Chiropterabats

Diversity

Bats are the second-most speciose group of mammals, after rodents. The approximately 925 species of living bats make up around 20% of all known living mammal species. In some tropical areas, there are more species of bats than of all other kinds of mammals combined. (Hill and Smith, 1984; Nowak, 1991; Vaughan, et al., 2000)

Bats are often divided into two major groups, usually given the rank of suborders, Megachiroptera and Microchiroptera. Although these groups probably do not represent monophyletic lineages (discussed in more detail below), there are several relevant ecological differences between them. These groups will be used throughout this account in describing the diversity of bat life histories. (Teeling, et al., 2002; Teeling, et al., 2005; Van den Bussche and Hoofer, 2004; Vaughan, et al., 2000)

Megachiroptera includes one family (Pteropodidae) and about 166 species. All feed primarily on plant material, either fruit, nectar or pollen. The remaining 16 families (around 759 species) belong to Microchiroptera. The majority of species are insectivorous, and insectivory is widely distributed through all microchiropteran families. However, many microchiropterans have become specialized to eat other kinds of diets. Some bats are carnivorous (feeding on rodents, other bats, reptiles, birds, amphibians, and even fish), many consume fruit, some are specialized for extracting nectar from flowers, and one subfamily (three species in the subfamily Desmodontinae) feeds on nothing but the blood of other vertebrates. Megachiropterans and microchiropterans differ in many other ways. Megachiropterans are found only in the Old World tropics, while microchiropterans are much more broadly distributed. Microchiropterans use highly sophisticated echolocation for orientation; megachiropterans orient primarily using their eyes, although members of one genus, Rousettus, are capable of a simple form of echolocation that is not related to echolocation in microchiropterans. Megachiropteran species control their body temperature within a tight range of temperatures and none hibernates; many microchiropterans have labile body temperatures, and some hibernate. (Hill and Smith, 1984; Nowak, 1991; Vaughan, et al., 2000)

Geographic Range

Bats are found throughout the world in tropical and temperate habitats. They are missing only from polar regions and from some isolated islands. Although bats are relatively common in temperate regions, they reach their greatest diversity in tropical forests. (Hill and Smith, 1984; Museum and Institute of Zoology, 2012; Vaughan, et al., 2000)

Habitat

Bats can be found in many terrestrial habitats below the polar regions. Typical habitats include temperate and tropical forests, deserts, open fields, agricultural areas, and in suburban and urban environments. Many bats forage near freshwater streams, lakes and ponds, preying on insects as they emerge from the water. Generally, if a terrestrial habitat provides access to sufficient roost sites and appropriate food, one or more species will be found there. Bats generally have very specific roosting requirements, which differ among species. They may roost in caves, crevices, trees, under logs, and even in human dwellings. Bats may also use different types of roosts at different times. For example, a species that hibernates in a cave during the winter may use crevices in tree holes as roosts during warmer months.

Physical Description

Bats are unmistakable. No mammals other than bats have true wings and flight. Bat wings are modified forelimbs, much as are bird wings, except in the case of bats the flight surface is covered with skin and supported by four fingers, while in birds the flight surface is provided mostly by feathers and is supported by the wrist and two digits. The flight membrane usually extends down the sides of the body and attaches to the hind legs. Bats also often have a tail membrane called a uropatagium. In order to accomodate powerful flight muscles, the thoracic region of bats is quite robust. In addition to providing power, a massive chest and shoulders maintains the center of gravity between the wings, making flight more efficient. The opposite is true of the posterior end of the body, which is small relative to the chest and back. The hindlimbs in particular are generally short and small, with sharp, curved claws that help bats cling to surfaces in their roost. (Hill and Smith, 1984; Vaughan, et al., 2000)

An important cranial characteristic for recognizing bat families is the nature of the premaxilla. (Hill and Smith, 1984; Vaughan, et al., 2000)

The suborder names, Megachiroptera and Microchiroptera, imply that megabats are all large and microbats are all small, which is is not always the case. The smallest bat is indeed a microchiropteran (Craseonycteris thonglongyai) and weighs only 2 to 3 grams. Likewise, the largest bats are among the Megachiroptera and can weigh up to 1500 grams. Size varies with each group, however, with the smallest megachiropterans weighing only 13 grams and the largest microchiropterans weighing nearly 200 grams. (Vaughan, et al., 2000)

There are several obvious morphological features that distinguish the two suborders. Megachiropterans rely on vision to orient in the dark of night, and thus have large, prominent eyes. All microchiropterans rely heavily on echolocation, and not vision, and generally have small eyes. Instead most microchiropterans have large, complex pinnae (external ears), including an enlarged tragus or antitragus. Megabats have claws on the second digits supporting their wings (with one exception); this is never the case in microbats. Microbats often have dentition or cheek teeth whose morphology can easily be related to dilambdodont teeth; megabats have simplified cheek teeth that are difficult to interpret. (Hill and Smith, 1984; Vaughan, et al., 2000)

  • Sexual Dimorphism
  • sexes alike
  • female larger
  • sexes shaped differently

Reproduction

Mating systems vary among bat species. Many temperate bats mate in the fall as they aggregate near their winter hibernacula. These bats are generally promiscuous. Pteropodids also tend to have promiscuous mating systems. These bats often aggregate in large groups in one or a few trees and mate with various nearby individuals. In many neotropical microchiropterans, one or two males defend small harems of females. Males secure all matings with their harem females until other males supplant them. While most species are either polygynous or promiscuous, there are some bats that are monogamous. In these cases, the male, female, and their offspring roost together in a family group and males may contribute to protecting and feeding the young. Examples include Vampyrum spectrum, Lavia frons, Hipposideros galeritus, H. beatus, Nycteris hispida, N. arge, N. nana, and some Kerivoula species. One megachiropteran species, Hypsignathus monstrosus, has a lek mating system, where males gather in a lekking arena to display to females, who then choose the most desirable of mates. Courtship behavior is complex in some species, while in others, it can be nearly nonexistent (e.g., males of some species will mate with hibernating females that barely react to the copulation event). (Hill and Smith, 1984; Vaughan, et al., 2000)

A large number of bats breed seasonally. Temperate species often breed before they enter hibernation while many tropical species breed in a cycle that is linked to wet-dry seasonality. All species that are not seasonal breeders occur in the tropics, where resources may not be as variable as in temperate regions. The function of seasonal breeding is to coordinate reproduction with the availability of resources to support newborn young. To this end, many species have also evolved complex reproductive physiology including delayed ovulation, sperm storage, delayed fertilization, delayed implantation, and embryonic diapause. Females generally give birth to one two two pups per litter, but in some species in the genus Lasiurus, litter sizes may reach 3 or 4 individuals (e.g. Lasiurus borealis, L.seminolus, and L.cinereus). (Hill and Smith, 1984; Nowak, 1991)

At birth, newborn bats weigh between 10 and 30% of their mother's weight, putting a large energetic strain on pregnant females. All newborn bats are completely dependent on their mothers for both protection and nourishment. This is true even in Pteropodidae, where pups are born with fur and open eyes. Microchiropterans tend to be more altricial at birth.

Aside from the few monogamous bat species, where males contribute to feeding and protecting young, all parental care in bats is provided by females. Some males defend feeding territories for their harems, thereby contributing indirectly to the survival of their young after birth. Bats cannot fly when they are born, so young bats either remain in the roost while their mothers forage, or cling to their mothers' during flight. Females of many species form maternity colonies while they are lactating and rearing young. When the young are left in the roost as the mother forages, they cluster together to keep warm. Upon their return, mothers and their respective infants can identify each other by their vocalizations and scent, and thus can successfully reunite. In some species, females will communally care for young, with "babysitters" caring for the cluster of young while their roost-mates forage.

Juveniles grow quickly and can usually fly within 2 to 4 weeks of birth. They are weaned shortly thereafter. Thus, lactation is relatively short, but metabolically demanding. (Hill and Smith, 1984)

  • Parental Investment
  • altricial
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • male
      • female
    • protecting
      • male
      • female

Lifespan/Longevity

Bats live surprisingly long lives. Typically, mammalian lifespans roughly correlate with their body size: smaller mammals live short lives, whereas larger mammals live longer lives. Bats are the only group of mammals that does not conform to this relationship. Despite the fact that bats are generally small mammals, many bats can live over 30 years in the wild. Where data on longevity is available, lifespans in the wild are often recorded from 10 to 25 years. Typically, a given species will live at least 3.5 times longer than other mammals of similar size. (Wilkinson and South, 2002)

There are several viable hypotheses to explain longevity in bats. Hibernation and daily torpor may restrict lifetime energy expenditure in individuals, allowing them to live longer. Lack of predation pressure on adults may also allow bats to live long lives. For their size, bats have low reproductive rates in a given breeding season. Typically, females give birth to only one or two young per year, but reproduce many times over a long life. By evolving a reproductive strategy that is more typical of large mammals, perhaps lifespans have evolved to match those of large mammals as well. (Wilkinson and South, 2002)

The longest-lived bat on record is a little brown bat (Myotis lucifigus). One banded individual was recaptured 33 years after it was originally tagged. These bats weigh only 7 grams as adults, roughly 1/3 the size of a house mouse. Myotis lucifugus is one of the most widely studied species worldwide; thus, it would not be surprising if other, less well-known species live even longer. (Kurta, 1995)

Behavior

The behavior that unifies all of Chiroptera is flight. Bats are the only group of mammals to have evolved powered flight (although many species glide), and only the third vertebrate group to do so. Depending upon the size and shape of their wings relative to their body mass, different species of bats may have different flight styles. Many species have large, broad wings and relatively small bodies, which allows them to fly slowly but with high maneuverability. This flight behavior is useful for chasing evasive insect prey and maneuvering within a dense forest at night. Some species with large, broad wings can even hover. This behavior is especially useful for bats that eat nectar or pollen from stationary flowers. Other species have long, narrow wings, which are useful for acheiving high speeds, but which restricts maneuverability. Many of these species forage in open spaces and may be able to fly long distances. These two wing morphologies represent the ends of a continuum, most species have wing morphologies that fall between these extremes. (Hill and Smith, 1984; Nowak, 1991; Vaughan, et al., 2000)

Many bats live in groups, while some species are solitary. Often, bats roost in colonies for some portion of the year. Living in a colony can serve many functions. For bats, one of the main purposes of group living is to collectively conserve heat. Bats are small and have high metabolic rates, so heat conservation is vital. Many bats hibernate during the winter and undergo daily torpor to conserve energy. Clustering together while roosting can further reduce heat loss. Some bats that roost together do so in groups of several individuals. Some groups (e.g. Tadarida) roost in caves in groups of thousands, or even millions. Some bat species migrate to hibernation sites or to follow a food source (flowering cacti, for example). Most bat species are not known to defend foraging areas, but this behavior is known from some tropical species. Territorial defense of roosting sites is also known in some species. (Hill and Smith, 1984; Vaughan, et al., 2000)

Communication and Perception

Echolocation is another signature life history strategy in bats. All microchiropterans rely heavily on echolocation to navigate through their environment and to find food. Bats call at frequencies that are typically higher than humans can hear. These sounds bounce off objects and produce echoes, which bats can hear and interpret. Bat calls vary in duration and structure. Some species use short calls (2 to 5 milliseconds) at a high rate of repetition, while other species use longer (about 20 milliseconds), but less frequent calls. The frequency (pitch) characteristics also vary within and among species. Differences in characteristics like frequency and duration affect the ability of an echolocation call to produce echoes from objects of different sizes, shapes, and at different distances. As a result, echolocation call structure can reveal quite a bit about the ecology and foraging strategy of a bat species. (Hill and Smith, 1984; Vaughan, et al., 2000)

Perhaps the biggest functional difference between vision and echolocation is that vision is a passive mode of perception, while echolocation is an active mode of perception. Vision typically relies on external sources of light energy. Echolocation is quite different in that the energy provided is by the animals themselves. Because bats have tight control over what kinds of sound they produce, bats can exhibit a high degree of control over what types of objects they can perceive. Echolocation calls vary among species, within species, and even within individuals. This variation in echolocation behavior reflects variation in the habitats bats are using and the food for which they are searching. Bats can also use "passive echolocation", detecting and locating prey based on prey-generated sounds, such as frogs calling or the sound of a beetle walking across sand. (Hill and Smith, 1984; Vaughan, et al., 2000)

Bats communicate with one another in a variety of ways. Although bats may be able to hear and interpret the echolocation calls of other bats, there is little evidence that those calls are used directly in communication. Bats employ a suite of communication calls, most of which are audible to the human ear. Some species use a diverse repertoire of social calls, which can be useful in intra-specific agression, mother-infant communication, and mating behavior. (Behr and von Helversen, 2004; Hill and Smith, 1984; Vaughan, et al., 2000)

Scent marks and pheromones are also important in bats, as they are in other mammals. Scent is used to communicate reproductive status and individual or group identity. Many species have special scent glands near their faces or their wings. One family, the sac winged bats (Emballonuridae), are so called because of a sac on the leading edge of their wing that may be a scent gland. (Behr and von Helversen, 2004)

Bats also communicate with visual displays, often during courtship. Some species have special markings on their wings or pelage, and engage in ritualized displays to attract mates. (Behr and von Helversen, 2004; Hill and Smith, 1984)

Food Habits

As a group, bats eat a wide variety of food types. The majority of species eat insects, either taking them on the wing or picking them off of surfaces. Species specialized for eating fruit, nectar, or pollen are especially abundant and diverse in tropical regions. Some bats eat vertebrates like frogs, rodents, birds, or other bats. Several species (e.g., Noctilio leporinus and Myotis vivesi) are specialized to trawl for fish. Three species of bats, the vampire bats subsist solely on the blood of other vertebrates. Although most stories related to mythical "vampires" originated in the Old World, there are no Old World bat species that feed on blood. Vampire bats occur only in the neotropics. Vampire bats eat blood by using their sharp incisors to make incisions in the skin of their their prey. An anticoagulant in their saliva keeps blood flowing while they lap it up. Only one of these three species eats the blood of mammalian prey, the common vampire bat (Desmodus rotundus). The other two species (Diaemus youngi and Diphylla ecaudata) are specialized for feeding only on birds. Although most bats tend to be specialized for a particular diet, most frugivorous bats also include arthropod prey in their diet when available. At least one extant species, the unusual New Zealand lesser short-tailed bat (Mystacina tuberculata), is omnivorous. (Hill and Smith, 1984; Nowak, 1991; Vaughan, et al., 2000)

The different food preferences of bats are widely distributed among families. Megachiropterans eat only fruit and nectar, but the entire range of diets can be found among microchiropterans. Insectivory is common in many families, and carnivory on vertebrates is exhibited by several. The New World leaf-nosed bats (family Phyllostomidae) in particular have undergone an extensive radiation in ecology and food habits. The entire range of diets exploited by all of Chiroptera can be observed in this single family, which also includes the only sanguivorous (blood feeding) bats. (Vaughan, et al., 2000)

Predation

Few studies have directly examined the effects of predators on bat populations. Most of this type of information comes from anecdotal observation of predation events or evidence of bats in the scat of predators. Groups that are known to eat bats are owls and other birds of prey, many carnivores, other bats, and snakes.

Bats are probably most vulnerable to predators as they roost during the day or emerge in large groups in the early evening. Predators like snakes or hawks often wait near the entrances of caves at dusk, attacking bats as they leave the roost. Juvenile bats that cannot yet fly are also at risk of predation if they fall to the ground. Individual bats flying in the dark of night are probably difficult to catch, even for owls, which can fly and locate prey well in the dark. Several species of bat have become specialized for preying on other bats, these include the New World species Vampyrum spectrum and Chrotopterus auritus, and two Old World species in the genus Megaderma. (Hill and Smith, 1984)

Bats generally avoid predation by staying in protected roosts during the day and through agile flight at night. Most bats are also cryptically colored.

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Because of their high metabolic needs and diverse diets, bats can impact the communities in which they live in a variety of important ways. They are important pollinators and seed dispersers, particularly in tropical communities. Also, carnivorous and insectivorous bats may significantly limit their prey populations. Bats may be keystone species in many communities, particularly in the tropics where they are most abundant and diverse. (Hill and Smith, 1984; Jones, et al., 2003; Nowak, 1991; Vaughan, et al., 2000)

Bats are associated with many kinds of internal and external parasites. They are known to harbor several protozoans that cause malaria (e.g., Plasmodium, Hepatocystis, Nycteria and Polychromophilus) although none of the malarial parasites found in bats cause malaria in humans. Trypanosome protozoans, that may cause a variety of diseases, such as sleeping sickness, are also found in a number of bat species. Many flatworms (Cestoda and Trematoda) and roundworms (Nematoda) spend at least part of their life cycle within the tissues of bat hosts. Bats commonly harbor external, arthropod parasites. Ticks, mites and insects such as true bugs and fleas are known to live and feed on bats. An entire family of flies, Streblidae, has co-evolved with bats. These flies have secondarily lost the ability to fly, living only in the fur of bats. Species that parasitize bats exhibit a range of host-specificity: some are found on one or a few bats, others occur on a wider variety of bat species, and still others can parasitize bats as well as other taxonomic groups. (Hill and Smith, 1984)

Commensal/Parasitic Species

Economic Importance for Humans: Positive

Although many people consider bats to be harmful pests, bats play pivotal roles in ecological communities and benefit humans in numerous ways. Many species of insectivorous bats prey heavily on insects that transmit diseases or are crop pests. In addition, bat guano (feces) is often used to fertilize crops. Many tons of guano are mined each year from caves where bats aggregate in large numbers. In other words, some species eat crop pests and excrete crop fertilizer! Evidence continues to accumulate in support of the immense economic benefit of insectivorous bats for the agricultural industries worldwide. Frugivorous bats are important seed dispersers, helping promote the diversity of fruiting trees in the tropics. Bats that eat pollen and nectar are important pollinators, and some plants they pollinate are economically important to humans, such as Agave and bananas (Musa). Larger bats, such as pteropodids are sometimes eaten by humans. (Hill and Smith, 1984; Nowak, 1991)

Recently, common vampire bats have become an important focus of medical research. Vampire bats are generally considered a significant threat to human interests because they regularly transmit rabies to cattle (and sometimes to people). However, the anticoagulant protein in their saliva ("Desmoteplase") is being studied in an effort to help prevent blood clots in humans, such as those being treated for stroke. (Reddrop, et al., 2005)

The increasing popularity of bats has led to a booming ecotourism industry, often surrounding large roost emergences, such as those of Mexican free-tailed bats. ("Bat Conservation International", 2004)

Economic Importance for Humans: Negative

Although bats are often perceived as much more of a threat to human interests than they actually are, bats may negatively impact humans in at least two ways. Some species roost in human dwellings and can become a nuisance. This is particularly true if a large colony takes up residence in a home, producing a great deal of guano and an unpleasant odor. Bats also carry and transmit rabies. In general, bats rarely transmit rabies to other species, including humans and domestic animals. Vampire bats, on the other hand, regularly transmit the disease to domestic cattle, representing a large financial burden for the cattle industry in the New World tropics. Rabies is transmitted through saliva and other body fluids and vampire bats exhibit several behaviors which make them especially effective vectors of the disease (e.g., social grooming and food sharing). Their feeding habits result in their saliva contacting the blood of other animals, which is an ideal situation for rabies transmission. (Hill and Smith, 1984; Vaughan, et al., 2000)

Conservation Status

Approximately 25% of all species within Chiroptera (nearly 240 species) are considered threatened by the International Union for the Conservation of Nature (IUCN)