When bats are active they are very active indeed, so to balance things out, they spend much of the rest of their lives asleep, to one extent or another, and their "sleep" may be profound. Bats are strongly heterothermic. That is, their body temperature is highly variable. Once, this led people to imagine that bats were poor thermoregulators. Now it is recognized, however, that their thermoregulation is exquisite and it is we "typical" mammals who are rather sloppy.
Bats achieve and maintain body temperatures appropriate to their current physiological and behavioral needs, thereby conserving energy. When they are actively feeding, metabolic activity and body temperature are high. When they are resting, both are low. Humans and most other mammals, by contrast, have nearly constant, high body temperatures, maintained by continual moderate to high metabolic activity, regardless of behavioral or physiologic "needs." It is as if typical mammals are furnaces with a simple ON/OFF switch. "ON" is alive; "OFF" is dead. Bats, by contrast, have a furnace regulated by a sophisticated thermostat. Temperature is adjustable to the variable demands of activity. This process of resting under lowered metabolic activity and body temperature is termed torpor. Bats may enter torpor for a few hours or several months. The intensity of torpor ranges from shallow to deep. Deep, long-term torpor in winter is termed hibernation.
Periods of torpor are spent in a roost, usually a place where bats can hang protected from dangers both biological and physical.
Most of Colorado's bats prefer caves, fissures, mines and old buildings as roosts. Several species, however, use tree roosts. Red bats of the eastern plains prefer deciduous trees near rivers and streams. The hoary bat and the silver-haired bat roost in deciduous riparian woodland but are also found in higher coniferous forests, where they blend with pine needles, bark and lichens.
Day and night roosts usually are separate and often distinctive. Night roosts serve as temporary havens for active bats. A number of local species show a biphasic pattern of foraging activity, in which individuals emerge to feed, then night-roost for a few hours, then feed again and finally retire to a day roost. Because night roosts are adjacent to feeding grounds and feeding areas may be in clearings near homes, porches, eaves and haylofts are frequent night roosts, often marked by tiny piles of droppings.
Day roosts usually provide more protection than night roosts, especially from changes in temperature. The day-roosting behavior of bats differs among species and sometimes between sexes. For example, in a summer colony of little brown bats in a barn, males tend to roost singly or in small groups, sleeping and conserving energy. Females, on the other hand, may gather in large nursing colonies near the roof, where it is warmer and more conducive to the growth of young. On cooler fall days, females and young cluster together in tightly packed groups, while males let their body temperatures drop to that of their surroundings and slide into deep torpor.
In some colonial bats -- like the Brazilian free-tailed bats of the San Luis Valley -- roosts of males, females and young are strongly segregated during the summer, apparently to increase available feeding opportunities. The separate nursery and "bachelor" colonies are often hundreds of miles from each other, allowing the bats to exploit different habitats and independent food resources.
Most Colorado bats migrate to some extent, although little is known about this behavior. The advantages of having separate summer and winter roosts must be substantial, however, to justify sometimes lengthy commutes. Brazilian free-tailed bats in the southwestern U.S. travel 1,000 miles or more into the interior of Mexico each fall, probably the longest migration of any bat in the world.
About half of Colorado's bats make only local migrations and hibernate in the state; the remainder apparently migrates to hibernate elsewhere. Hibernacula (hibernation roosts) of some species never have been located. Hibernacula are occupied much longer than other roosts -- up to four months or more in some species, during the coldest months. Hibernation usually is a bat's deepest torpor, and the rate of fuel consumption is greatly reduced. In some species, body temperature may drop to near freezing, and in hibernating little brown bats in Wisconsin, temperatures have been recorded at 30° F, with icicles developing on the bat's fur. The heart rate of hibernating bats may drop to 10 beats per minute, compared with well over 1,000 when the animal is in flight.
Because of their ability to slow their metabolism so markedly, torpid bats have survived submersion in water for an hour at a time with little effect. Laboratory researchers sometimes keep bats hanging in the refrigerator for upcoming projects.
Bats roosting in caves often fly near the entrance of their hideouts just before dark, apparently checking the time. Some biologists believe that these bats may be synchronizing their internal clocks with the solar cycle, re-setting them if necessary. This flight is also thought to help synchronize members of the population with one another by shifting forward the activity cycles of late-risers.
Migratory instincts are apparently triggered by day length. The direction of migratory patterns seems to be inherited to some extent, but details are unclear. Several species of bats orient visually toward familiar mountain ranges during their annual journeys, whereas other species, in controlled experiments, are able to travel to distant sites even if blindfolded. According to Merlin Tuttle, founder of Bat Conservation International, some bats probably learn migration patterns from older bats. "Gray bats that breed and hibernate in a single cave often come from all directions to a place they have never seen," he said. "They could not be genetically programmed to go in one direction, and their sonar reaches only a few yards at best -- yet these bats travel hundreds of miles and find cave entrances too small and obscure to be detected from more than a few feet away." One study has even suggested that bats can synthesize magnetite (the mineral from which magnets are made) in their brains, perhaps for use in some sort of an internal compass.