All about bats

0

Bats are usually cast in a league with vampires, witches and the supernatural. This is a phenomenon that’s widespread and has been going on for centuries in cultures around the world. In the U.S., some folks credit a certain book with virtually cementing the ink between bats and a perception of danger. You know the one: Bram Stoker’s 1897 book, Dracula, the first in English literature to link bats to human vampires. Stoker’s antagonist, Count Dracula, who could shape-shift into a bat,¹ caught the public’s imagination and vampire books and movies since have played on the theme that bats are evil, blood-drinking creatures. And, that’s completely false. 

First, let’s dispense with the creepy misconception that bats are blood-thirsty predators. In actuality, only three species of bats drink blood and none of these favor human blood. Bats represent a quarter of the mammals on earth, with about 1,200 species, so the population of “vampire” bats is relatively very small. Besides, of the 47 species of bats known to inhabit North America, none are vampire bats (you’ll read more about them further down this page.)

What we have here in the U.S. are three small, flower-eating species who spending part of the year in the Southwest, migrating between the U.S. and Mexico, and 44 others spread out across the U.S. who feed on insects. If you have bats silently patrolling your backyard wildlife habitat at night, count yourself lucky.

The Southwest is home to the greatest diversity of U.S. bats, with Texas hosting more than 30 species. Texas’ Bracken Cave near San Antonio, is home to the largest colony of bats in the world. Twenty million Mexican Free-tailed bats swoosh out of the cave at dusk and by the time they return at dawn they’ve feasted on 250 tons of insects. Bracken Cave, owned by Bat Conservation International, is closed to the public for the protection of the bats. You can see images and video of the bats leaving the cave here.

Bat (Icaronycteris) fossil. (Andrew Savedra Wiki cc by-sa 2.0)

Icaronycteris bat fossil, 5.5 inches long, found along Green River in the U.S. (Andrew Savedra Wiki cc by-sa 2.0)

The earliest known bat species, Icaronycteris index, lived in the early Eocene Epoch, about 52.2 million years ago. Scientists know, however, that bats are older than that, as other fossil evidence points to an ancestor (also shared by humans) dating back 80 million years. Bat fossils show a creature not very different from the bats of today.

Bats stay quietly hidden by day and come out at night to silently skim the nighttime sky, making them seem mysterious and dangerous. As you read on, you’ll find yourself viewing them very differently. You may even grow to like them.

Myths, be gone
Bats aren’t dangerous. To repeat: Bats aren’t dangerous. They couldn’t be more different from their fictional counterparts. In real life they’re timid and non-aggressive — so much so, even one with rabies won’t attack.

Bats aren’t blind. Day and night, their vision is excellent. Researchers have discovered that at least two nocturnal bat species even have photoreceptors for color vision.

Bats aren’t dirty. They’re actually very clean, spending about a third of their resting time scouring their entire body, much like a cat does.

Bats don’t get tangled in people’s hair. Most bats are insectivores and it’s true they might occasionally get close to us — even very close — while catching flying insects. But they don’t want to touch us and they see far too well to do it by accident. This leads to another common myth, the “blind as a bat” one. Bats have very good vision in the daytime and at night.

Major bat groups
Bats belong to the order Chiroptera (ki-ROP-ter-uh). Within this are two major groups — the Microchiroptera and the Megachiroptera. About 200 species are Megachiropterans, known as Old World bats or fruit bats. They’re found only in tropical areas of Europe, Asia and Africa. Some Megabats are as small as 2 inches long, but this group also includes the very large “flying foxes,” who are bats with fox-like faces and large eyes. Megabats are eaters of fruit, nectar and pollen. Most don’t use “radar” (echolocation). Instead, they use their excellent vision and a good sense of smell.

The rest of the bats, Microciropterans, inhabit all areas of the world except Polar Regions and extreme deserts. It’s a Microbat we see patrolling the warm nighttime skies above a backyard wildlife habitat. These bats use echolocation. Most are insect-eaters, although some species feed on small mammals and fish.

The nitty-gritty on vampire bats

#

White-winged vampire bats (Diaemus youngi) (© Daniel K. Riskin) / http://www.noctilio.com/bats)

Vampire bats are Microbats in the family Phyllostomidae, subfamily Phyllostominae. These bats are very small, with a body only about 3 inches long and a wingspan of around 8 inches. They drink blood, as their name suggests, but they seek it from animals like cattle, pigs, horses, birds and sometimes other bats. Occasionally they’ll bite a human who happens to be snoozing uncovered in the great outdoors, but we aren’t their usual target.

Their prey is typically fast asleep when it happens, a bite so tiny with teeth so sharp the victim seldom stirs. The amount of blood loss is so little it doesn’t affect the host, and the bats don’t suck it in, like Count Dracula — they lap it up, about a teaspoonful, over a period of 30 or 40 minutes. Don’t worry yourself with mental images of a vampire bat attacking your big toe if it escapes from under covers while you’re camping — that is, unless you plan to do it in Mexico or Central and South America, where they live.

Vampire bats are smart and they can be tamed and trained — National Geographic online reports a researcher who has vampire bats who come when called by their name. Bats are also charitable: Females have been observed feeding new mothers for two weeks after they give birth.

Physical characteristics
Bats aren’t birds. They’re mammals, the only ones who actually fly as opposed to gliding from a higher elevation to a lower one, such as the “flying” squirrels. The name of their order, Chiroptera, is from the Greek for “hand wing” and it’s an appropriate name — their hands, complete with thumbs and elongated fingers, serve as the bony structures of the bat’s wings. Their wings are webbed and move with all the flexibility of hands (and then some), giving them amazing maneuverability in flight. For instance, they can turn 180 degrees in less than half the span of their wings and perform aerobatics that would crash a bird into the ground. Bats have speed, too, able to fly at 35 to 40 miles per hour.

Mexican Long-tongued Bat (Choeronycteris mexicana).

Mexican Long-tongued Bat (Choeronycteris mexicana). Note how the bones of the arms, finger, legs can be easily seen through the thin patagium. (Ken Bosma / Wiki; cc by 2.0)

The wings are covered by the patagium (puh-TAY-gee-um), a membrane of skin so thin you can see light through it. It appears to be bald, but with at least some species it’s coated with tiny, inconspicuous hairs thought to play a role in aerodynamics or as sensors. The patagium extends from the tips of the fingers, back along the arm bones to the shoulder and then all the way down the body to the ankle. The membrane also stretches between the legs, but there is called the interfemoral membrane or uropatagium. With some species, the uropatagium completely encloses the tail.The thumbs have a claw positioned at the front edge of the wing. They’re used for holding food, climbing and clinging.

Bats range in size from what is possibly the world’s smallest mammal, Asia’s bumblebee-sized Kitti’s Hog-nosed bat (Craseonycteris thonglongyai), up to the Gigantic flying fox (Pteropus giganteus), a handsome, Asian fruit-eating species with a wingspan of about 6 feet! There’s nothing close to that in the U.S. Wingspans here range from around 8-1/2 inches up to the 22 inches displayed by the Greater Mastiff bat (Eumops perotis), whose range includes the Southwestern U.S.

California Leaf-nosed Bat (Macrotus californicus), the only leaf-nosed species in the U.S. (© Daniel K. Riskin/ http://www.noctilio.com/bats)

Just as they vary in size, they also look different from each other in the face. Some have elongated snouts suitable for sipping nectar in flowers. Others have stubby faces for chewing up fruits. There are bats with pig-like snouts and others with intricate folds and wrinkles on their face that are thought to aid with their radar navigation in the dark. Some bats emit sounds through their nose and have a flap of skin called a noseleaf around their nostrils; it’s believed this helps them focus their radar. Many bats are, as you might guess, rather homely, but others are actually cute. The flying foxes, for instance, are so-called because of their handsome fox-like faces.

Ear size and shape varies from small and round to large and pointy. Bats have excellent hearing and many can rotate their ears to gather sound even better. Bat calls can be very loud, so to keep from deafening themselves they close their ears when sounding a call.

#

Bat skeleton (“Brehms Tierleben,” 1887 / Wiki; PD)

Bats’ legs are differently oriented from those of other animals. Indeed, very differently: The knee bends in a backward orientation that enables bats to comfortably hang against a surface, usually head down, without knees getting in the way. Bats hang by their five toes and a system of tendons clench when the weight of their body pulls against them — it’s this that allows them to sleep without losing their grip. Weirdly, if a bat dies while roosting, his body will hang there until something knocks it loose. (Birds’ feet also have tendons that hold them in place, but in their case in an upright position.)

Bat hair color is usually black or brown, but there are exceptions. Some examples are the Silver-haired bat (Lasionycteris noctivagans), who has dark hair tipped with silver, giving him a frosted look; the Eastern Red bat (Lasiurus borealis), who has a reddish-orange coat; the Northern Yellow bat (Lasiurus intermedius), who’s often yellowish; and the Spotted bat (Euderma maculatum) who has huge pink ears and a black coat with three large white spots on the back.

The upside to upside-down
You’ve probably seen photos of bats hanging upside-down. Do you wonder why? It’s because their wings don’t produce enough lift for takeoff from a standstill and they can’t run fast enough to gain speed for lift. It takes much less effort for bats to simply let go from an elevated position and fall into flight. If you were to place a bat on the ground he would walk to the nearest vertical structure and climb it to gain some height before taking wing. Now, that said, it’s going to surprise you that bats don’t always rest head-downward. They hang in many different positions.

Bat-speak
Bats emit ultrasonic calls in order to “see” prey within their environment. They use a different set of vocalizations for communicating with other bats. Surprisingly complex ones, at least with Mexican free-tailed bats, according to Dr. George Pollak. He’s a bat expert who has studied the vocalizations of a captive colony of these bats in Austin, Texas. He describes the bats’ vocalizations as made up of syllables and phrases, with “what seems to be some kind of syntax.” He says they’re also “situation-specific.” This complexity probably extends to other species as well. Pollak attributes this to bats’ need to communicate in the dark, where they can’t transmit physical clues. Listen to bats.

Sonar: Echolocation
Most people know bats have the ability to fly in the dark without smacking into things because they use some kind of sonar (which are radio waves). There’s a specific term for this when used in reference to animals — echolocation.

While flying, a bat emits constant high-frequency, short-wavelength sounds which are produced in the larynx and emitted through his mouth (usually) or the nose. The sounds, or calls, are often described as clicks or cheeps and they can be very loud. Up to 120 decibels — that’s louder than a low-flying jet plane. (Fortunately for humans, most of these ultrasonic sounds are beyond our range of hearing.) As the sound waves strike objects in their path they bounce echoes back to the listening bat, who’s able to interpret them to form a mental picture of what’s ahead. It’s a picture so well-formed, they can detect a single hair or home in on a mosquito on a moonless night.

The volume, frequency, tone and duration of calls vary depending on the habitat. Most are high-frequency sounds we don’t hear, but sometimes calls are made within our hearing. The Western Mastiff bat (Eumops perotis) regularly calls in a lower range. (You can listen here to bat sounds captured by ultrasonic recorders called “bat detectors.”) In a bit of evolutionary logic, bats who use echolocation tend to have small eyes, while those who don’t use it have large eyes and better vision. Bats aren’t the only species that use echolocation. Whales, dolphins, porpoises and certain species of shrews are some of the other animals who do.

Humans are using rudimentary echolocation when we shout into a canyon and listen to the sound waves as they bounce off the canyon walls. Some blind people use echolocation very successfully in their daily lives, making “clicking” sounds, just as bats do, although at a lower, slower frequency.

#

Bats roosting in a cave (© George Burba)

Roosting
The human vampires in literature and movies would have us believe bats burst into flames if caught in sunlight! Of course, they don’t, although they are nocturnal, with the exception of some tropical bats who eat in the daytime. When not feeding, bats “roost,” which just means they find a safe place to groom and rest. Roosts may be in caves, abandoned mines, rock crevices or under bridges. Also in abandoned buildings, attics, under louvers or siding, and other such places, where they cling to vertical surfaces. Any sliver of an opening is all they need for entry. Some bats roost under loose bark, within hollow trees or clinging to a trunk or even simply hanging from a limb.

Some bat species are solitary, but most are highly social and roost in colonies of a few up to millions. Males and females of some species stay separated except during mating season. Other species roost together except in summer when females form “maternity colonies.”

Reproduction
Sexual maturity begins at anywhere from eight or nine months old up to two or three years, depending on the species. Mating usually occurs once or twice a year and the gestation period ranges from about six weeks up to six months or more with some of the fruit bats.

Bats don’t construct nests, even when pregnant. Pregnant females roost together in maternal colonies, sometimes very large ones. At delivery time the mother may separate from the group. Often she turns herself head up and forms a pouch with her interfemoral membrane (uropatagium). She catches her baby in it as he (or she) emerges.

Lesser Short-nosed Fruit Bat (Cynopterus brachyotis), less than 12 hrs old ( Anton Croos Wiki cc by-sa 4.0

Lesser Short-nosed Fruit Bat (Cynopterus brachyotis), less than 12 hrs old. ( Anton Croos / Wiki cc by-sa 4.0

The “pup” — usually one, occasionally two, rarely four — is born feet first. He’s naked, sightless and already a quarter or more the size of his mother. He’s relatively strong, too: As his mother licks him clean, head to toe, he climbs up her body to suckle. This is a perilous time for him — if he loses his hold, the fall could kill him. If he survives, he’ll call out loudly for his mother. She has a strong maternal instinct and will do her best to rescue him. She’ll even move him to a different location if her roost site is disturbed.

Some bat species carry their pup with them for the first few days while foraging, the baby clinging tightly to mother’s underside. But as pups put on weight and become too heavy, they’re left in the roost. You may be wondering how his mother can pick him out of dozens, hundreds or thousands of other pups packed tightly together in a maternity colony. She identifies him through odor and vocal communication.

Within a week of his birth the pup will open his eyes, in a couple more he’ll have fur. He’ll start flying in three to six weeks and in a few months he’ll be fully weaned. Bats can live 20 to 30 years, but the mortality rate is very high in their first year from predation, accidents, failure to put on enough fat to last through hibernation, and forced abandonment of young caused by human harassment. This combined with bats’ low birth rate means it takes a very long time for a partially destroyed colony to rebuild itself, if ever.

Hibernation
Most bats are insect eaters, so those living in colder climates hibernate the six or so months of the year when insects aren’t readily available. Their winter quarters (hibernaculum) must be cold enough to considerably lower their body temperature, but not to the freezing point. When hibernating, bats allow their body temperature to drop to match the temperature of their environment. This dramatically reduces their energy requirements. The resting heart rate slides from over 200 beats per minute down to 12. Breathing slows to perhaps one breath every 45 minutes. When they hang from their toes that also saves energy because muscles aren’t being used.

Bats awaken periodically through the winter to urinate or change position. Before hibernating, they fatten up so they’ll have enough “fuel” to carry them through to spring. Each awakening consumes enormous amounts of energy. So, it comes at a great cost when their hibernation site is disturbed. The consequence is death if a bat runs out of fat stores before insects are available. It can easily happen, here’s a reason why: A Canadian study concluded that each time a Little Brown bat (Motif lucifugus) is awakened from hibernation he expends 67 days’ worth of fat!

Why does waking up consume so much energy? We humans think of “waking up” as gradually becoming mentally alert and opening our eyes. This may occur over a period of a few minutes or even seconds. But for bats, coming out of hibernation requires igniting their internal furnace, slowly raising their temperature, increasing their heartbeat and drawing more frequent breaths. It’s a long process — 44 minutes for the Little Brown bat.

Bats are beneficial
More than half the U.S. species of bats are endangered or threatened, mostly because of human activities: encroachment, harassment, deliberate killing and habitat destruction. Worldwide about 70 percent of bats eat insects. Some are important predators of crop pests, while others include nuisance insects, like mosquitoes, in their diet. Just one bat can devour hundreds of insects an hour — up to 8,000 in a night — often just scooping them into their mouths as they fly. (Not all bats grab their prey out of the air. Some seek out insects on the ground or on trees. Some bats use their feet to snatch insects. A few species are carnivorous, feeding on frogs, lizards, birds, small mammals, generally killing them very quickly. Bats are voracious feeders, eating up to half their body weight each night to meet their energy needs.

Old World species are important for dispersing seeds, either spitting them out as they eat fruit or eliminating them in their feces. Fruit bats are the single most important factor in reforestation of rainforests. Some types of trees owe their continued existence solely to bats. Bat activities also pollinate fruit plants, such as banana, fig, date and mango. Another benefit of bats is their droppings (guano), which make an excellent fertilizer. Guano, however, must be collected responsibly, if at all. Hauling it from caves and other bat roosts removes the food source of other creatures living in that ecosystem, threatening their lives. Also, too-frequent disruption of resting bats can drive them permanently away.

Bats and rabies
It’s commonly thought that any bat seen in daytime must be rabid. That isn’t true. Some bat species rest in caves, attics or other dark and hidden resting areas in daytime. But, others rest in crevices, under eaves or even hanging from tree limbs, where they might be seen.

Bats can contract rabies, but it’s very rare. According to the most recent Rabies Surveillance Report by the U.S. Centers for Disease Control, across the entire U.S., there were only 1,380 reported incidents of rabies in 2011. Any case of rabies, of course, is one too many, but to put that figure into perspective, consider that there are millions and millions of bats living across the U.S. Of course, a bat should never be touched, unless it needs help. And, even then, never bare-handed.

¹ Stoker’s Count Dracula could shape-shift into other animals, as well.
* Mariana Fruit Bat juvenile (Pteropus mariannus mariannus), a species of Guam and the Commonwealth of the Northern Mariana Islands. (USFWS-Pacific Region; cc by 2.0)

Share.