QUESTIONS AND ANSWERS: Bats

Content Updated: 7th September 2008

QUESTIONS:

What is echolocation and how do bats use it?
How is moth evolution linked to bat echolocation?
What bat species are found in the UK?
What it the “Pipistrelle Split”?
Is the expression “blind as a bat” justified?
What should I do if I find an injured bat?

Q: What is Echolocation & How do Bats Use it?

A: Echolocation can be broadly described as “seeing with sound”. As early as 1793, Italian researcher Lazzaro Spallanzai demonstrated that, while blinded bats could find their way around their enclosure, deafened bats lost their sense of direction. The term “echolocation” was first coined by the late Harvard zoologist Donald R. Griffin who, back in 1938, used a microphone sensitive to ultrasound to listen to bats. Echolocation is effectively the ability to localize (find) objects based on how they reflect sound. In the case of our visual system, we rely on light reflected back from objects around us in order to see – bats rely on sound reflected back from objects around them in order to “see in the dark”. Bats emit a variety of chirps and squeaks during flight and listen for the echoes. Sound striking close objects will be reflected back sooner and be louder than sound striking a more distant obstacle. Similarly, by listening for changes in the phase of the echo, bats can determine the type of surface from which the sound was bounced back – a hard, continuous object (such as a wall) will produce a sharper echo than softer objects (such as foliage).

The figure (left) shows the basic principle of echolocation – a sound is produced, bounces back from the first object (the moth in this example) and then, a fraction of a second later, bounces back from a second, third, fourth etc. object (e.g. trees, walls, hedges, etc. in the vicinity). If the bat knows how fast this ‘block’ of sound is travelling, it can calculate -- based on the time separating the two returning echoes -- the distance between the two objects. Moreover, the bat can vary the harmonics, rate, length, intensity and components of the call to gain an extraordinary amount of information about its surroundings.

The frequency -- measured in kilohertz (kHz), or thousand’s of cycles per second -- of bat calls varies with species and, it is generally considered that, high-frequency sounds give the bat lots of detail but over a short distance, while low-frequency sounds give less detail but over a longer range. Although the rare Short-eared Trident bat (Cloeotis percivali) of South Africa can call at frequencies as high as 212 kHz -- bearing in mind that humans can only hear sounds as high as about 20 kHz -- frequencies of between 20 and 60 kHz are more common. Frequencies lower than about 20 kHz have a wavelength larger than most insects (so the sound wave moves around the insect, rather than striking it and bouncing back) while frequencies above 60 kHz attenuate (weaken) rapidly in air, which lessens their range.

Bat calls can generally be classed into two groups: narrowband and broadband. Narrowband calls (sometimes referred to as Constant Frequency, or CF, calls) are those of almost constant frequency, while broadband calls (sometimes referred to as Frequency Modulation, or FM, calls) sweep a large range of frequencies in a very short time (i.e. from 100 kHz down to 20 kHz in a couple of seconds). Broadband calls are used to scan the landscape, while narrowband calls are used to identify and provide information on potential prey items. The search calls tend to be intense (with some 10 to 15 calls -- or pulses -- every second in some species), getting faster and faster (up to 200 calls per second) to home in once an insect has been detected.

In his New Encyclopedia of Mammals, David Macdonald points out that bats tend to have a “Duty Cycle” when echolocating, which represents the proportion of time actually spent generating the sound. In other words, bats only spend a certain period of their time (e.g. about 20%) echolocating, because they can’t listen for returning echoes whilst shouting new pulses. This is not true of all bats; Horseshoe bats (Rhinolophidae) can apparently shout and listen at the same time, allowing them to spend as much as half their time echolocating. This is achieved through something known as “Doppler Shift Compensation” or DSC. The example I was always given at school to explain the Doppler Shift was the ‘train approaching a station’ illustration – we hear a high pitched sound as a fast moving train approaches the station platform (because the sound waves are being compressed by the approaching train) and a lower pitched sound as the train races past and away from the station (because the sound waves are being stretched). Thus, the Doppler Shift may be described as the change in pitch (i.e. frequency) of a sound produced by a moving object. How the bats use this DSC is actually rather complicated and involves varying their call frequency according to their flight speed. Sufficed to say that Horseshoe bats send (as ultrasound) and receive (as echoes) sounds on different bandwidths, enabling them to separate their outgoing, echolocative, calls from the returning echoes.

The bat larynx (voice box) is large and reinforced with bone, allowing a high tension on the vocal chords to be maintained (permitting the production of high-frequency vibrations). According to A.A. Wardhaugh’s 1995 book, Bats of the British Isles, sounds are produced in concentrated beams that are directed through a gap in the upper incisors of most species. Horseshoe bats are slightly different – rhinolophids have a flap of skin adorning their nose, which is used to concentrate the stream of sound pulses (in a similar way to a megaphone). This flap of skin seems to afford the Horseshoe bats extra sensitivity, allowing them to detect insects at distances of some 10m (30 ft), while the vesper (or "evening") bats can only detect insects at distances of about 1m (almost 3.5 ft). Once the call has been emitted, the ear muscles relax and await the returning echo. Many microbats have a ‘spike’ of cartilage sticking up from the base of their ear, which scientists believe help give the bat better sound detection in a given plane. The echolocation of bats is impressively accurate. An intriguing paper, by Sabine Schmidt at the University of Munich in Germany and two colleagues in October 2000, found that Gleaning bats (Megaderma lyra) were able to find silent and motionless prey on the ground and use their broadband echolocation calls to reject dummy food items whilst hovering over them.

It seems that the ability to echolocate is largely a characteristic of the microbats; megabats (Fruit bats, or Flying foxes), with few exceptions, don’t echolocate because they have sufficiently good vision to find fruit by sight (scent is probably also involved). One exception to this is the Egyptian Fruit bat (Rousettus aegyptiacus), which apparently uses echolocation to find its way about in caves. Indeed, a study by Dean Waters at the University of Leeds and Claudia Vollrath at the University of Freiburg in Germany, found that R. aegyptiacus used echolocation in both light and dark conditions while flying within a tunnel.

Newborn bats appear to pick up echolocation rapidly. In their 2003 paper to the Journal of Neurophysiology, Marianne Vater and five co-workers report that two-week-old Mustached bat (Pteronotus parnellii) pups were capable of spontaneously producing CF and FM signals. Dr Vater and her colleagues also report that the ability of these bats to utilize DSC was evident from about four weeks old! It seems that, as well as a rather rapid development of echolocation calls, the call structure can vary according to geography. A 2003 study by Fanni Aspetsberger at the University of Cape Town and two colleagues, found that echolocation calls of Little Free-tailed bats (Chaerephon pumilus) in the Amani Nature Reserve of Tanzania, were of a lower frequency and had longer gaps between pulses than in those individuals of the same species living in South Africa. These differences are probably related to differences in feeding ecology between the populations.

Bats are not the only animals that use echolocation to find their way about and locate food. Echolocation is perhaps best known in the Odontoceti (toothed whales), especially the Delphinidae (dolphins). In the case of dolphins, sounds (in the form of rapid, high pitched clicks) of about 120 kHz are generated in the nasal sacs, after which the melon (the bony surface of the skull) focuses the sound into a narrow band and projects it forwards. Returning echoes are received by the pan bone of the lower jaw, and are then transmitted to the middle ear by fatty tissue located just behind the jaw; from the ear the sound is transmitted to the brain. (Photo: Bats aren't the only mammals to make use of echolocation. Members of the dolphin family, which includes the Killer whale, also use sound to locate prey.)

The observation that sound travels four-and-a-half times faster in water than in air suggests that the dolphin’s brain must be extremely well adapted to make sense of the returning echoes, which arrive more rapidly than they do for bats. This may explain why dolphins tend to transmit each click after receiving the echo from the previous one. Some authors have even suggested that the dolphins’ echolocation may have a healing effect on humans. It has been postulated that the ultrasound emitted by dolphins may have a mechanical and/or electro-mechanical effect on the endocrine (hormone) system, positively stimulating it and providing some relief from certain psychological and psychosomatic illnesses. Research into this idea by Karsten Brensing, Katrin Linke and Dietmar Todt at the University of Berlin, however, rejected the idea that dolphins exhibit a behaviour that leads to patients being exposed to ultrasound in doses comparable to those in medical treatments. (Back to Menu)

Q: How is Moth Evolution Linked to Bat Echolocation?

A: Inextricably, in some species. In his fascinating contribution to Recent Advances in the Study of Bats, James Fullard at the University of Toronto in Canada reports that the auditory system of Noctuid (Cutworm) moths evolved as a direct result of predation by bats. Dr Fullard notes that this species doesn’t use sound socially, but their tympanal (middle ear) organs are often most sensitive to the frequency of calls emitted by echolocating (and, therefore, hunting) bats. In other words, these moths have evolved to hear the bats that feed on them coming! Indeed, Fullard has conducted numerous studies into how moth hearing has evolved in relation to the echolocative calls of the bats that feed on them. Over the years, he has come to realize that most moths that fly in the same airspace as hunting bats avoid being eaten by using their ears, which are syntonic with the hunting calls of bats – that is to say, moths have evolved to hear in the same sound range that bats have evolved to hunt with. In one particular study, published in the Proceedings of the Royal Society of London back in 2001, Fullard reports on the predation of moths by Hawaiian Hoary bats (Lasiurus cinereus semotus) on the Hawaiian island of Kaua’i, observing that the endemic Hawaiian Cutworm moths (Haliophyle euclidias) were preferentially eaten by this bat, compared to other endemic and introduced species. Fullard concluded that this moth -- which has hearing that is less sensitive to bat calls than the other species of moth he looked at -- suffers higher predation because it is drawn away from its normal habitat, enticed by the man-made lights that are now favoured hunting grounds for bats.

Intriguingly, although the hearing of certain moths (especially noctuid moths) is related to bat predation, the emergence of nocturnal activity in moths seems unrelated to bat activity. In a 2000 paper, James Fullard reported on the day-flying butterflies in Polynesian bat-free habitat, comparing the activity of three species of nymphalid (Brush-footed) butterfly on the bat-free Pacific island of Moorea with three nymphalids in Queensland, Australia (where bats actively prey on moths). Fullard found that nocturnal flight activity and the number of active individuals did not differ significantly between the two locations, leading him to conclude that living in a bat-free environment did not produce nocturnal flight in these insects. This is strange, especially considering that during the daytime, moths are at risk of predation from birds and are competing for nectar with butterflies; thus exploiting the night-time in a habitat where there are no major nocturnal predators (i.e. bats) would seem the best course of action.

Fullard considered three possible reasons for the daytime activity seen in the butterflies from bat-free habitats: that bats weren’t important nocturnal predators; that the insects are somehow constrained to the day by some physiological reason; or that the Moorean butterflies haven’t spent enough time in genetic isolation. The first of these suggestions is unlikely – bats are a major predator of insects in almost every forested region of the world, and it seems doubtful that they weren’t an important influence in this study. The second and third ideas are both plausible – of these, Dr Fullard considers that the second is most probable and these insects are constrained by some physiological parameter (either temperature or light).

Subsequent research has documented hearing sensitivities at the range of bat calls in other, non-noctuid, moths. Annemarie Surlykke at Denmark University and four colleagues, for example, report that the ear of Drepanid (Hooktip) moths is tuned to ultrasonic frequencies between 30 and 65 kHz. Such an observation suggests that drepanid hearing resembles that of other moths, in that the main function is bat detection. (Back to Menu)

Q: What Bat Species are Found in the UK?

A: There are currently 16 species of bat known from the UK, six of which are considered -- conservationally-speaking -- “Vulnerable”, four are “Rare”, two are “Endangered” and only four are not threatened to a sufficient extent to warrant adding to a conservation list. The 16 UK species and their conservational status are as follows (S = Scotland; W = Wales; I = Ireland; E = England):

Horseshoe bats (Family: Rhinolophidae)
Greater Horshshoe bat (Rhinolophus ferrumequinum) (W, E)
Lesser Horseshoe bat (Rhinolophus hipposideros) (W, I)

Vesper (“Evening”) bats (Family: Vespertilionidae)
Pipistrelles Common Pipistrelle (Pipistrellus pipistrellus) (S, W, I, E)
Nathusius’s Pipistrelle (Pipistrellus nathusii) (W, I, E)
Pygmy Pipistrelle (Pipistrellus pygmaeus) (S, W, I, E)

House bats (Serotines)
Serotine (Eptesicus serotinus) (W, E)

Noctules
Leisler’s Bat (Nyctalus leisleri) (S, I, E)
Noctule (Nyctalus noctula) (S, W, E)

Little Brown bats
Bechstein’s Bat (Myotis bechsteini) (W, E)
Brandt’s Bat (Myotis brandtii) (S, W, E)
Daubenton’s Bat (Myotis daubentoni) (S, I, W, E)
Natterer’s Bat (Myotis nattereri) (S, I, W, E)
Whiskered Bat (Myotis mystacinus) (S, I, W, E)

Barbastelles
Barbastelle (Barbastella barbastellus) (W, E)

Long-eared bats
Brown Long-eared Bat (Plecotus auritus) (S, I, W, E)
Grey Long-eared Bat (Plecotus austriacus) (E)

Up until the January 1990, when it was declared extinct in Britain, the Common Mouse-eared bat (Myotis myotis) was the 17th member of this list. Since its removal, there have been occasional sightings of hibernating Myotis myotis in the UK. Such sightings have, however, been of single individuals and I'm not aware of any evidence to show that the species has begun recolonising the British Isles. Additionally, the regions above should be considered tentative. In Wales, for example, there are only isolated records of P. nauthusii, E. serotinus, and M. bechsteini. Similarly, species identification is easier for some bats than others, and some species are very difficult to tell apart. M. brandtii vs. M. mystacinus or P. pipistrellus vs. P. pygmaeus, for example, are often recorded as simply 'brandt/whiskered' and 'pipistrelle', respectively. (Back to Menu)

Q: What it the “Pipistrelle Split”?

A: It was the realization that, what was once thought to be a single species (Pipistrellus pipistrellus), is actually two different species (P. pipistrellus and P. pygmaeus) that probably diverged some five to ten million years ago. Bat workers had known for some time that the Common pipistrelle (P. pipistrellus) occurred in two apparently different forms; differences in appearance and in the peak frequencies of their echolocation calls had been documented. It was not until the early 1990s, however, that anyone actually set about categorizing these differences and looking into the possibility that they might be different species.

The first investigations were conducted by Gareth Jones at Bristol University and one of his students in 1992 – the results showed that not only did the two different forms use different maternity roosts, but they also had different call frequencies: 45 kHz and 55 kHz. Subsequent experiments, conducted by John Altringham, Gareth Jones and Kirsty Park, looked at the mating behaviour of the two “phonic” (i.e. with different frequency calls) pipistrelles. Prof Altringham and his team found that the two types did not share mating roosts but were thus considered to be reproductively isolated (i.e. they don’t interbreed with one another). A plethora of ensuing papers reported various differences in the morphology and feeding ecology of the two phonic types and, eventually, DNA analysis was conducted in a bid to find out whether the two types were actually separate species. The analysis was conducted by a team led by Liz Barratt and Mike Bruford at London Zoo using a sample of wing tissue. The results, published in the journal Nature in 1997, showed that the two types couldn’t interbreed – in other words, they were different species. Thus, the two types were reclassified as different species: P. pygmaeus (the Brown, Pygmy, Soprano or 55 kHz pipistrelle) and P. pipistrellus (the Common, Bandit or 45 kHz pipistrelle).

Unfortunately, while DNA analysis has shown that we now have two species where we originally considered there to be only one, this doesn’t mean that they are easily separated. Although there are differences in overall appearance and behaviour of the two pipistrelles they are still VERY similar. The Common pipistrelle has a darker face and ears than the Pygmy pipistrelle, giving the appearance of a mask and leading to some giving it the vernacular name “Bandit pipistrelle”. There is also no single feature that is significantly different in their teeth arrangement or biometric measurements (i.e. the size of various body parts) and moreover, some individuals overlap in the peak frequencies of their echolocation calls, making identification in the field complicated. Just to muddy the waters even further, in his A Guide to the Identification of Pipistrelle Bats, Henry Schofield of the Vincent Wildlife Trust in Herefordshire notes that “there is suggestion that the overall appearance of the two species may vary geographically…making them easier to separate in some areas of the country than in others”. Henry also states that some people think that the bats “smell” different, although this has yet to be subjected to rigorous scientific study!

Unfortunately, separating the "Common" and "Pygmy" Pipistrelle is not as easy in the field as it is in the genetics laboratory!

As something of a sideline, it seems that our British pipistrelles are not the only ones subject to tangled taxonomics! There is considerable debate in the US about the taxonomic status of Hollister’s bat (Myotis occultus). Over the last few decades, bat researchers have been trying to figure out whether M. occultus is a discrete species, or whether it is actually a synonym of the Little Brown bat (Myotis lucifugus). In 1999, Michael Bogan at the University of New Mexico and three of his colleagues carried out tests on the two species and, based on the high similarities between the two and little divergence from the Hardy-Weinberg equilibrium (the idea that populations are in “genetic equilibrium”), concluded that the two bats are nominal taxa and M. occultus should be regarded as a subspecies of M. lucifugus. If the two bats were that similar, however, why keep M. occultus as a subspecies? Why not just say that the two are the same species? Well, despite the genetic similarities, there are still rather obvious morphological differences between the two bats – thus, Dr Bogan and his team suggested M. occultus as a subspecies.

Data published in the Journal of Mammology by a team of American geneticists again looked at the discombobulated taxonomy of these little bats. This time, a team led by Antoinette Piaggio at the San Francisco State University looked at two genes from mitochondrial DNA (that is only inherited along the maternal bloodline) and found that M. occulatus represents “an evolutionarily distinct monophyletic lineage” – in other words, Piaggio and his team support the idea that M. occultus and M. lucifugus are separate species. The jury is, however, still out and, as is now commonplace for most things of taxonomic ilk, it is up to the reader to decide whose line of evidence he or she finds most compelling! (Back to Menu)

Q: Is the Expression “Blind as a Bat” Justified?

A: The short answer is: No! Indeed, while most bats (i.e. the microbats) have monochromatic vision (are colour blind), some (i.e. megabats) may see in colour.

Flying foxes (megabats) have exceptionally large eyes, and -- considering the lack of feeding-orientated echolocation in these bats -- vision obviously plays an important role in finding food, avoiding obstacles and perhaps finding a mate. Royal Melbourne Institute of Technology biologist Mal Graydon published a fascinating summary of Fruit bat vision in the June 1997 issue of Friends of Bats newsletter. In his article, Dr Graydon notes that Flying foxes can very easily adapt to their daylight surrounds, a skill afforded by the rapid contraction of the iris.

Studies by the esteemed scientist Gerhard Neuweiler during the 1970s concluded that Fruit bats have a visual acuity far superior to ours in dim light. Moreover, it seems that the “bob and sway” observed in flying megabats may be related to “two-eye analyses”. Using both eyes when assembling information about a visual target allows the brain an opportunity to compare two sets of information about the object (one from each optic nerve) and produce a final image that is more accurate with respect to textures, distance and shape. (Photo: Grey-Headed Flying fox, Pteropus poliocephalus.)

Assessing the presence of colour vision in animals is tricky, and normally relies on a combination of retinal scans and psychological tests. Humans have two types of cells on their retina: rods and cones. Rods are used to detect changes in light levels and contrast (i.e. serve a monochromatic function), while cones are used to collect and transmit information about colour. Rods and cones have been documented in many different species and are, generally speaking, considered to have much-the-same function across the species barrier. In Fruit bats, the retina is almost entirely covered with rod cells. Strewn in amongst these rods, however, are cells that don’t look entirely rod-like, nor do they look entirely cone-like, although they look more like cones than rods. The number of these cells on the retina is minor, but it does leave the question of whether megabats have full colour processing ability open to debate. More recent studies on the retina of two megabats by a team of eight scientists, led by Daryi Wang at Academic Sinica in Taiwan, has revealed that these bats have the gene associated with detection of red light, which the researchers suggest might aid the bat in discriminating between fruit and foliage. Ergo, the results suggest at least some colour processing by these Fruit bats.

The role that echolocation plays in object avoidance and hunting in the microbats has reduced the need for high visual acuity. Consequently, with few exceptions, microbat eyes are proportionally smaller than their megabat kin and their roughly spherical lenses suggest a short focal distance, good depth of field and probable hyperopia (far-sightedness). A series of experiments by the late Martin Eisentraut in the 1970s found that Brown Long-eared bats (Plecotus auritus) were able to discriminate different targets, but not different shapes (i.e. they could tell the difference between black squares and white squares, but not a circle and a cross). Dr. Eisentraut’s experiments were, however, carried out in bright light, which may have affected the outcome. Various studies have revealed that bat vision works better in dim light and studies on Phyllostomid (New World Leaf-nosed) bats, by Roderick Suthers (currently at the Indiana University) and his co-workers, in the late 1960s and early 1970s revealed more sophisticated discrimination of patterns in these chiropterans. Intriguingly, papers in the journal Animal Behaviour in 1981 and 1983 looking at the escape behaviour of the Geffroy’s Tail-less bat (Anoura geoffroyi), reported that this microbat used visual cues alone when selecting an escape route from the experimental setup, discarding acoustic cues.

The above studies and on-going experiments on microbats suggest that they may rely on vision more than was originally considered. A fascinating study by Johan Eklof of Goteborg University in Sweden and Gareth Jones at Bristol University published in Animal Behaviour during 2003 revealed that visual cues were more important than acoustic ones to foraging P. auritus. The scientists even found evidence for spatial memory in their subjects – the bats were observed to hover over places where dishes of worms were once placed but had subsequently been removed, suggesting that they remembered that there had once been food there. Elkof and Jones observed more feeding attempts at dishes that provided only visual cues, compared with those that provided only sonar cues, suggesting that not only were the bats able to locate food by sight, they also seemed to ‘prefer’ using visual cues rather than acoustic ones.

Recently, it has become clear that many bat species seem to have a sensitivity to ultraviolet (UV) light, which is more abundant at dawn and dusk. Indeed, York Winter at the University of Munich in Germany and two of her co-workers were able to demonstrate that the Long-tongued Nectar bat (Glossophaga soricina) is sensitive to UV down to a wavelength of 310 nm. York and her colleagues -- Jorge Lopez at the Universidad de San Carlos in Guatemala and Otto von Helversen of Erlangen University in Germany -- also conducted behavioural experiments that revealed a sensitivity in the green (max. 510 nm) and UV (above 365 nm) spectra. Moreover, the team found that the same photoreceptor (light-sensing cell) is responsible for both peaks (i.e. in the green and the UV) – this is interesting because in all rodents and marsupials (pouched mammals) where colour vision has been established, there is a separate receptor to deal with UV light. Indeed, the mechanism described for UV vision by Dr Winter and he co-workers has never been demonstrated in intact mammal visual systems before! (Back to Menu)

Q: What Should I do if I Find an Injured Bat?

A: Ideally, phone the National Bat Helpline, run by the Bat Conservation Trust (BCT), on (0845) 1300 228 or e-mail the BCT on enquiries@bats.org.uk. The Helpline is, however, only staffed Monday to Friday from 9am to 5.30pm (GMT). Ergo, should you find a grounded bat -- or should your family feline bring one home for you -- there are a few things you can do to make the creature a little more comfortable while you contact the necessary authorities. The Bat Conservation Trust has a list of guidelines on their website regarding the caring for of grounded and/or injured bats. Alternatively, you could contact your nearest bat group, who may offer advice over the telephone or send a qualified bat handler out to you. There are upwards of 90 volunteer bat groups in the UK, 33 of which have websites – a list of local bat groups can be found on the Bat Conservation Trust’s website. The following is a summary of the information provided by the BCT. First, and foremost is DO NOT pick the bat up with your bare hands, use decent gloves (gardening gloves are excellent) or a cloth.

Care for a grounded bat:

1. If the bat has injuries and you are going to be keeping the bat for longer than a couple of hours, prepare some suitable housing.

2. Housing should be something such as a shoebox or large margarine tub, with sufficient air holes (but no gaps larger than 5mm / one-quarter in.).

3. Line the housing with kitchen towel or soft cloth and place it in a warm spot – a dark airing cupboard is ideal.

4. Offer water regularly on a small clean paintbrush, cotton bud or in a teaspoon – don’t put a pot of water in with the bat.

5. Bats may be enticed to feed on small meaty chunks of cat food.

It is very important that you do NOT harm the bat – the law protects all species of UK bat and, under the Wildlife and Countryside Act of 1981, causing injury to them (or their roosts) is a criminal offence. (Back to Menu)

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