Bat Caves and Bat Species
When you think of bats, you think of bat caves....but they don't look like Batman's cave...they are dark, warm or cool and full of guano (bat poop)...often the ammonia smell is suffocating. Bats live in a variety of places, but many species make caves their seasonal homes. Surprisingly, they are quite picky about the kind of cave - not all caves have bats. Some bats will hibernate in one cave, and raise their pups in another. Some bats use caves to moderate temperatures so they don't have to migrate. Here are just a few examples:
The combined mass of bats in Bracken Cave in Texas amounts to an amazing 270 tons. They share body heat, and by doing so, they are able to raise the cave’s temperature by over 20 degrees. In any bat nursery, warmth is essential to growth and survival of young. Those that congregate in very large numbers can dramatically increase the heat of an entire cave. Most bats live in smaller colonies, though, and these must find nursery roosts that are already warm (mostly between 80 and 90°F). This is problematic for most temperate-zone species and prevents them from rearing young in caves because their groups are not large enough to heat the cave sufficiently.. In warm climates, even a few bats often can rear young by selecting small heat-trapping cavities in ceilings. In a Caribbean cave, small colonies of Jamaican fruit bats (Artibeus jamaicensis) rear young in cavities that are carefully guarded as valued commodities by the males. Clustering and trapping of body heat is important for many bats, which explains their tendency to form large colonies and protect the roosts when they find them..
In tropical and subtropical areas, where most caves are relatively warm, and food and water are often nearby, bats can use nearly any cave that is safe from predators. But here in the US, cave entrances must be large enough for bats to enter and exit quickly in groups to avoid snakes, carnivorous mammals, and birds of prey. Florida has rat snakes that wait for bats to emerge from a single cave entrance. This is an unnatural situation created because many southeastern myotis (Myotis austroriparious) roosting caves have been destroyed by careless human development and disturbance, forcing ever larger numbers to congregate in fewer and more vulnerable caves. In one incident, a quarter of a million of these bats took refuge in a single wet cave where they later drowned in a flood. Such unnatural crowding increasingly threatens bats worldwide.
In the United States and Europe, especially at higher latitudes and elevations, only a few species use caves, or even rock or erosion crevices, for rearing young. There are exceptions, such as northwestern Canada where bats are able to rear young in rock crevices that are geothermal heated by hot spring water. They remain safe despite being at ground level, only because there are no predators.
In desert areas, bats often must find rock or erosion crevices that receive just the right amount of heat from the sun. The tiny western pipistrelles (Pipistrellus hesperus) uses cliff-face crevices year-round. They look for warm (but not extremely hot) crevices to rear young in summer, and cool (but not sub-freezing) ones for winter hibernation. These tiny bats wedge into spaces only a half-inch or less wide. This enables them cling tightly to the rock surface for temperature control and move deeper to avoid extremes. Bizarrely, they prevent attacks from snakes by simply opening their mouths to fill the space so completely that the snake has no room to maneuver.
The western small-footed myotis (Myotis ciliolabrum) rear their young alone, in ground-level cavities that are easily accessible to predators; however, because they occupy only one in many thousands of openings in extremely rocky or eroded landscapes it is not worth a predator’s effort to look for them.
Most temperate-zone bats are forced to migrate or hibernate to avoid winter weather extremes. The western pipistrelle, survives winter by moving deep into cliff-face rock crevice where temperatures are low, but above freezing - ideal temperatures for hibernation.. Prior to the arrival of modern humans, caves were by far the best places to find ideal conditions. Many bats are now so well adapted to hibernating in specific temperature zones of caves that they cannot change and have to find alternate cave-like situations.
Bats living in temperate climates typically spend the summer in tree cavities, man-made structures, or rock crevices that are sufficiently solar-heated to permit rearing of young. However, in fall they must travel varying distances to reach the relatively rare caves that are capable of providing cool, stable temperatures (between 32 and 52°F) for hibernation. Long migrations can consume as much energy as an entire winter of hibernation, so bats can afford long travels only where hibernation caves are ideal.
Bats have developed a variety of winter survival strategies in response to the temperature gradients found in hibernation caves. Bat species, having differing physiological and behavioral adaptations, divide their roosting habitat in a manner that minimizes competition.
Eastern pipistrelles (Pipistrellus subflavus) cannot survive below-freezing temperatures. Thus they must enter hibernation early and leave late, often remaining asleep except for brief arousals, from mid-September to early May. Indiana myotis (Myotis sodalis) have an intermediate tolerance. They hibernate from late October through March, while big brown bats (Epetesicus fuscus), which are extremely hardy and can arrive during severe snowstorms, only have to hibernate from December through February.
Bats will spend close to 80 percent of their winter fat supply fighting dehydration and changing temperatures. Big brown bats roost near cave entrances, where incoming air is cold and dry. They reduce their exposure to dry air and cold by wedging into tight crevices. Rapid changes in temperature and humidity force them to wake up and move more often than other species .They spend less time in hibernation, so are able to use a wider variety of hibernation sites, and avoid a long migration. Packing themselves into dense clusters, they save energy.
In hibernation the bat's body temperature drops and heart beat and respiration slow. The breathing of a hibernating bat is imperceptible, and its body is cold to touch. Its heartbeat drops from roughly 400 beats per minute when awake, to about 25 in hibernation, and its body temperature drops to within a few tenths of one degree of the surrounding cave. The bat's fat stores are slowly metabolized during hibernation which can last eight months. Even a few moments of disturbance during this time can waste 30 to 60 days worth of fat reserves.
The kinds of exceptionally large, complex, and multi-entranced caves that bats require are also those most sought after for human recreation such as for spelunking or tours....so there has been a slow extinction happening due to loss of good cave habitat.. Prior to the arrival of European settlers in America, the Indiana bat’s intermediate hibernation strategy had become extremely successful. By forming large, tightly packed clusters in just a few ideally structured caves, where it occupied a relatively narrow middle range of temperature, this species had minimized hibernation needs enough to permit long migrations. It could build enormous colonies--an estimated 10 million or more in Mammoth Cave, Kentucky, alone--but could not survive in the much wider variety of caves tolerated by big brown bats and eastern pipistrelles.
Cold-air-trapping caves, provide temperatures that can be 10° below the annual average at the surface. This variance means that bats arriving as early as September can enter hibernation at an ideal 45°, though outside temperatures may still be close to 75°. By entering hibernation at this time, bats can avoid the danger of being trapped in the open in sudden fall storms. If this cave’s contours were reversed, it would trap warm summer air, providing potential nursery roost sites, though it would be unsuitable for hibernation.