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 Marías Islands, Nayarit, Mexico. American Museum Novitates 1973, 1-14.