Vesper bats. Well done with sticking with it all so far – I have lots of non-bat stuff I want to cover, but (for reasons soon to be explained) I need to get this series finished. With this article – part XI in the series (XI) – we are not at the end. But we are at the beginning of the end. Look at the cladogram below to see where we are, and follow the links below if you want to know what happened before. We arrive now at Antrozoini…
The Pallid bat or Desert bat Antrozous pallidus of western North America and Cuba is a large (wingspan 37-41 cm; mass 20-35 g), fairly scary-looking, big-eared vesper bat with proportionally large eyes, a square-shaped muzzle and creamy or yellowish dorsal fur [image above by Anne-Marie, formerly of the apparently defunct Pondering Pikaia]. It superficially resembles the Old World desert long-eared bats (Otonycteris), though this resemblance most likely represents convergence. Semi-circular ridges surround each nostril and its snout glands produce a skunk-like odour.
Like some megadermatids (false vampires and ghost bats), it flies low to the ground (from 50 cm to 2.5 m up), searching for low-flying or grounded insects, arachnids, rodents and lizards, and (in another parallel with megadermatids), it may use passive acoustic cues to find prey rather than echolocation. It may alight on the ground when capturing prey; its pelvic and hindlimb bones are said to be especially robust and hence to reflect this behaviour, though I don’t know if this has been properly demonstrated. It’s a desert specialist and can go for a month without drinking (all water coming from prey animals); also notable is the large amount of co-operation recorded among females: “Female roost mates have been observed helping each other through labor and helping to locate lost young” (Graham 2002).
The Pallid bat is not a phylogenetic singleton: it seems to be closely related to the peculiar Van Gelder’s bat Bauerus dubiaquercus of Mexico, Belize, Honduras, Guatemala, El Salvador and Costa Rica. In fact, Bauerus has been included in the genus Antrozous by some. While both are somewhat similar, Bauerus is darker with smaller ears; the two also differ substantially in skull shape and tooth morphology. In real contrast to Antrozous, Bauerus is a tropical forest bat, inferred from head and wing shape to be a pursuit predator of insects. In contrast to Antrozous, there’s no indication from the pelvic skeleton that it might alight on the ground when hunting (though both the thumb and foot are long and robust). [Drawing of Bauerus below from Emmons (1997)].
Van Gelder’s bat was named in 1959 and remains poorly known. Its memorable species name – which means something like ‘doubt oak’ – is said on wikipedia to denote the fact that collector Oakes A. Plimpton (then of the American Museum of Natural History) was sceptical of this bat’s distinction as a new species. That’s such a nice story that I wish it were true but, in fact, the ‘dubia-‘ bit of the name honours the other collector, Richard G. Zweifel (and, in German, ‘zweifel’ means doubt) [thanks to Dartian for corrections: see comments below]. The generic name honours Harry J, Bauer, the sponsor of the expedition that led to the discovery of the bat (Engstrom et al. 1987).
Antrozous and Bauerus are so distinctive relative to most other vesper bats that they’ve conventionally been given their own ‘subfamily': Antrozoinae. Some authors have gone even further and have suggested that this deserves to be promoted to ‘family': Antrozoidae (Simmons 1998, Simmons & Geisler 1998). In fact these authors also argued that antrozoids were likely not vesper bats at all, but a distinct lineage more closely related to molossids (the free-tailed bats or mastiff bats). However, genetic data shows that these bats belong within the vespertilionine radiation (Teeling et al. 2005, Miller-Butterworth et al. 2007, Roehrs et al. 2010), in which case their small clade should be termed Antrozoini [Pallid bat below by M. Siders, from wikipedia].
Fossils of Antrozous are known as far back as the Middle Miocene: some of these have been identified as the living Pallid bat (indicating a pretty surprising duration for a single species) while others have only been labelled as Antrozous sp. (Czaplewski 1993, Czaplewski et al. 2008). Anzanycteris anzensis from the Pliocene of California has been identified as an antrozoin by some authors (White 1969, McKenna & Bell 1997).
A particularly large fossil vesper bat from the Early Miocene of Florida – Karstala silva – possesses a lower molar tooth character (that is, a “groove alongside an inclined lingual cingulid”) unique to Antrozous and Bauerus. However, in other characters it resembles the hairy-tailed bats (Lasiurus) while in humeral shape it differs strongly from antrozoines and hairy-tailed bats. Its affinities remain uncertain (Czaplewski & Morgan 2000). With an estimated mass of 20-22 g – similar to a Hoary bat L. cinereus or European serotine Eptesicus serotinus – it’s North America’s largest known fossil bat (Czaplewski & Morgan 2000).
Another North American fossil vesper bat – Simonycteris stocki from the Pliocene of Arizona – is also somewhat similar to Antrozous, but it’s similar to serotines as well: based only on a skull fragment with teeth, it can’t (yet) be identified precisely (Czaplewski 1993).
Little yellow bats: where the cryptic species are
The little yellow bats or rhogeëssa bats (Rhogeessa) primarily occur in Mexico, but are also found throughout Central America as well as in Bolivia, Venezuela, Colombia, Brazil and Trinidad. As suggested by the name, they’re typically small (between 3 and 10 g) and often yellowish-brown dorsally. 11 species are currently recognised. Molecular studies have shown that populations that look just about identical are highly divergent in genetics, and indeed this is one of the first bat groups in which so-called cryptic species were discovered (Baker 1984). [The adjacent yellow bat belongs to a new species, the name of which was up for auction in 2008. I don’t know how this panned out (lots of species names have been put up for auction in recent years)].
They aren’t all “just about identical”, however; several of the species can be distinguished based on measurements of the toothrow, finger bones and so on, and large sagittal and occipital crests give some of the species a pronounced raised area at the back of the skull. Little yellow bats have fairly large, robust teeth: the lower jaw has a robust appearance with a particularly deep ‘chin’ region [skull diagrams below from Baker (1984)].
The wing membranes of these bats are said to be unusually thick for bats of this size (Roots & Baker 1998, Vonhof 2000): it would be interesting to know if this correlated with any particular facet of behaviour. They have broad, naked interfemoral membranes. Males possess large, strongly smelling glands on the dorsal surfaces of the lower parts of the ears: as is the case for many vesper bat taxa, females are bigger than males (Audet et al. 1993). Little yellow bats are typically associated with humid, well vegetated lowland habitats, but populations of R. minutilla have been reported from semi-arid thorn forests. Czaplewski et al. (2005) suggested that this species – one of the world’s smallest mammals – might have become isolated as a relict during the late Pleistocene. Numerous fossils of Rhogeessa are known from the Pleistocene of Venezuela, though it hasn’t been possible to work out whether they represent new species or are referable to modern ones (Czaplewski et al. 2005).
Within Rhogeessa sensu lato, R. alleni has long been regarded as the ‘most divergent’ in terms of morphology and hence worthy of its own ‘subgenus’, Baeodon Miller, 1906. Genetics confirms this distinction, but shows that R. gracilis should be included in Baeodon too (Baird et al. 2008) [in the adjacent montage, B. alleni is on the left and R. hussoni is on the right. Both images are from Professor Paul’s Guide to Mammals].
Hoofer & Van Den Bussche (2003) regarded Baeodon as properly (viz, ‘generically’) distinct from Rhogeessa and included both taxa within Antrozoini. In their large molecular analysis of vesper bats, Roehrs et al. (2010) found Rhogeessa to form a clade with both Antrozous and Bauerus, but the position of Baeodon was less well resolved: in some trees compiled from nuclear DNA data, Baeodon was the sister-taxon to the hairy-tailed bats (Lasiurus). However, the authors noted that this position was likely incorrect.
Tomopeas: get out, the party’s over, you ain’t no vesper bat
Having mentioned molossids, it’s worth discussing another bat once thought by many bat workers to be somehow intermediate between this group and Vespertilionidae.
It’s the Blunt-eared bat Tomopeas ravus, a tiny (3 g) Peruvian bat of arid habitats, often included among the vesper bats even though a list of anatomical features (including an overhanging upper lip, tubular nostrils, tail-tip that extends beyond the edge of the uropatagium, and fused seventh cervical and first dorsal vertebra) are more reminiscent of molossids (Miller 1907, Davis 1970). Despite this obvious early evidence for non-vespertilionid status, most authors continued to treat Tomopeas as a vesper bat, albeit one deserving of its own ‘subfamily’ (Tomopeatinae). However, Davis (1970) did suggest that it might best be given its own ‘family’ positioned in between Vespertilionidae and Molossidae.
Molecular studies eventually confirmed that Tomopeas is indeed a molossid, and in fact one of the ‘most basal’ members of the group (Sudman et al. 1994). This recovered position is consistent with the hypothesis that vesper bats and molossids are sister-taxa, since it seems logical that primitive members of both groups might exhibit symplesiomorphies (shared primitive characters).
For previous Tet Zoo articles in the vesper bats series, see…
- Introducing the second largest mammalian ‘family': vesper bats, or vespertilionids
- The vesper bat family tree: of myotines, plecotins, antrozoins, and all those cryptic species (vesper bats part II)
- Bent-winged bats: wide ranges, very weird wings (vesper bats part III)
- Of southern African wing-gland bats, woolly bats, and the ones with tubular nostrils (vesper bats part IV)
- The many, many mouse-eared bats, aka little brown bats, aka Myotis bats (vesper bats part V)
- Long-eared bats proper: Plecotus and other plecotins (vesper bats part VI)
- Desert long-eared bats – snarling winged gremlins that take scorpion stings to the face and just don’t care (vesper bats part VII)
- Hairy-tailed bats: a tale of furry tails, red coats, cold tolerance, migration and sleeping out in the open (vesper bats part VIII)
- Robust jaws and a (sometimes) ‘greenish’ pelt: house bats (vesper bats part IX)
- Australasian big-eared bats, and how to (perhaps) single-handedly wipe out an entire species, 1890s-style (vesper bats part X)
And 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 – –
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Baird, A. B., Hillis, D. M., Patton, J. C. & Bickham, J. W. 2008. Evolutionary history of the genus Rhogeessa (Chiroptera: Vespertilionidae) as revealed by mitochondrial DNA sequences. Journal of Mammalogy 89, 744-754.
Baker, R. J. (1984). Mammalian sympatric, cryptic species: a new species of Rhogeessa (Chiroptera: Vespertilionidae). Systematic Zoology, 32, 178-183
Czaplewski, N. J. 1993. Late Tertiary bats (Mammalia, Chiroptera) from the southwestern United States. The Southwestern Naturalist 38, 111-118.
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– ., Rincón, A. D. & Morgan, G. S. 2005. Fossil bat (Mammalia: Chiroptera) remains from Inciarte Tar Pit, Sierra de Perijá, Venezuela. Caribbean Journal of Science 41, 768-781.
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