Bats are fundamentally flying shrews, and shrews are insectivores. The teeth of ancestral bats (and many living forms as well) are like those of shrews, with numerous sharp cusps adapted to piercing insects' chitinous exoskeleton, then macerating the contents. The results of efficient chewing are tiny bits of food delivered to the gut, ready for digestion. Smaller pieces have a higher surface area compared to their volume, and hence digestion is rapid. This is important to mammals as small and active as bats, which need to fuel their hard-working cells as quickly as possible.
Most bats continue the insectivory of their ancestors, but not all bats pursue insects the same. Specializations have evolved in methods of capturing prey, selecting it (by size or taxonomic group, for example) and foraging. In Colorado, the long-legged myotis and the little brown bat forage on continuous, repeated circuits, whereas the long-eared myotis and Townsend’s big-eared bat flutter through the vegetation, hovering to glean insects from leaves. The little brown bat captures single in-sects, first knocking them off balance with a wing-tip, catching them in the uropatagium, then pulling them to the mouth with a curl of the tail. By contrast, hoary bats take larger insects directly into their mouths.
From bats original insectivory, food habits have diversified greatly. For example, a number of bats of diverse affinities have specialized in fishing. When watching bats drinking or filter-feeding on insects low over water, it is not hard to envision the step from insectivory to fishing. New World bulldog bats are strongly adapted to fishing with long feet and gaff-like claws that drag the water for top-feeding fish. However, similar piscatorial adaptations are seen in Myotis vivesi (a not-too-distant relative of Colorado's mouse-eared bats), which occur along the coasts of Baja California and Sonora.
Similarly, first steps in the shift from insectivory to frugivory are not difficult to imagine. Insects of many kinds are drawn to fruit and flowers; perhaps bats moving to a diet of fruit or pollen were simply cutting out the middle man, eating fruit directly rather than eating fruit-eating insects.
Much of the diversity of chiropteran food habits is exhibited in New World leaf-nosed bats, the family Phyllostomidae. Many phyllostomids eat fruit, hovering or landing on plants to take their meal. Recall that fruits are meant to be eaten. Bats help dispersal. Frugivores often pass seeds through the digestive tract unharmed, thus spreading the plant's offspring out of competition with the parent. The teeth of frugivorous bats are fewer than their insectivorous ancestors, and the cusps are less pronounced. From frugivory, more specialized forms of vegetarianism evolved. The long-tongued phyllostomids have essentially converged with hummingbirds. Their greatly elongated muzzles and tongues serve as straws to extract nectar from flowers. In the process, the bat's head is covered with pollen. The pollen is carried to the next flower as the bat continues on its nectar-feeding rounds, and cross-pollination of the plant is ensured.
Pollination systems are a frequent arena for co-evolution, the mutual evolutionary response of species to each other's presence in a biotic community. Plants that have co-evolved with bat pollinators tend to have white flowers that open at night. Familiar examples are saguaro and cardon cacti of the desert Southwest, which are pollinated by long-nosed bats.
Note that bat vegetarians specialize in eating the high-energy parts of the plant, such as fruit and nectar. No bat is specialized to eat leaves (folivory). Leaves are an abundant resource, but their energy mostly is tied up in cellulose, which mammals cannot digest. A bulky diet of cellulose is incompatible with a bat's high-energy lifestyle.
Leaf-nosed bats have not stuck with vegetarianism. Fruit-eating bats gave rise to specialized carnivores. The so-called "false vampires bats" of the New World tropics are not vampires at all but are carnivores that eat rodents, bats and birds. Not surprisingly, given their aerial foraging habits, a number of large phyllostomid bats capture other bats (and so do some of the larger vespertilionids, by the way, including Colorado's hoary bat). The fringe-lipped bat is specialized to locate and capture frogs, especially noisy males calling for mates.
Finally, true vampire bats -- three species in their own subfamily -- arose from fruit-eating, leaf-nosed ancestors. Vampire bats, all from Latin America, eat only blood. Two species feed mostly on the blood of birds, and one -- the common vampire -- specializes on mammals. These bats locate their host with a variety of techniques, including echolocation, smell, heat sensitivity and vision. They are as stealthy as they are efficient. After making a quick incision with their razor-sharp incisors on an exposed surface (often the neck, shoulder, ears, hips, legs or around the hooves), the bats inject an anticoagulant, drink their fill and are gone usually without awakening the host. These bats drink less than a half ounce of blood at a meal, but this adds up to about three gallons of blood annually. With their habits of forming rather large colonies and individuals returning to the same host on successive nights, vampire bats can have a significant impact on livestock. They also are important vectors of disease, including bacterial and other parasitic infections and rabies.
The Old World flying foxes, the Megachiroptera, are specialized frugivores. Like fruit-eating microchiropterans, they have fewer and simpler teeth. However, unlike other bats, they are active by day, and sight and smell are the principal senses used to find food.
All Colorado bats are insectivores. Little brown bats, common in Colorado, have been known to catch and eat more than 150 mosquitoes in less than 15 minutes. These mammals couple efficient harvesting with speedy processing. They chew their food up to seven times per second.
Bats generally fly low over lakes or streams to drink, although some bats living in deserts do not drink at all, relying instead on insects to provide them with the water they need.