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Illuminating Secret Shadows
By Don E. Wilson

As if small migratory birds didn't face enough travail on their seasonal journeys, a new predator on some of these avian sojourners has recently been identified by a team of Spanish scientists. Millions of songbirds that breed in Europe funnel through the Iberian Peninsula en route to winter havens in Africa. As they fly high in the sky in the dark of night, they risk becoming the victim of another winged creature—a bat with a wingspan as wide as 18 inches and long, sharp canine teeth, according to a study published by Spanish and Swiss scientists this year in the journal PLoS ONE.

short-tailed bat
A short-tailed bat (Carollia perspicillata) feeds on watermelon. (Jessie Cohen/NZP)


One of the largest bats in Europe, the giant noctule (Nyctalus lasiopterus), is also one of the few bats known to eat birds. What's more, it is the only animal that snatches birds out of midair at night and, scientists believe, somehow manages to eat them on the wing. (This occurs so high, and in such darkness, that no one has seen it happen—but it must.) All other bats known to eat vertebrates, such as frog-eating bats, tackle their prey on the ground or other substrate. All other bats known to catch prey on the wing, a foraging pattern called aerial hawking, hunt flying insects, as the giant noctule does between its seasonal songbird feasts.

This finding astonished the scientific community (or at least that part of it concerned with bats and birds). But to Peter Marra, of the Smithsonian National Zoo's Migratory Bird Center, "It makes complete sense. There's so much biomass it's not really surprising that there's a species taking advantage of it."

Taken in the context of all the world's bats, however, it's hardly surprising that the giant noctule has evolved to fill an unusual ecological niche—after all, there are a huge number of bats and they occupy every terrestrial ecosystem on Earth except for extreme polar regions. In the most recent (2005) edition of Mammal Species of the World, which Dee Ann M. Reeder and I edited, we listed a total of 5,416 species of mammals. Of these, 1,116 are bats; only rodents boast a larger diversity of species.

For example, the genus Myotis, sometimes called little brown bats or mouse-eared bats, is more broadly distributed than any other terrestrial mammal genus. These relatively small insect-eaters occur from Alaska to Labrador and from Norway to eastern Siberia. They range southward to Chile and Argentina in the New World, and as widely as Southeast Asia, Australia, and Africa, to the Cape of Good Hope, in the Old World. Myotis also represents an ancient, primitive lineage of bats: There are Myotis fossils from Europe dating to the Eocene (52 to 34 million years ago), when early bats rapidly diversified. Myotis is therefore the most widespread genus of bats in both space and time. The familiar little brown myotis (M. lucifugus) lives throughout North America and is the continent's most abundant bat species.

Bats range in size from some of the smallest known mammals to giants with wingspans as wide as some adult humans are tall. The world's smallest bat, the hog-nosed bat (Craseonycteris thonglongyai), was discovered in Thailand in 1973. It weighs less than a penny and could rest comfortably on two fingers of your hand. The largest are the so-called flying foxes, which are not actually foxes but fruit-eating bats (Pteropus spp.) with wingspans of up to five feet. The hog-nosed bat and some species of flying foxes live in Southeast Asia, where the diversity of bats is very high, second only to that found in the tropics of South America (although no flying foxes live in the New World.)

Bats also have a variety of food habits. In addition to the many species that eat insects or fruit, others specialize on sipping nectar from flowers; hunting fish, frogs, or a variety of small vertebrates; or even feeding solely on blood, as three species of true vampire bats do.

Despite their incredible diversity and distribution, bats are generally so little-known that the scientists who study them discover new things all the time. Much about them remains mysterious, even though we now understand a great deal more about their biology than we did in the 1960s, when my own interest in them began.

Flying Machines
Although it is fascinating to watch bats as they hang upside-down in their roosts, it is on the wing that these animals become truly impressive. If you have ever watched a small bat deftly circling the trees in your backyard as it catches insects on the fly, you must have marveled at its ability to change directions, avoid obstacles, and intercept prey. Flight is the single characteristic that sets bats apart from all other mammals, and there are no flightless bats.

Bats belong to the order Chiroptera, a name meaning "hand wing" in Latin, because the bones of a bat's elongated fingers, along with its forelimbs, support the membranes of its wings. Another interesting evolutionary modification is that bats' knees are rotated 180 degrees from those of most mammals. So, if they are hanging upside-down with their bellies toward the wall, they can take flight immediately by simply dropping and flapping away.

Although both birds and bats fly by flapping their wings, bat flight is considerably different from bird flight, or any other animal flight, for that matter. The unique attributes of bats' flexible wing membranes yield flight characteristics that would be very useful to human-designed flying machines, an international team of scientists reported in Science this year.

G. R. Spedding, an aerospace engineer at the University of Southern California in Los Angeles and a coauthor of the study, outlined an exciting research program to make detailed measurements of bat wings and add to a growing body of data about bat flight. "Bats are agile hunters, capable of plotting and executing complex maneuvers through cluttered environments," he said. "These are the traits we'd like our unmanned air vehicles to have because there are so many complex rural and urban environments in which we could use them."

Bats and Echolocation
Bats are not blind; they can see as well as we can, and better than some mammals that spend most of their time underground. However, most species have essentially supplanted vision with echolocation, a process of sending out pulses of high-frequency sound and using the returning echoes to detect objects and to navigate. This allows them to specialize on haunting the same dark night skies that also shroud them in mystery. Echolocation is so foreign to our own experience that we have trouble even conceiving of it. Yet the images formed in the brains of echolocating bats are probably not terribly different from the images our brains form from our own visual system.

fringed myotis
A fringed myotis (Myotis thysanodes) in the palm of a person's hand. (Jennifer Purvine/USDA Forest Service)

Bats' exceptional ability to orient themselves with their hearing rather than their vision was first demonstrated in a series of elegant experiments performed in 1793 by an Italian scientist named Lazaro Spallanzani. He inserted small tubes into the ear canals of bats so that, by stopping up the tubes, he could block their hearing. He showed that the bats became disoriented and less able to avoid obstacles with even one ear plugged.

But the idea that bats produce ultrasonic sounds was not confirmed until about 70 years ago, when a Harvard biology student named Donald Griffin used an extremely sensitive microphone to detect bats' high-frequency calls and show that the rate of their calls increases as they approach obstacles. Since then, the study of echolocation has played a prominent role in our efforts to learn more about bats and how they have adapted to their environments. Scientists now use an electronic apparatus known as a bat detector to distinguish between bat calls and translate them into audible signals for analysis.

The evolution of echolocation has led to bats having a very complex inner ear anatomy, which gives them supersensitive hearing. A group of bats called foliage gleaners listen for the very faint noises large insects make as they move about on the ground, or on the surface of vegetation. By homing in on these sounds, which we cannot hear, they easily locate their evening meals.

These sophisticated listening skills can lead to some startling adaptations. A 2006 study published by German scientists Udo Gröger and Lutz Wiegrebe of Munich's Ludwig-Maximilians-Universität shows how common vampire bats (Desmodus rotundus) suck blood from the same people night after night. The scientists' experiments demonstrated that the bats can detect differences in the breathing sounds of three sleeping human subjects. By listening to potential prey and detecting its heat in the form of infrared radiation, the bats can discriminate between individuals.

Bat Social Systems
The social systems of bats are as varied as the places they live and the foods they eat. Some species live in huge colonies. Millions of pregnant female Mexican free-tailed bats (Tadarida brasiliensis), for example, roost together in Bracken Cave near San Antonio, Texas, each spring. When the females give birth in early summer, the population of the cave rises to an astounding 20 million individuals—the largest known maternal bat colony in the world. Other species, such as the eastern red bat (Lasiurus borealis), are solitary, and males and females come together only to breed.

Some species form harems in which groups of females roost together and are attended by a single male. By huddling together, harem females maintain a consistent body temperature, allowing them to conserve energy for flying and pup rearing. And unlike solitary or colony-roosting bats, harem females enjoy protection by their attending male from predators and other suitors.            

Social dynamics within harems can be simple or quite complex. Female spear-nosed bats (Phyllostomus hastatus), for example, tend to remain loyal to one harem for years at a time, but female Jamaican fruit-eating bats (Artibeus jamaicensis) switch harems more frequently. In 2006, National Zoo geneticist Jesús Maldonado and Jorge Ortega from the Instituto de Ecología in Mexico published a study of the social dynamics of Jamaican fruit-eating bat harems. They discovered that females roosting in the central core of the harems were dominant over females in the middle or outskirts. These fortunate females were groomed and licked more frequently by the females in the middle than other members of the harem, while those on the outskirts were subjected to more aggressive behavior such as wing flicks and grunts and were more often expelled from the harem by the females in the middle than any other members of the group.

Biologists from the United Kingdom reported in Nature in 2005 that greater horseshoe bats (Rhinolophus ferrumequinum) have a mating system that seems a bit bizarre to us. Mothers, daughters, and grandmothers sometimes mate with the same male, leading to some curious relationships. Using sophisticated genetic techniques, the scientists showed that a female and her maternal half-aunt were also half-sisters on their father's side. Despite these tangled familial ties, which increase the number of ancestors that individual greater horseshoe bats have in common, the study's authors found no similar increase in the level of inbreeding in the species.    

A bat's social system, and especially its mating systems, may have interesting effects on its anatomy. Gerald Wilkinson of the University of Maryland, College Park, Scott Pitnick of Syracuse University in New York, and Kate E. Jones of Columbia University in New York found that in species in which females are promiscuous, males have larger testes and smaller brains proportionate to their body size than in species in which the females are more faithful. For instance, female silver-tipped myotis (Myotis albescens) are promiscuous, and males have testes that comprise 6.7 percent of their body weight, while their brains comprise only 3.2 percent. This suggests there is an evolutionary price for successful mating when females can choose among many mates. The same principle has been linked to primates as well: Chimpanzees, for example, are promiscuous and the males have relatively larger testes than male gorillas, which maintain harems of dependent and faithful females.

Regardless of their mating system, male bats don't contribute to caring for their pups—in fact, paternal care has been documented in only two species. As in all mammals, female bats feed their young with milk produced by their mammary glands. Pups are born blind and naked, and require a month or two of maternal care, with daily bouts of nursing.

Mother bats go out and forage each night, and almost always leave their pups behind in the roost. On their return, they may use a variety of complicated signals, including high-frequency vocalizations, to relocate their own young from among what may be thousands of others if they live in a large colony. In 2006, German scientists from the University of Erlangen-Nuernberg in Erlangen, Germany, published a study of mother-young communication in the journal Naturwissenschaften. The scientists discovered that baby sac-winged bats (Saccopteryx bilineata) attempt to mimic their parents' vocalizations by making nonsense sounds that are probably comparable to the babbling of human babies.

Roosting Habits
Although you may see bats whispering their way through the summer nighttime skies, most of them spend the daytime resting in secluded roosts. The most common type of day roost is caves, and most families of bats have at least some species that frequent them whenever possible. Caves protect bats from the sun and predators, and their relatively constant temperature and moisture conditions help bats conserve energy.       

Bats control their body temperature by a process called facultative heterothermy: As the ambient temperature falls, they can allow their body temperature to decrease until they go into a state of inactivity called torpor. They may do so daily, dropping their body temperature to match those of their cool day roosts. In addition, bats of many temperate-zone species sleep the winter away in hibernation torpor. An active bat's body temperature may range from 95 to 104oF, but it can fall as low as 32oF in hibernation. Maintaining a constant body temperature is energetically costly for mammals, so bats save significant amounts of energy through daily and hibernation torpor.            

Another result of spending half or more of the year in hibernation might be an extended lifespan. Compared with most small mammals, bats are amazingly long-lived: In general, they tend to live 3.5 times longer than comparably sized small mammals in other orders, according to a study by the University of Maryland's Gerald Wilkinson and Jason M. South that was published in Aging Cell in 2002. Some individuals may live for 30 or more years (although the average life span is undoubtedly much shorter than that). The maximum age varies greatly from species to species and is heavily influenced by geography. For instance, the study's authors noted that a male Brandt's bat (Myotis brandti) from Siberia holds the longevity record of 38 years, and 23 other males from this area lived 25 years or more. They suggest that the duration of hibernation, which lasts longer the higher the latitude, may increase longevity compared with bats that have shorter hibernations.

Bat Conservation
Threats to bat species are increasing globally due to the relentless onslaught of human activities, including habitat destruction, which limits foraging areas and roosting sites for bats worldwide. Although some bat species have adapted well to humans and their environmental modifications, most have not. For every species like the common little brown myotis of North America, which takes advantage of man-made structures like attics, barns, and other buildings for roosting sites, there are many more that find human encroachment on their natural habitats incompatible. The World Conservation Union's (IUCN) Red List includes 1,024 bat species—almost all of them—as being of some conservation concern. Excluding those deemed of "least concern" still leaves 546 species, of which 172 are called vulnerable, 44 endangered, and 32 critically endangered. Nine species are listed as extinct, a number that will likely grow.

At the annual meeting of the American Society of Mammalogists in June 2007, Smithsonian's National Museum of Natural History scientists Lauren and Kristofer Helgen reported on two newly identified species of flying foxes that once occurred on Samoa. Known only from museum specimens collected in the mid-19th century, these two join at least three other species of flying foxes that have gone extinct on Pacific Islands due to expanding human populations.

On the other hand, conservation action can stem such losses. My colleague, John Engbring of the U.S. Fish and Wildlife Service, and I studied another Samoan flying fox (Pteropus samoensis). This species is exceptional among bats because it is not nocturnal; instead, it forages over the small islands of Samoa during the daytime, searching for trees with ripe fruit. This unusual diurnal feeding behavior has likely evolved because of a lack of predators on Samoa, at least until humans arrived there.

When John and I were studying these flying foxes in the late 1970s and early '80s, their numbers were in severe decline, because people were harvesting and selling them in markets as food. Many were exported to Guam as delicacies. Protection under the Convention on International Trade in Endangered Species, which prohibited this trade, has allowed their populations to recover, albeit slowly, in the face of natural disasters such as typhoons, which have been increasing in frequency.

Although people pose a much greater threat to bats than the reverse, bats are associated with diseases transmissible to humans. Rabies, a viral infection of the central nervous system, is by far the biggest public health concern. Like all mammals, bats are capable of contracting rabies; they also suffer the symptoms of the disease and eventually die from it. The disease can manifest itself in two distinct ways: as paralytic or as furious rabies. Animals with paralytic rabies become immobilized and may seem less threatening even though they are equally dangerous. Animals with furious rabies may wander dazedly, biting fiercely at whatever they encounter. Bats rarely exhibit this form of the disease and typically bite only if handled.   

Recent research also implicates some bat species as carriers and transmitters of emerging zoonoses, which are animal-borne diseases that can be transmitted to humans. These diseases are appearing as a result of both climate change and our increasing encroachment on animal habitats, which brings animals and people in greater contact. Three species of horseshoe bats (Rhinolophus spp.) carry the virus that causes Severe Acute Respiratory Syndrome (SARS), while other bat species are natural hosts of the Nipah and Hendra viruses that recently hit people in Asia and Australia. This argues strongly for an enhanced research program on bats, although by some estimates, fewer than two percent of human pathogens are found naturally in bats.           

Greater understanding of bats and how they interact with their environments is critical to human health as well as to the continued existence of bats in all their diversity. Apart from their small role in carrying human pathogens, bats provide us with many ecosystem services, such as eating insect pests and pollinating important food plants— including the plant from which tequila is made. So raise a toast to bats as they grace the late summer skies, eating the mosquitoes we are only too willing to share.

—Don E. Wilson is a senior scientist and curator of mammals at the Smithsonian's National Museum of Natural History in Washington, D.C.

 

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ZooGoer 36(5) 2007. Copyright 2007 Friends of the National Zoo.
All rights reserved.

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