All about bats

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Let’s first dispense with the creepy notion that bats are blood-thirsty predators. In actuality, only three species of bats drink blood, and none of them favor human blood. Bats represent a quarter of the mammals on earth, with about 1,200 species, so the percentage of vampire bats is quite low. Besides, of the forty-seven species of bats known to inhabit North America, none are vampire bats. You can rest easy!

Background

The oldest known bat species, Icaronycteris index, lived in the early Eocene Epoch, about 52.2 million years ago. Other fossil evidence points to an ancestor (that’s also shared by humans) dating back eighty million years. As you see here, bat fossils show a creature not very different from the bats of today.

A reddish-brown fossil of the bat species Icaronycteris index imbedded in tan colored stone.

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



Inhabiting the US are three small, flower-eating species that spend part of the year in the Southwest, migrating between the US and Mexico, and 44 others spread out across the US that feed on insects. If you have bats patrolling your backyard wildlife habitat at night, count yourself lucky. List of U.S. bat species

The Southwest is home to the greatest diversity of US bats, with Texas hosting more than thirty 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 down when they return they’ve feasted on 250 tons of insects! Bat Conservation International owns the cave. It’s closed to the public for the protection of the bats, but you can watch them leaving it.

Micro and Mega

Bats belong to the order Chiroptera (ky-ROP-ter-uh). Within this are two major groups—the Microchiroptera and the Megachiroptera. About two hundred 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 two inches long (5 cm), but this group also includes the very large “flying foxes,” which are bats with fox-like faces and large eyes. Megabats eat fruit, nectar, and pollen. Most don’t use echolocation. Instead, they use their excellent vision and a good sense of smell.

The rest of the bats, Microchiropterans, inhabit all areas of the world, except Polar Regions and extreme deserts. They use echolocation to find prey. Most are insect-eaters, although some species feed on small mammals and fish.

The world’s smallest bat (and possibly smallest mammal) is the bumblebee-sized Kitti’s Hog-nosed Bat, (Craseonycteris thonglongyai), of Thailand and Myanmar, which is only 1.2–1.3 inches long (3.0–3.3 cm) and weighs just 0.071 of an ounce (2.0 g). At the other extreme is the Giant Golden-crowned Flying Fox (Acerodon jubatus), an inhabitant of the Philippines, which is 7–12inches long (18–30 cm), with a wingspan of 5.0–6.0 feet (1.5–1.8 m). 

The smallest bat in the US is the Western Pipistrelle, Pipistrellus hesperus, with a wingspan of 7.4–8.5 inches (19–22 mm) and a body length of 2.4–3.1 inches (6–8 cm). The largest US bat is the Greater Mastiff Bat, Eumops perotis, 5.5–7.5 inches long (14–19 cm), with a wingspan of over 22 inches (56 cm). 

Myths, be gone!

Bats stay quietly hidden by day and come out at night to silently skim the nighttime sky, which makes them mysterious and seem dangerous. But, here are the facts:

  • Bats aren’t dangerous. To repeat: Bats aren’t dangerous. They couldn’t be more opposite their fictional counterparts. In real life they’re timid, gentle and non-aggressive—so much so, even one with rabies won’t attack.
  • Bats aren’t “blind as a bat!” Day and night, their vision is excellent. Researchers have discovered that at least two nocturnal bat species even have color vision.
  • Bats aren’t dirty. They’re 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.
  • Rabies among bats makes sensational news, but it’s rare: one-half of one percent.² Also, rabies does not get transmitted through the air; so being near a bat is not a risk.

The nitty-gritty on vampire bats

Two young White-winged Vampire Bats swaddled in a white towel.

Young 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 small, with a body only about three inches long (7.6 cm) and a wingspan of around eight inches (20 cm).

They do drink blood, as their name says, 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 an ounce in thirty to forty 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 where they live, Mexico and Central and South America.

Vampire bats are smart and can be tamed, and trained—NationalGeographic.com 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 fly, but they aren’t birds. They’re mammals, the only ones that truly fly rather than glide from a higher elevation to a lower one, such as “flying” squirrels do.

Bats don’t all look alike. In fact, they can look very different from each other. For example, some have elongated snouts suitable for sipping nectar in flowers, while others have pig-like noses or intricate facial folds.

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 echolocation. Some bats are cute. Flying foxes, for instance, are so-called because of their fox-like faces. (There’s one at the top of this chapter.)

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. Their calls are extremely loud, and they have to close their ears when calling to keep from deafening themselves! 

Wings

“Chiroptera,” the name of bats’ order, is a word from the Greek cheir, meaning hand. and pteron for wing. Hand-wing. It’s well-chosen because their wings have elongated bony structures that work like the fingers of a hand. They also have a thumb on each wing tipped with a claw they use for holding food, climbing and clinging. 

Common Pipistrelle Bat, Pipistrellus pipistrellus, lying on a man's thumb.

Common Pipistrelle, Pipistrellus pipistrellus. Its thumb is the pointy projection on the folded wing. (Gille San Martin / Wiki; cc by-sa 2.0



The wings are covered by the patagium (puh-TAY-gee-um), a membrane of skin so thin that light can be seen through it. It gives the appearance of being hairless, but with at least some species it’s coated with tiny, inconspicuous hairs thought to play a role in aerodynamics or as sensors.

Mexican Long-tongued Bat drinking nectar from a hummingbird feeder.

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 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. This membrane also stretches between the legs, where it’s then called the uropatagium or interfemoral membrane. With some species, the uropatagium completely encloses the tail.

Legs

Bats’ legs are oriented differently from those of other animals. 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.

Black and white illustration of bone structure of a bat, with labels.

(Popular Science Monthly Volume 9, 1876 via Ineuw / wiki; PD

Bats have five toes and hang from them using a system of tendons that clench when the weight of the body pulls against them. That allows bats to sleep without losing their grip. Weirdly, if a bat dies while roosting, its 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.)

Bats usually have black or brown hair, but there are exceptions. For example, the Northern Yellow Bat (Lasiurus intermedius) is often yellowish; the Silver-haired Bat (Lasionycteris noctivagans) has dark hair tipped with silver, giving it a frosted look; and Eastern Red Bats (Lasiurus borealis) have a reddish-orange coat.

Flight

Bat wings move with all the flexibility of hands (and then some), giving them astonishing 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 35–40 miles per hour (56–64 kmh). In at least one instance, even faster: In 2009, scientists attached a transmitter to a Mexican Free-tailed Bat, Tadarida brasiliensis, and clocked it at ninety-nine miles per hour (159 kmh), a new record for horizontal flight among bats (and birds, too).

The upside to upside-down

You’ve seen photos of bats, and they always appear to be hanging upside-down. There’s an anatomical reason for that. It’s because their wings don’t produce enough lift for takeoff from a standstill and they can’t run fast enough to lift themselves into the air. So, bats simply let go from an elevated position and fall into flight. If you were to place a bat on the ground, it 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 face head-down when they rest.

Bat-speak

Bats use vocalizations for communicating with other bats. And, they’re surprisingly complex, at least with Mexican Free-tailed Bats. Dr. George Pollak, a physiologist at the University of Texas in Austin, has been studying the vocalizations of a captive colony of these bats. He describes their 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 bat species, as well. Pollak attributes this to bats’ need to communicate in the dark, where they can’t transmit physical clues.  

Echolocation

Most people know bats can fly in the dark without smacking into things because they use “radar” (electromagnetic radio waves). That’s the right idea, but not entirely accurate. What they use is sonar (sound waves). There’s a specific term for sonar when used in reference to animals–echolocation.

Cover your ears!

A bat emits constant high-frequency, short-wavelength sounds while flying. They’re produced in the larynx and emitted through the 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—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, which can interpret them to form a mental picture of what’s ahead. The image is so well-formed, the bat can “see” 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 bat’s surroundings. 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.

Not all bats use echolocation. In a bit of evolutionary logic, bats that don’t use it have larger eyes and better vision than does that do. (Bats, by the way, aren’t the only species that use echolocation. Whales, dolphins, porpoises and certain species of shrews are some of the other animals that use it.)

We humans use rudimentary echolocation when we shout into a canyon and listen to the sound waves as they bounce off the 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.

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 really, although they are nocturnal (except for some tropical bats that 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. You may discover a bat roosting under loose bark, within a hollow tree, clinging to its trunk or just hanging from a limb. Where you won’t find them is on any surface that prevents them from falling downward to take flight.

Small group of Seba's Short-tailed Bats roosting in old building.

Seba’s Short-tailed Bats, Carollia perspicillata, roosting in old building. (Bernard Dupont / wiki; cc by-sa 2.0)



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 maternal, or nursery, colonies.

Reproduction

Sexual maturity begins at anywhere from eight or nine months old up to two or three years, depending on the species. It 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 huge ones. At the birthing 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, less than 12 hrs old, sleeping on a human's finger.

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. It’s naked, sightless and a quarter or more the size of its mother. She licks it clean, head to toe. The pup is strong, a good thing because it must climb up its mother’s body to suckle. It’s a perilous journey for the tiny baby—if it loses its hold, the fall could kill it. If it survives a fall, it’ll call out loudly for its mother. She has a strong maternal instinct and will do her best to rescue her pup. She’ll move it to a different location if her roost site is disturbed.

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

Within a week of its birth, the pup will open its eyes; in a couple more, it’ll have fur. It’ll start flying in three to six weeks, and a few months after that, it’s completed weaned.

Lifespan

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. All this, combined with bats’ low birth rate, means it takes a very long time for a partially destroyed colony to rebuild itself, if ever.

Food sources

Northern Yellow Bat, Lasiurus intermedius, in flight.

Northern Yellow Bat, Lasiurus intermedius, catching flying insects. (USFWS / Wiki; PD)



Bats, depending on the species, eat beetles, moths, mosquitoes and other insects, as well as nectar and fruit. A few species are carnivorous, feeding on frogs, lizards, birds, small mammals or fish, generally killing them very quickly. Vampire bats feed only on blood.

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 frigid enough to lower their body temperature considerably, but not to the freezing point.

When hibernating, they allow their temperature to drop to match that of their environment. That dramatically reduces their energy requirements. The resting heart rate slides from two or three hundred beats per minute down to ten or twelve. Breathing slows to perhaps one breath every forty-five minutes. They hang by their toes, which also saves energy because it doesn’t require the use of muscles.

Before hibernating, bats fatten up so they’ll have enough “fuel” to carry them through to spring. They awaken periodically through the winter to urinate or change position. Each awakening consumes enormous amounts of energy. So, it comes at a high cost when their hibernation site is disturbed. The consequence is death if one runs out of stored fat before insects are available. That can easily happen. For example, a Canadian study concluded that each time a Little Brown Bat (Motif lucifugus) wakes from hibernation, it expends sixty-seven days worth of fat!

Little Brown Bat being held in a man's hand.

Little Brown Bat, Myotis lucifugus. (Mike Leveille / EOL; cc by-nc 4.0)



Why? Where a human becomes fully alert in seconds or a few minutes, a bat coming out of hibernation must ignite its internal furnace, slowly raise its temperature, increase its heartbeat and draw more frequent breaths. It’s a long process—forty-four minutes for, say, a Little Brown Bat.

Predators

Numerous animals prey on bats. Raccoons, snakes, weasels, and other predators crawl or reach into places where bats are roosting in the daytime and eat them. Fish catch them as they’re skimming above water. Hawks and owls snatch them out of the air when they’re leaving or going to their roosting places at dawn and dusk. Large spiders, like tarantulas, also occasionally catch bats.

Encroachment, harassment, deliberate killing, habitat destruction, and wind farms threaten more than 50 percent of US bats.

Bats are beneficial

Worldwide about 70 percent of bats eat insects. Some are important predators of crop pests, such as cucumber beetles, while others include nuisance insects, like mosquitoes and gnats, in their diet. Just one can devour hundreds of insects an hour—up to eight thousand in a night—often just scooping them into their mouths as they fly. Some bats seek insects on the ground or trees. Others use their feet to snatch insects. Bats are voracious feeders, eating up to half their body weight each night to meet their energy needs.

Old World bats are important for dispersing seeds, either spitting them out as they eat fruit or eliminating them in their feces. Fruit bats are the most critical 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 is their droppings, called guano, which make excellent fertilizer. However, hauling it from caves and other roosts removes the food source of other creatures living in that ecosystem. Also, too-frequent disruption of resting bats can drive them permanently away.)

Bats and rabies

It isn’t true that any bat seen out in the daytime must be rabid. While some species sleep in caves, attics or other dark and hidden resting areas in the daytime, others rest in crevices, under eaves or even hanging from tree limbs, where they might be seen.

Rabies among bats is rare. According to the most recent Rabies Surveillance Report (2015) by the US Centers for Disease Control, across the entire US, there were 1,704 confirmed incidents of rabies among bats submitted for testing. 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 across the country. Bracken Cave alone (near San Antonio, Texas) has 1,500,000 bats. Carlsbad Caverns has 400,000. And, those are just two locations. Of course, never touch a bat unless it needs help. And, even then, never bare-handed. 

*Top photo:Mariana Fruit Bat juvenile, Pteropus mariannus, USFWS-Pacific Region; CC BY 2.0)

More reading:

In your yard: beetles 
In your yard: flies      
All about birds   

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