly isn’t a reason to wipe out bats. We don’t have any way of ridding the world of harmful viruses, and if we did, we would have no idea about the impact on the environment. Our natural resources hold a balance far more complex than our recent ancestors realised when they discovered the power of modern tools to re-sculpt the evironment. 

     There are strong reasons for suspecting that human intervention is the biggest root cause of a virus jumping species. There is evidence for example that a combination of deforestation and economic pressures on legal and illegal bushmeat in Africa are pushing up the risk of viruses jumping species. The human population now is bigger than it has ever been before, our reach is further than ever before. The problem isn’t so much that we are being contaminated by an external threat. Environmental scientists say that there isn’t a single patch of the planet that hasn’t been affected by human activity. We are the contamination, not the other way round. 

Cedar, a viral cousin of the deadly hendra

    With the case of SARS it is well documented that crowded markets were vital to creating the circumstances that unleashed the virus from its bat host. In the past some policy makers would have taken that as enough reason to fire-bomb bat-caves, but surely it would have made more sense to burn-down the markets.

     In a world without bats what would we do with up to 4,000 mosquitoes per bat per night that carry on living and breeding? Mosquitoes are reckoned to be biggest killers on the planet with up to a quarter of a million human deaths per year attributed to their microbe-spreading behaviour, the vast majority of those fatalities being caused by malaria. They are even more dangerous than humans, the next item on the top killers list and by far the most lethal mammal on the planet.

    Bats are one of the most important natural weapons against mosquitos and other insect pests. One study has estimated that bats are worth about 23 billion dollars a year to the US agricultural industry through savings on pesticides. A colony of 150 bats in Indiana eats its way through 1.3 million insects a year, according to researchers at the University of Tennesee. Without bats, on top of additional economic costs, there would be a huge environmental impact from spraying tonnes of life-destroying chemicals into the air, and on to the land, all leaching through to waterways. 

     Fruit bats also play an important role in maintaining and regenerating forests by pollinating trees and dispersing seeds. And for centuries farmers have exploited bat guano, the name for the collected mass of bat droppings, for high quality fertilizer. It is a simple fact that humans have benefited from the existence of bats throughout our history.

     Have bats benefited from us? It isn’t easy to say that, especially as we know for sure that humans have cut down vast swathes of their natural forest habitats and hunting grounds. But in Hong Kong there is a case to be made that bats have at least adapted to our environment, and quite possibly have flown over from other places to make use of our structures.

     Hong Kong’s only natural caves have been carved by ocean surf, and are dotted around the craggier parts of the coast. Yet we host 11 species of cave dwelling bats inland, in water tunnels, disused air raid shelters, and old mine shafts, according to CT Shek at the AFCD. Water channels have been recognised as roosting sites and sections have been fenced off by the Water Works Deparment specifically to protect bats. An AFCD study in 2003 counted a total of 21,178 bats hanging upside down mostly in man-made homes. In the Lin Ma Hang abandoned lead mine alone there were two thousand bats on a count in 1994, and at least eight species in the vicinity.

     Shek is certain that artificial structures are contributing to bat biodiversity here:

     “The water tunnels or abandoned mines in the countryside are excellent roosting habitats for bats and that’s why most of the cave dwelling species are abundant and widespread throughout Hong Kong.”

     I suppose that my long neglect of the torn canopy gave one bat an artificial habitat for some weeks. If that one was eating 4,000 mosquitoes a night it was surely earning its keep, but it is a shame I never did get a positive identification.