The name Chiroptera is derived from the Greek words cheir (=hand) and pteron (=wing), referring to the hand-like wings of bats, the primary adaptation for flight (Feldhamer et al. 2007). The wing is formed from skin (patagium) stretched between the arm, wrist, and finger bones, which are light and slender. The forearm (radius and ulna), metacarpals, and fingers are elongated and, with the patagium, they form a structure that is “strong like an umbrella” (Neuweiler 2000). The thumb remains free and is the only digit that retains a claw in extant microbats. The wing membranes usually attach along the sides of the body and, besides their primary function in flight, they also aid bats in thermoregulation – the thin, vascularized membranes dissipate excess body heat generated during flight (Feldhamer et al. 2007).
Many skeletal features are associated with a need for strong or specialized attachments for the powerful flight musculature, which has influenced the architecture of the vertebral column, pectoral girdle, sternum, and first rib (Vaughan 1970). On the other hand, the hindlimbs are smaller than the forelimbs, presenting a 90-degree or more rotation around the longitudinal axis of the body, so the knees point backwards (Czaplewski et al. 2008). This is important to the upside-down roosting posture and aids in several flight maneuvers. A special locking tendon allows bats to cling surfaces without spending energy (Feldhamer et al. 2007).
Microbats encompasses representatives of the suborder Yangochiroptera plus Rhinolophoidea (Yinpterochiroptera) and are differentiated from megabats (Pteropodidae; Yinpterochiroptera) by the following features: tragus often well developed; nose or facial ornamentation often evident; no claw on second digit; cervical vertebrae modified; head is flexed dorsally from main axis of body when roosting; tail and tail membrane often evident; generally small body size; eyes generally small; mandible with well-developed, long, and narrow angular process; postorbital process usually absent; palate usually does not extend beyond the last upper molars; and presence of a neomorphic calcar (Gunnell and Simmons 2005; Feldhamer et al. 2007).
There is a great range of size variation among microbats, from the small Craseonycteris thonglongyai (2–3 g) to the large Vampyrum spectrum (130–235 g). Since they are nocturnal animals, a living coloration would be worthless and therefore most bats present only variations between black and brown, with some reddish or yellowish species (e.g., Lampronycteris brachyotis, Noctilio leporinus). Even white fur may occur in some species (e.g., Diclidurus spp., Ectophylla alba) although this does not appear to increase its predation (Reis et al. 2007). The pattern of coloration is an important taxonomic tool that helps on the identification of some species, such as the tufts of white hair on the dorsal part of the forearm and the longitudinal stripes on the dorsal fur in Rhynchonycteris naso (Emballonuridae), the different facial strips patterns on some stenodermatines (e.g., Platyrrhinus spp.), the remarkable pattern of body fur coloration in Niumbaha superba (Vespertilionidae), and the stunning pattern of wing coloration in Kerivoula picta (Vespertilionidae) (Fenton and Simmons 2015; Reis et al. 2017). Besides taxonomy, the color pattern may function as a camouflage in roosting situations, potentially protecting bats from visually oriented predators. An example occurs in some reddish and yellowish lasiurine bats (Lasiurus borealis and L. intermedius) that roosts in the foliage of deciduous trees (Kunz and Fenton 2005).
Microbats usually have small eyes, large ears, well-developed tragus, nasal, and facial ornamentations, features primarily related to their echolocation system. The pronounced nose leaf of phyllostomids, for example, is important on the orientation of the ultrasonic pulses that come out from their nostrils. A projection from the lower margin of the ear, called tragus, is also important in echolocation (Feldhamer et al. 2007). On the other hand, the larger ears of gleaning bats, including some phyllostomids, are important to hear the sounds generated by insects or small vertebrates (Kunz and Fenton 2005; Reis et al. 2017). As the coloration patterns, the facial features are important to taxonomy: the flesh nose leaf present in Phyllostomidae, Nycteridae, Rhinolophidae, and Megadermatidae; the lips with folds and hairs around the muzzle that look like a mustache in Pteronotus spp.; the upper lip divided by two vertical grooves in Noctilionidae (Feldhamer et al. 2007; Gardner 2008).
Important structures to flight, the wing and uropatagium morphology also varies in bats. When viewed from above, wing shape varies from short and broad, to long and thin, being important to aerodynamics of flight. The uropatagium is the membrane between the hindlimbs that encloses the tail and may contribute to lift and to help on the stabilization of the body during turns and other maneuvers when bats are flying. A cartilaginous process, the calcar, helps to support the uropatagium. The size and shape of the tail membrane varies from extremely reduced (e.g., Desmodontinae) to well-developed, being sometimes even greater than the body length (e.g., Natalidae), and may completely enclose the tail (e.g., Vespertilionidae) or just a part of it (e.g., Molossidae) (Feldhamer et al. 2007; Reis et al. 2017).
The bats skull is generally long and low with a rather long and narrow rostrum and somewhat inflated braincase. However, the configuration of the skull and jaws varies greatly in some species, reflecting primarily the strikingly diverse feeding habits, but also roosting habits, echolocation and flight styles, and other morphological correlates (Vaughan 1970; Czaplewski et al. 2008). The rostrum of nectar-feeding bats (e.g., Glossophaginae and Lonchophyllinae), for example, is narrow and elongated, while in frugivorous bats (e.g., Stenodermatinae), it is short and broad. Some bats have the skull strongly flattened which facilitate the access to small spaced roosts (e.g., Tylonycteris robustula lives inside of bamboo culm; molossids roosts in rock crevices) (Kunz and Fenton 2005), while bats that roost by hanging have skulls with well-rounded braincases (Vaughan 1970). Regarding to dentition, bats do not have the 44 mammalian pattern teeth, being 38 the greatest number of teeth that are usually present in most insectivorous groups (e.g., Natalidae, Vespertilionidae) and 20 the fewest number of teeth, present in the vampire bat Desmodus rotundus (Vaughan 1970). The incisors and canines retain their typical form in most bats, but the former can be absent in some species (e.g., Megadermatidae). The first premolars are absent and additional lost occurs in many genera. The molars are W-shaped in most insectivorous species, being probably a primitive condition for bats (Czaplewski et al. 2008). The large neotropical family Phyllostomidae, however, presents a wide variety of molar morphology, according to its diet. The last molar is greatly reduced or lost in some members, usually associated with frugivorous and sanguivorous habits (Vaughan 1970). Like most mammals, newborn bats have a set of deciduous teeth that are precursors of the incisors, canines, and premolars. These teeth are slender, pointed, and sharply recurved and are used by the young bats to grip the mothers’ nipples (Vaughan 1970; Neuweiler 2000; Fenton and Simmons 2015).
The axial skeleton of bats is similar to that of other mammals. In bats, the spine is usually made of 7 cervical vertebrae, 11 thoracic vertebrae, and up to 10 caudal vertebrae. The pectoral area is highly developed and modified, and some features seem to be related to flight in these mammals. The pectoral ring, which anchor the wings, is formed by the fusion of the manubrium, the first two ribs, and the last cervical and first two thoracic vertebrae. The thoracic vertebrae are tightly connected to one another to form a rigid column. The articulating scapula and proximal end of humerus are also highly modified for flight. The sternum presents a keel for the insertion of strong pectoral muscles, the clavicula is robust, and the scapula is parallel to the vertebral column, holding the muscles that controls the wing-beat cycle during flight. The body of bats is short and broad, due to both an anteroposterior compression of the cervical and thoracic vertebrae, and to a strong dorsal arching of the thoracolumbar section of the vertebral column. Therefore, their rib cages are large and often broader than deep. Also, the cervical vertebrae are twisted so the bat’s head can stand raised during flight. The pelvis has undergone several adaptations that make it specially well suited for flight. In microbats, the ilium and the sacrum are fused together up to the level of the acetabulum (the socked into which the head of the femur fits), limiting the mobility at the iliosacral joint (Vaughan 1970; Neuweiler 2000; Feldhamer et al. 2007; Reis et al. 2007).
The bones of the forelimbs form the wings of bats. The large and strong humerus is suspended from the scapula. The elongated lower arm is consisted of two bones: the radius, which is thick and sturdy, and the ulna, which is thin. There are six carpals, five metacarpals, and five sets of phalanges. The second to fifth metacarpals are greatly elongated and the thumb is free and is the only clawed digit of the manus. The hindlimbs primarily serve to support the wing membranes, so they usually are small, thin, and not very muscular, but there are few exceptions from species that have quadrupedal behavior (e.g., desmodontines, molossids, mystacinids). The femur is roughly the same length as the tibia, its head is large and slightly offset from the long axis of the shaft, which allows a great freedom of movements at the hip joint, important during hanging, flighting, and crawling behaviors. The tibia is the only bone in the lower leg, once the fibula is vestigial in most bats. The feet are usually short, the phalanges and claws are laterally compressed, and the calcar is usually well developed and projects into the adjacent border of the uropatagium, which is important to keep the posterior part of the tail membrane extended (Vaughan 1970; Neuweiler 2000).
It is worth noting that studies on the postcranial anatomy may increase and improve the descriptions and identifications of bats that usually are made based on cranial characters. Recent researches on this area have been showing a great potential on taxonomic, phylogenetic, evolutionary, and morphofunctional studies (Gardner 2008; Gaudioso et al. 2017). From the paleontological view, such studies are also noteworthy, allowing the identification of extinct and extant specimens from fossil records, which many times are composed by fragmented bones (e.g., Czaplewski et al. 2008).
Despite the general description given above for both external and skeleton morphology of bats, each family presents a set of features that allow their differentiation from other families. Bellow, we describe a brief characterization of each family of microbats, based on Hill (1982), Hoofer and Van Den Bussche (2003), Garbutt (2007), Feldhamer et al. (2007), Lack et al. (2010), Happold and Happold (2013), Fenton and Simmons (2015), Stuart (2015), Armstrong et al. (2016), and Reis et al. (2017).
Cistugidae: The wing-gland, pimple-winged, or hairy bats formerly were grouped in the large genus Myotis (Vespertilionidae). The unique feature that distinguishes them from other bats is the presence of one or two pronounced pimple-like glands on the plagiopatagium of each wing, just near the forearm, that are evident in living animals, but may not be observed in dried museum specimens. There are only two species representing this family: Cistugo lesueuri and Cistugo seabrae. They are small bats and have a great color variation of their dorsal dense pelage. The tragus is long and narrow but not sharply pointed. Their tail is long and enclosed in the uropatagium.
Craseonycteridae: This monotypic family comprises the smallest species of bat, the hog-nosed bat Craseonycteris thonglongyai. They have large ears and tragus, and a distinctive plate on the nose. The tail and the calcar are absent, the uropatagium is large, and the wings are broad.
Emballonuridae: The sac-winged bats have this name in association to their most remarkable feature – wing sacs on the ventral surface of the wings, near the elbow. These sacs, that are more developed in males, exude a red, odiferous substance that is probably important in pheromone production and attraction of females. The ears are short and the long uropatagium is drilled in the upper face by the tail, which has its end free.
Furipteridae: The species of this family are known as smoky or thumbless bats. They have large and well-separated ears with a small triangular tragus, and dense fur. Their thumb appears to be absent, once it is very small and mostly enclosed in the wing membrane. The tail is long and enclosed in the long uropatagium.
Hipposideridae: They have an elaborate nose leaf (leaf-nosed bats), which consists of fleshy protrusions above the nostrils. These ornamented nose leaf present a great diversity in size and shape, varying among species. The size of the ears varies and the tail can be absent.
Megadermatidae: Known as false vampire bats by the false belief that they feed on blood, megadermatids have large ears that are united on the forehead, a divided tragus, and a large, erect nose leaf. All species lack upper incisors. The uropatagium is extensive, but the tail is short or absent.
Miniopteridae: They are known as long-winged or bent-winged bats and were first considered as vesper bats. However, some apomorphies distinguish them from all other vespertilionids. They have a prominent rostrum, the second phalanx of the third finger is about three time as long as the first, the baculum is absent, and there is a supplementary vestigial tooth between the upper canine and the first premolar. They have a long tail that is enclosed in the long uropatagium.
Molossidae: The free-tailed bats have a long and robust tail that extends well beyond the outer edge of the uropatagium. They have large ears that point forward and a tragus that is nearly rounded. The wings are long and narrow, the thumbs are short and thick, with a swollen base.
Mormoopidae: The species of this family are known as mustached, ghost-faced, or naked-backed bats. They have reddish-brown or dark brown coloration, the ears are short, and the tail projects dorsally from near middle of the long interfemoral membrane. The lips are enlarged and surrounded by numerous stiff hairs (“mustached”). Some species have the wings meeting middorsally, obscuring the fur (“naked-backed”) and some have conspicuous, leaflike appendages on the chin (“ghost-faced”).
Mystacinidae: The New Zealand short-tailed bats have a long snout, and their slit-like nostrils are located in a pad covered with bristles. The ears are moderately developed with a long and pointed tragus. They have very sharp claws on the hind feet and thumb, and a thick membrane along the side of the body where they can hide their wings when walking on the ground. The short tail projects from midpoint of dorsal surface of a medium-sized uropatagium.
Myzopodidae: The sucker-footed bats have suction disks on their wrists and ankles, not related with those in thyropterids. They have a unique mushroom-shaped process at the base of their large ears. Their toes have only two phalanges, and the thumb is small with a vestigial claw. The tail is long and extends beyond the well-developed uropatagium.
Natalidae: The funnel-eared bats are small bats with a long dorsal pelage that varies from yellow to reddish-brown. The tragus is short, and the large ears are funnel-like, giving these bats their common name. The forehead region is domed, and there are no ornaments on the face, although the males have a mass of glandular sensory cells (called natalid organ) on their muzzle. They have a well-developed uropatagium, which encloses the long tail.
Noctilionidae: Known as bulldog bats, noctilionids have a remarkable yellow to brown pelage coloration and a cleft upper lip. The hindlimbs are well developed, as the feet and claws, and the tail extends halfway through the long uropatagium. The ears are separated, narrow, and pointed.
Nycteridae: The slit-faced bats have this name due to the unusual longitudinal groove throughout the facial region, given by the deep concavity between the eye orbits. They have large ears and small tragus. The uropatagium is long and encloses the tail, which forms a unique T-shape on the tip.
Phyllostomidae: The leaf-nosed bats have a nasal ornament usually in the form of leaf or lance, that varies from highly developed (e.g., Chrotopterus and Lonchorhina) to extremely reduced and modified (e.g., Desmodus, Diaemus, and Diphylla, which have a horseshoe-shaped nose leaf). Their ears vary greatly in size and shape, and the tragus is always present. The tail may or may not be present, but when present is completely enclosed in the uropatagium. Some species have a well-developed uropatagium (e.g., Macrophyllum macrophyllum), but it can be almost absent in another species (e.g., Desmodus rotundus and Sturnira lilium).
Rhinolophidae: The horseshoe bats are named by their distinctive facial ornamentation, which varies from simple to very ornate, and includes a nose leaf above the nostrils, and horseshoe-shaped flaps bellow and on the sides of the nostrils. Between the horseshoe and the nose leaf there is a median projection called sella, which differs them from hipposiderids. The ears are large and pointed, lacking the tragus. The wings are broad and rounded, the uropatagium is well-developed, with the tail extending up to its end.
Rhinonycteridae: The trident bats and the diamond-faced bats are named by one of their most conspicuous external features, a complex nose leaf, which have three dorsally oriented fleshy processes in the posterior rhinarium in most genera (trident bats), and a diamond-like nose leaf, with the anterior leaf more or less pentagonal in outline in Rhinonicteris (diamond-faced bats). They have a flattened muzzle, and the ears can be short or large, usually pointed. The tail is long and its tip may exceed the margin of the long uropatagium in some species.
Rhinopomatidae: Also known as mouse-tailed bats, they have a long tail, nearly equal to their body length and even longer than their forearm. The uropatagium is short and most of the tail is free. The ears are large and connected across the forehead by a flap of skin. They have dark dorsal fur and paler ventral fur, being some areas of the face, rump, and abdomen, naked.
Thyropteridae: The disk-winged bats have this name due to their round and concave suctions disks at the base of the thumbs and on the soles of the feet. They are small bats with a long and slender snout and have small warts above their nostrils. The ears are moderately large and funnel-shaped, and the tragus is present. Their feet are tiny, with fused toes. The tail is long and extends beyond the well-developed uropatagium.
Vespertilionidae: The vesper bats present a great variation of size, pelage coloration, and morphology features. They have small eyes and ornaments on the nose are usually absent, although their face may be adorned with swollen glands and other structures. The ears vary in size and shape, some species with small rounded ears (e.g., Lasiurus cinereus) and other ones with elongated ears (e.g., Histiotus). A well-developed tragus is usually present. Their tail is well developed and extends to the edge or beyond of a V-shaped uropatagium.
Morphological characteristics of bats may represent functional adaptations related to the habitat use, foraging strategies, styles of terrestrial locomotion, and roosting behavior. The wing morphology, for example, is related to the flight pattern and habitat use of bats. A long and narrow wing, with small surface area, adapts a species for sustained, relatively fast flight, as is seen in noctilionids and many molossids, that forage high above the ground, predominately in open unobstructed environments, as the region above the forest canopy or over water. A short and wide wing, with large surface area, is present in bats with slower, more maneuverable flight, as seen in carnivorous bats (e.g., Chrotopterus auritus, Vampyrum spectrum, Nycteris grandis, Cardioderma cor), that forages more often in habitats with dense, obstructing understory vegetation. Intermediate patterns of wing morphology, present in some gleaning frugivores (Stenodermatinae and Carolliinae), indicate flexibility in the use of space and resources (Feldhamer et al. 2007; Marinello and Bernard 2014).
Some bats have specializations related to roosting behavior, having morphological features that allow them to occupy different habitats. Neoplatymops mattogrossensis, for example, have the forearm with small wart-like granulations on dorsal side, which is related to their crevice-dwelling habits. Specimens of Thyropteridae and Myzopodidae have rounded, concave, suction disks at the base of the thumbs and on the soles of the feet that make possible for them to cling to stems, furled leaves, and other smooth surfaces; these disks, however, differ anatomically, and evolved independently in each family, being an example of convergent evolution (Kunz and Fenton 2005; Feldhamer et al. 2007).
Regarding to foraging strategies, Noctilio leporinus and Myotis vivesi have very long and large hindlimbs, rake-like feet with well-developed claws to scoop up aquatic and terrestrial prey. Among sanguivorous bats, Diphylla ecaudata is the only one that retains a functional calcar. The behavior of this vampire indicates that it is an arboreal hunter, and its calcar can be used as an opposable sixth digit to facilitate branch-grasping during arboreal locomotion, while they are stalking its prey (Schutt 1998).
Some features are related to the quadrupedal habit of some bats. Desmodontines and molossids have metacarpal pads to support the hindlimb during the terrestrial locomotion (Schutt and Simmons 2006). Mystacinids have very sharp claws on the hind feet and thumb, each with a small, basal talon. They also have a thick membrane along the side of the body, where the wings can be folded up, during the quadrupedal behavior (Feldhamer et al. 2007).
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