Bats are one of those groups of animals that I've come back to on several separate occasions, yet have never dealt with in satisfactory fashion (that is, comprehensively). Seeing as the group includes over 1110 living species, I hope that this is forgivable. But I have plans, and over the last few weeks a number of coincidental and unrelated events have caused me to do a lot of thinking and writing about the enormous, hugely successful, globally distributed microbat group known as Vespertilionidae.
The bats in this group are variously known collectively as common bats, plain-faced bats, evening bats or vesper bats. I prefer the last of these names and will be using it here. About 410 living vesper bat species (in about 48 genera) are currently known. This accounts for about one third of all bat species, making this group the largest 'family-level' mammal group after Muridae (the rats and mice). [Image above, from l to r: Kerivoula (woolly bat, from Francis et al. (2007)), Plecotus (long-eared bat, by Mauro Mucedda, from wikipedia), Antrozous (pallid bat, by M. Siders, from wikipedia). Image below: Common pipistrelle Pipistrellus pipistrellus in flight. By Barracuda1983, from wikipedia].
Molecular data indicates that Vespertilionidae sensu lato originated in the Eocene, with the divergence of this lineage from Molossidae, the free-tailed bats, occurring early in the Eocene (Miller-Butterworth et al. 2007).
While there are some supposed vesper bat fossils that are about this old (like Stehlinia from the Eocene of France and the UK), fossils of the group are rare until the Miocene. Some authors have said that other bat groups (like molossids and emballonurids) were preeminent prior to the Miocene, but then became disadvantaged as conditions became cooler and more seasonal. A post-Oligocene explosion of vesper bat lineages is indicated by molecular data.
In the series of articles that'll follow this one I'll be going through all the vesper bats of the world - yes, all of them... unless I get distracted. Long-time readers will know that I have a nasty habit of starting a series of articles on a group of animals, only to apparently move on and leave it incomplete (examples: anurans, temnospondyls, toads). This doesn't happen because I lose interest - rather, incomplete articles in the middle of the series slow things down and demand that I have to move on, and I then fail to find the time to finish the problem sections. To ensure that this didn't happen here, I've made a point of finishing the whole series. If you love bats you're in for a treat. If you don't... well, there are plenty other blogs out there. None as good as this one, of course, but you get the point.
Vesper bat basics
Vesper bats tend to have short, plain snouts (they mostly - but not entirely - lack the nose leaves present in many other microbat groups). In the skull, a V-shaped palatal or narial emargination is present along the midline of the snout [obvious here, in a Pallid bat Antrozous pallidus skull; from Van Gelder (1959)]; it means that the upper incisors are typically reduced in number down to one or two pairs. Vesper bat ears and tragi (the cartilaginous projections located inside the external ear. Singular = tragus) tend to be simple in shape, though ribbing on the ears is common and enormous ears (sometimes as long as or longer than the body) have evolved several times within the group.
The vast majority of vesper bats are insectivores that pursue moths, beetles, flies and such insect prey near water or around vegetation. However, some species are specialised gleaners that pick prey off vegetation or even from the ground: some of the species that do this (like the Pallid bat) superficially resemble the megadermatids (ghost bats or false vampire bats) of Asia, Africa and Australasia, but are typically nowhere near as large. Other vesper bats are pursuit predators of open spaces, in cases even catching small birds in flight. One or two species (like the Australasian big-eared bats (Nyctophilus) and the juveniles of some Myotis species) have been reported to hunt from perches in the manner more typical of horseshoe bats and megadermatids. Two African vesper bats (the Mimetillus species) have bizarrely short wings and a peculiar head and body shape - they're among the strangest bats of them all.
An enormous diversity of wing shapes is present in vesper bats: they span most of the diversity present in bats as a whole, with extremely short, broad wings and low wing loading being present in slow-flying gleaners (like the long-eared Plecotus bats), and long, slender wings and high wing loading being present in fast-flying hawking bats (like the bent-winged bats and some hairy-tailed bats) [the composite above shows the plecotin Corynorhinus (image by BLM, from wikipedia) compared with a bent-winged bat. The graph below - from Norberg & Rayner (1987) - plots wing loading against aspect ratio in various vesper bats and also natalids, thyropterids, mystacinids and molossids. I know it's hard to make out the details, but the vesper bats are represented by the open circles. Among the more noteworthy data points: long-eared bats and woolly bats are at bottom left, serotines mostly cluster near the middle, 'pipistrelles' are scattered all over the place (in part because 'pipistrelles' of tradition are polyphyletic), noctules are at extreme right, barbastelles are right down at the bottom, and Mimetillus is on its own at extreme bottom right].
The largest vesper bats (like the Great evening bat Ia io) have wingspans of just over 50 cm and can weigh 63 g, while the smallest (like the Lesser bamboo bat Tylonycteris pachypus) have wingspans of about 19 cm and can be adult at just 2 g. This size is about similar to that of Craseonycteris thonglongyai (the Bumblebee bat or Kitti's hog-nosed bat: NOT a vesper bat), meaning that some vesper bats are contenders for 'world's smallest mammal'.
In keeping with their morphological diversity, the echolocation calls used by vesper bats span the range from high frequency, broadband FM (good for slow hawking or gleaning in and around vegetation) to low frequency, narrowband FM or CF calls (good for long-distance detection of objects in uncluttered habitats). Some species (e.g., Geoffroy's bat Myotis emarginatus) are highly flexible in terms of call structure, using different durations and bandwidths according to whether they're commuting across distance, hunting in foliage, or hunting in the open, and some gleaning species (like the long-eared Plecotus bats) rely on prey-generated noises and eyesight as much as echolocation.
Vesper bats tend to have tails that reach the end of the interfemoral membrane (the membrane that stretches between the back legs, also known as the tail membrane or uropatagium) [the adjacent image showing a noctule's membranes is borrowed from here]. In cases the tail's tip slightly protrudes beyond the membrane's margin. Many species use the membrane to scoop up prey during flight, at the same time encircling it with the wings and reaching down to grab it in the mouth. The trailing edge of the interfemoral membrane is typically supported by a long, highly mobile calcar. The calcar is particularly long in species that use their feet to capture prey from the water surface, perhaps because these bats need to lift the interfemoral membrane up away from the water. A small post-calcarial lobe is often present (it's notably absent in the Myotis species and some others).
Hibernation and migration
Many vesper bats are well known for using caves, houses and hollow trees as roost sites, and hibernation is a common habit in the species of cool habitats. A common pattern is for vesper bats to use trees as summer maternity roosts, and houses and caves as winter hibernacula. Caves are frequently preferred as hibernacula because they are cold (but not too cold), typically with a stable microclimate, and often humid. However, there are some species that hibernate in cavities in soil, in leaf litter among tree roots, or hanging from branches, sometimes even right out in the open. The American hairy-tailed bats (Lasiurus) provide the best examples of such behaviour, and their densely furred interfemoral membranes presumably help insulate them from cold air [see photo below of a Hoary bat L. cinereus, from wikipedia. Hopefully the thick, extensive coat is obvious. The bat doesn't look very happy].
The biology of hibernation is a pretty huge subject that I won't do justice here. Bats typically enter hibernation after laying down fat reserves that account for 20-30% of their body weight; ambient temperature and day length are most likely the main factors that induce this behaviour. A bat in full hibernation may slow its heart rate to something like 10-16 beats/minute (compared with 250-450 beats/minute during stationary periods at other times of the year) and can go for 60-90 minutes without taking a breath (Altringham 1999).
How might hibernation have evolved? Many vesper bats of temperate zones (and a few of tropical zones) are facultative heterotherms: that is, they allow their body temperature to rise or fall with the ambient temperature - a great energy saving strategy, so long as they stay within a 'safe' thermal zone. Low temperatures allow these heterothermic bats to enter torpor: a physiological condition where oxygen consumption and heart rate are reduced, and where blood flow is restricted to the vital organs, but where arousal occurs independently of ambient temperature. Torpor commonly occurs on a daily basis among temperate vesper bats, and it essentially seems that hibernation is an extended form of torpor, lasting days, weeks or even months.
Some vesper bats are highly migratory, in cases crossing the greater part of continents and making one-way trips of over 1000 km. An interesting thing is that these migratory habits are typically inferred, rather than observed, but I don't think there's doubt that they really do occur. Migratory species tend to be tree-roosters of temperate climates, but uncertainty over the timing and geography of vesper bat origins renders it difficult to determine whether they started their history in temperate or tropical regions. Anyway, neither model of origins tells you anything useful about the presence or absence of a migratory habit: species that moved from tropical climates into the temperate zone might still migrate back to the tropics to winter, and species that originated in the temperate zone might have evolved migration in order to avoid cold winter temperatures.
Bisson et al. (2009) mapped the migratory habits of vesper bats onto a phylogeny and found that migration had evolved at least six times independently. They used the Jones et al. (2005) supertree for their phylogeny (it differs in structure from more recent appraisals of vesper bat phylogeny, but that's ok since the clades with migratory species are still well separated in other, more recently generated trees). Teeling et al. (2005) proposed that bats originated in Laurasia (perhaps in North America): this inspired Bisson et al. (2009) to suggest that migration evolved as a response to falling Cenozoic temperatures, not as a seasonal return to ancestral tropical climes. [Image below: noctules, by Eduard Oscar Schmidt, from wikipedia].
Putting vesper bats in the bat family tree
As usual for enormous groups that have expanded over time to contain hundreds of species, unique diagnostic characters have never been identified for Vespertilionidae: many bats identified as vesper bats have only been identified as such because they more resemble other vesper bats than members of other groups, and because they lack the obvious peculiarities of other groups. This strategy has (it seems) mostly worked out, but it has also led to a few failures, as we'll see. Because good character evidence linking all vesper bats has been elusive, some authors have suggested that the group might not be monophyletic. Having said all this, molecular studies have generally found Vespertilionidae as traditionally conceived (that is, Vespertilionidae sensu lato) to be a clade.
There's no question to begin with that vesper bats belong within the bat clade Yangochiroptera, and not within Yinpterochiroptera (the clade that includes megabats, horseshoe bats and kin)* (yes: mostly** gone are the days where bats could be neatly divided into megabats and microbats). Within Yangochiroptera, the majority of recent phylogenetic studies have found Vespertilionidae (sensu lato) to be the sister-group to Molossidae, the free-tailed bats (e.g., Simmons & Geisler 1998, Van Den Bussche & Hoofer 2004, Teeling et al. 2005, Gu et al. 2008). Natalids appear to be the sister-group to the vespertilionid + molossid clade; the name Vespertilionoidea is used for (Natalidae + (Vespertilionidae + Molossidae)) (Hoofer et al. 2003, Teeling et al. 2002) [see molecular timescale for all Chiroptera below, from Miller-Butterworth et al. (2007)].
* Hutcheon & Kirsch (2006) argued that terms such as Yangochiroptera and Yinpterochiroptera "no longer embody the authors' intended groups or have been so frequently redefined as to be positively misleading" (p. 1). They therefore proposed the new names Vespertilioniformes (for the 'core microbats') and Pteropodiformes (for megabats, horseshoe bats and kin).
** I did originally say "long, long gone", but then Agnarsson et al. (2011) was published. In some topologies, these authors did find a megabat-microbat split at the base of crown-Chiroptera. However, this tree is based on data from a single gene, stands in real contrast to the majority of other modern phylogenetic studies, and was noted by its authors as representing a "a crude estimate of the bat species tree".
As is typical for large, speciose groups, unravelling the relationships within Vespertilionidae has been difficult and an enormous amount of (sometimes conflicting) character information means that workers have disagreed about detailed relationships. The reason I've been referring to "Vespertilionidae sensu lato" is because a small number of notably distinct lineages, traditionally included within Vespertilionidae, now seem to be outside the clade subtended by all other vesper bat lineages; for this reason, these divergent lineages (they include the bent-winged bats [Miniopterus] and wing-gland bats [Cistugo]) are now excluded from Vespertilionidae by many authors. For the sake of completeness I will, however, be including them in the series of articles that's about to follow. It's the thorny issue of vesper bat phylogeny and classification that we'll be looking at next.
PS - as usual, getting good images of some of these animals has proved difficult or impossible. If you have any good vesper bat pics you're able to share with me - preferably of obscure species - please do make contact (eotyrannus at gmail dot com).
For previous Tet Zoo articles on bats, see...
- Desmodontines: the amazing vampire bats
- Giant extinct vampire bats: bane of the Pleistocene megafauna
- Camazotz and the age of vampires
- Dark origins: the mysterious evolution of blood-feeding in bats
- A new hypothesis on the evolution of blood-feeding: food source duality involving nectarivory. Catchy, no?
- Oh no, not another giant predatory flightless bat from the future
- The most terrestrial of bats
- I stroked a pipistrelle
- Red bats
- We flightless primates
- Big animalivorous microbats
- Hidden in plain sight: discovering cryptic vesper bats in the European biota
- PROTOBATS: visualising the earliest stages of bat evolution
Refs - -
Agnarsson, I., Zambrana-Torrelio, C. M., Flores-Saldana, N. P. & May-Collado, L. J. 2011. A time-calibrated species-level phylogeny of bats (Chiroptera, Mammalia). PLoS Currents 011 February 4; 3: RRN1212. doi: 10.1371/currents.RRN1212.
Altringham, J. D. 1999. Bats: Biology and Behaviour. Oxford University Press, Oxford.
Francis, C. M., Kingston, T. & Zubaid, A. 2007. A new species of Kerivoula (Chiroptera: Vespertilionidae) from Peninsular Malaysia. Acta Chiropterologica 9, 1-12.
Gu, X.-M., He, S.-Y. & Ao, L. 2008. Molecular phylogenetics among three families of bats (Chiroptera: Rhinolophidae, Hipposideridae, and Vespertilionidae) based on partial sequences of the mitochondrial 12S and 16S rRNA genes. Zoological Studies 47, 368-378.
Hoofer, S., Reeder, S., Hansen, E., & Van Den Bussche, R. (2003). MOLECULAR PHYLOGENETICS AND TAXONOMIC REVIEW OF NOCTILIONOID AND VESPERTILIONOID BATS (CHIROPTERA: YANGOCHIROPTERA) Journal of Mammalogy, 84 (3), 809-821 DOI: 10.1644/BWG-034
Hutcheon, J. M. & Kirsch, J. A. W. 2006. A moveable face: deconstructing the Microchiroptera and a new classification of extant bats. Acta Chiropterologica 8, 1-10.
Jones, K. E., Purvis, A., MacLarnon, A., Bininda-Emonds, O. R. P. & Simmons, N. B. 2002. A phylogenetic supertree of the bats (Mammalia: Chiroptera). Biological Reviews 77, 223-259.
Miller-Butterworth, C. M., Murphy, W. J., O'Brien, S. J., Jacobs, D. S., Springer, M. S. & Teeling, E. C. 2007. A family matter: conclusive resolution of the taxonomic position of the long-fingered bats, Miniopterus. Molecular Biology and Evolution 24, 1553-1561.
Norberg, U. M. & Rayner, J. M. V. 1987. Ecological morphology and flight in bats (Mammalia; Chiroptera): wing adaptations, flight performance, foraging strategy and echolocation. Philosophical Transactions of the Royal Society of London B, 316, 335-427.
Simmons, N. B. & Geisler, J. H. 1998. Phylogenetic relationships of Icaronycteris, Archaeonycteris, Hassianycteris, and Palaeochiropteryx to extant bat lineages, with comments n the evolution of echolocation and foraging strategies in Microchiroptera. Bulletin of the American Museum of Natural History 235, 1-182.
Teeling, E. C., Madsen, O., Van Den Bussche, R. A., de Jong, W. W., Stanhope, M. J. & Springer, M. S. 2002. Microbat paraphyly and the convergent evolution of a key innovation in Old World rhinolophoid microbats. Proceedings of the National Academy of Sciences 99, 1431-1436.
- ., Springer, M. S., Madsen, O., Bates, P., O'Brien, S. J. & Murphy, W. J. 2005. A molecular phylogeny for bats illuminates biogeography and the fossil record. Science 307. 580-584.
Van Den Bussche, R. & Hoofer, S. R. 2004. Phylogenetic relationships among recent chiropteran families and the importance of choosing appropriate out-group taxa. Journal of Mammalogy 85, 321-330.
Van Gelder, R. G. 1959. Results of the Puritan-American Museum of Natural History expedition to western Mexico. 8. A new Antrozous (Mammalia, Vespertilionidae) from the Tres MarÃas Islands, Nayarit, Mexico. American Museum Novitates 1973, 1-14.
Mention of bat hibernation these days should acknowledge the 'white-nose' fungal disease that is wiping out entire hibernacula in NE N America. It's a very serious conservation crisis in progress.
You're quite right - but I didn't mention it here because vesper bat decline, conservation etc. is discussed much later on in the series. I was googling for images of hibernating bats and seeing those many photos of 'white-nosed', infected bats is pretty horrible.
Comment unrelated--Darren someday, you should do an post on how we know whether prehistoric animals were nocturnal or diurnal. Eye size obviously, but what other evidence is there.
Of course--I should know by now to Wait For The Series!
On the subject of WNS, however, this just out in Conservation Biology (ha! crack that one, Dr. Glamor Mags!)
Hutcheon & Kirsch (2006) argued that terms such as Yangochiroptera and Yinpterochiroptera "no longer embody the authors' intended groups or have been so frequently redefined as to be positively misleading" (p. 1). They therefore proposed the new names Vespertilioniformes (for the 'core microbats') and Pteropodiformes (for megabats, horseshoe bats and kin).
In addition to those chiropteran clade names we have, of course, 'Megachiroptera' as well as the (probably) non-monophyletic 'Microchiroptera'; so, which (if any) of these names should we use if we follow the PhyloCode?
Is it the largest family? How about Muridae, if Cricetidae are included? Who wins, Mickey's or Batman's folk?
I am such an idiot. The text clearly states that Muridae is the biggest 'family' (over 600 species, in the sense that excludes Cricetidae), yet I forgot this when writing the article's title. Thanks for catching this.
There are a few places where you can observe a spectacular fall migration of Vespertilionids. Probably the best is the Dnepr Delta in Ukraine: at dusk on warm September evenings, hundreds of bats fly southwest along the southern bank of the main channel. It's always single bats, never groups.
Sorry, but you said "entangling the relationships within Vespertilionidae" was difficult. I think you mean 'unraveling' or maybe 'teasing out'. I think the relationships within this group are entangled enough, don't you think? ;)
Several potentially dumb questions from the back:
1) When a bat is walking around on all fours, how do the wing fingers fold up? Where do they go? In the illustration above, it looks like they disappear entirely.
2) Does the calcar have a bony core?
3) The calcar looks a bit like the pteroid bone. Do you think it provides a nice model for how the pteroid bone evolved?
4) Is body size in bat evolution constrained more through diet or competition from birds...if at all?
Zach, I'm sure Darren will pitch in with some answers, but here is my two cents, as well:
1) The fingers mostly flex, but slightly rotate, to fold tightly behind the antebrachium and elbow. They roll medially a bit, which is why it looks like the fingers vanish.
2) Yes, the calcar has a bony core in at least most taxa, but it also has a distinct cartilaginous component. It is especially large and bony in Noctilio (fishing bats)
3) The calcar might have similar development to the pteroid, but the functional implications are quite different, so it's more of a developmental model than a selection model (though in both cases, at a general level, there are control surface manipulation actions)
4) No one really knows what is constraining body sizes in bats. They certainly have not hit their theoretical mechanical limit, but few animals do. They would be more constrained than birds, mechanically, because bats are not pneumatic. This means that at large sizes the strength:weight ratio of the long bones would be insufficient. However, no known bats are anywhere near the size where that would really matter.
@Zach: I'd nominate respiration as the biggest constraint on bat size. Unlike birds and Pterosaurs, bats have fairly simple lung and no air-sacs. If I recall the calculations, fruit bats are near the upper bound for workable bats, at least under current oxygen levels. Personally, I don't entirely understand where there are no gorilloid flightless bats, but that's neither here nor there.
Hope you'll delight us with some discussion and pictures of Mimetillus and other "strange" bats as well as the general overviews. Weird animals are always fun to read about, even if you have to explain to us just *why* a certain animal is weirder than it looks ;)
PhyloCode and bats (comment 5): this depends on who decides to propose formal phylogenetic definitions for the respective clade names. At the moment, strict definitions are hard to find and (to my knowledge) nobody has determined historical precedent. Hutcheon & Kirsch (2006) explained how Vespertilioniformes and Pteropodiformes would (obviously) correspond to the clades including Vespertilio and Pteropus, respectively, but didn't formulate specific definitions.
Chris (comment 13): yes, Mimetillus and all the others are going to be dealt with in full :) I had to draw my own Mimetillus pictures - there's nothing available anywhere, except in such books as Walker's Mammals of the World.
Re: "and enormous ears (sometimes as long as or longer than the body) have evolved several times within the group"
Extra! Extra! This just in! Otopteryx is not a hopsorhinid Rhinogradentian in which the ears have functionally replaced the wings (and the nose leaf has evolved into a rudder)...
Seriously, the huge ears on some bats MUST have an effect on their aerodynamics. Has this been investigated? Perhaps with reference to the question of what crests might have done for or to Pterosaurs?
Oops! The words "but a derived Microchiropteran" should be inserted after the word "Rhinogradentian" in my last post.
At least according to John Altringham, echolocation and physics thereof is the body size limiter in bats. [warning: physics ahead!]
When flying, bats inhale on the wing upstroke, and exhale on the downstroke. Echolocation calls are energetically hugely expensive, so the bats only normally call on the downstroke (a feeding buzz is different, but not sustained for very long). So to "see" where it is going and where its prey are, a bat's echolocation must cover slightly more distance than the bat flies between each downstroke.
The limit of resolution for any wave-based detection system is about the wavelength of the wave used. For light, this is down in the nanometer range and so irrelevent, but for sound this limitation becomes very relevent. Pipistrelles normally echolocate around 45 KHz or higher; this limits their resolution to objects a few millimeters across, forcing them to emit "scanning" calls to get better resolution for hunting small flies, etc. When homing in on an insect, a bat always emits a feeding buzz, which is a lot of very high-pitched calls close together to let it resolve the object much better; this works close-in but is useless for navigation in flight.
The bigger the bat, the slower the wingbeats and the longer between normal calls. The longer between calls, the longer-ranged the echolocation has to be, and the lower frequency it must be. The lower the frequency, the larger the minimum size of object resolved. So, over a certain size a bat won't be able to resolve small twigs with echolocation and will have to forage in open environments, and eat something other than insects.
Fishing bats grow bigger than insectivores, but fly mostly out over rivers. Over a river, they can navigate visually so need only echolocate to look for fish sticking up through the water surface; a much, much easier echolocation target than an insect in a complex forest environment. So, these bats can grow much bigger than insectivores.
Nice try Dan... but the biggest bats (fruit bats) don't echolocate (well, with one exception). This constraint would only apply to rhinolophoids and yangochiropterans.
One other thing - some species belonging to echolocating lineages seem not to practise echolocation much at all, mostly relying instead on super-sensitive hearing to find prey (Altringham even has a subheading on p. 107: "Some microbats do not echolocate - hunting by other means"). I can agree that echolocating bats might be constrained to small size, but this can't work for the respective clades as a whole, since species might lose the echolocating habit altogether. In fact some people have suggested that this is exactly what happened in fruit bats (it's been called the "deaf fruit bat scenario"): rhinolophoids and yangochiropterans echolocate, yet fruit bats (closer to rhinolophoids than to yangochiropterans in most current phylogenies) don't. Parsimony suggests that echolocation evolved once at the base of the crown and was later lost in fruit bats - though independent origins of echolocation in rhinolophoids and yangochiropterans have been suggested by some. Incidentally, the one case of echolocation present in fruit bats (Rousettus) must have evolved independently from that present in other bats. For detailed discussion (and support of loss of echolocation in fruit bats), see...
Jones, G. & Teeling, E. C. 2006. The evolution of echolocation in bats. Trends in Evolution & Ecology 21, 149-156.
Why is there any mystery about size limitation? Fruit bats get pretty damn big - larger than most eagles, at least in terms of wing area. Most birds which are bigger have lifestyles very different from those adopted by any bat and few if any are nocturnal. There doesn't seem to be any reason to expect a condor-sized bat.
the huge ears on some bats MUST have an effect on their aerodynamics. Has this been investigated?
Yes, it has. See:
Gardiner, J.D., Dimitriadis, G., Sellers, W.I. & Codd, J.R. 2008. The aerodynamics of big ears in the brown long-eared bat Plecotus auritus. Acta Chiropterologica 10, 313-321.
Fruit bats get pretty damn big
Indeed. For example, in terms of both body mass and wingspan, the Indian flying fox Pteropus giganteus overlaps with the herring gull Larus argentatus and the northern raven Corvus corax - and neither of these birds is usually thought of as being 'small'.
I distinctly remember seeing Pteropus species apparently dwarfing a white bellied sea eagle Haliaeetus leucogaster over islets of Pulau Handeleum in Java. A quick look at Wikipedia suggests that 'dwarfing' must be an exaggeration of either my memory or perception but that the wingspans are comparable, though the bat is lighter. I suspect the bat has a much larger wing area than the eagle. Or conceivably Wikipedia's measurements for the eagle are for Australian individuals which could be larger than Indonesian ones (though I have no evidence to that effect). I'll look it up later.
Does any bat-eating bat exist?
I hope that one can recognize from a skull whether external ears are present. When in mammal fossil history does the external ear start to appear?
Does any bat-eating bat exist?
Yes. Large phyllostomids (e.g., Vampyrum spectrum) and megadermatids (e.g., Megaderma lyra) are known to prey on smaller bats. I'm not aware of any bat-eating vesper bat species, though.
On bat-eating bats, I previously used a nice photo here. It features the predatory phyllostomid Chrotopterus auritus. As Dartian said, megadermatids (= false vampire bats and ghost bats) are confirmed predators of other bats too (including of smaller megadermatids). As for vesper bats, I bet that the two confirmed avivores among them do eat smaller vesper bats on occasion. More on that later.
On external ears (comment 24): nope, you cannot ever be sure that external ears - meaning pinnae - are present when all you have is the bones. However, a set of bony features that correlate with high-frequency hearing are widespread across mammaliaforms, and suggest that pinnae have been ancestrally present across the entire clade (and not just limited to the crown). Pinnae have, of course, been lost a few times in aquatic and fossorial groups.
Thanks, both, for the update on bat-eating bats. I'm sorry I missed that post.
As to the pinnae, I wondered as Wikipedia gives a sort of reconstruction of Adelobasileus with small ears visible, under Mammaliaformes. So, just like the hair. Ida seems to have nice round ears, on her photographs.
External pinnae are primitive for at least the crown-clade mammalia. Echidnas have them (buried in fur). The loss in aquatic and fossorial clades is derived as Darren states.
Platypodes do have external pinnae, just very heavily modified for their aquatic existence.
About bats' constrains:
- insectivore size is limited by size of its insect prey, and this by insects' tracheal breathing. For primates, the border was determined as ca 200g (if I remember), above this a primate runs into time constraint of catching individual insects, cannot meet energetic demands and switches to other food.
- Bats' sonar and poor eyesight forces them to be nocturnal, bats flying at daytime are very easily caught by birds (for example, from behind).
I hope this won't be posted twice.
On phylogenetic nomenclature, I suggest the community (or as many people as possible) get together, agree on something, and publish that together. That's what we've done for Amphibia and Lissamphibia... except you won't see it before the companion volume to the PhyloCode will be published. Lissamphibia is a very carefully defined crown group, Amphibia its total group.
independent origins of echolocation in rhinolophoids and yangochiropterans have been suggested by some
Isn't there good evidence for echolocation in all stem-bats except Onychonycteris?
Platypodes do have external pinnae
Wow! Details, please!
On size constraints (comment 29): sure, insectivory is going to act as a size-limiter for insectivorous species. But this isn't "the" answer for size constraints in bats since (1) when members of insectivorous lineages become big, they can switch to other prey (including small terrestrial vertebrates and night-flying birds), thereby potentially becoming free from the energetic constraints associated with insectivory, and (2) insectivory is not ubiquitous in bats. Frugivory, nectarivory and carnivory have all evolved more than once.
I reckon it's a combination of things: competition from birds and thermoregulatory constraints perhaps being most important. I'm planning a series of articles on megabats at some stage and will be returning to this issue.
On the distribution of echolocation (comment 30), yes, some stem-bats were apparently echolocaters, which obvious supports the hypothesis of a single origin for echolocation somewhere on the stem (sorry, not at the base of the crown as I said above). There's then a single loss in fruit bats, followed by a novel acquisition within fruit bats.
And on PhyloCode definitions... I agree: getting the right people together is the thing to do. However, I tried (and eventually succeeded) in assembling a team to do Theropoda for the volume, and it was surprising how many noted theropod workers refused to be involved (their main argument being that we just don't need no PhyloCode!).
Oh - and I'm pretty sure that Ornithorhynchus lacks pinnae (I've checked myself). They're definitely present in echidnas, however.
Interesting. So a range of different factors are proposed as limits on bat size. It would be interesting for someone to do a book on theoretical upper limits in different classes of animals. There has to be a structural limit to each type of animal wing just like there is to each type of airplane wing (over a certain size, you have to add more spars, different construction methods, stronger and lighter materials, etc., and this is easier to solve with fixed wings than with flapping wings). If pterosaurs could get to 10m+ with a wing that seemed less well supported and braced than that of bats, though, I always wondered why bats didn't get bigger.
@31 sorry, I thought that flap that seals the eyes and ears was a heavily modified pinnae combined with eyelids. My mistake.
A further food for thought-comment regarding the question why there are no 'really large' bats:
The largest extant chiropterans are almost exactly as large as the largest extant passerines (see comment #21). In terms of species richness, global distribution and ecological diversity, the passerines are an even more successful group than the bats - and yet there are no pelican- or turkey-sized passerines. Is that because there are biomechanical constraints that prevent passerines from becoming larger than a raven? Or is it that passerines could get significantly larger than they are, but that they just haven't (for whatever other reason)?
I love this line of discussion, all of these questions are still outstanding in bat biology and we are still trying to uncover them. This discussion makes me very happy that all of the questions that drive my research are still of great interest to many people!