Bats

by Elisabeth K. V. Kalko, Ph.D.

Bat Wings | Bat Eyes | Roosting | Group Composition | Reproduction | Bat Predators | Echolocation

Much has been learned in recent years about the usefulness of bats and the important roles they play in maintaining the health of bat ecosystems. Understanding the amazing diversity of bats will change the common belief that all bats are ugly and blood-thirsty. On the contrary, bats are highly beneficial animals. Fruit-eating and nectar-drinking bats play indispensible roles in maintaining forest diversity and in facilitating forest regeneration through seed dispersal and pollination. Insect-eating bats remove large quantities of insects from the environment, including many that damage plants or are regarded as pests to humans.

Bats are mammals and thus, female bats possess mammary glands which produce milk. When baby bats are born, they are taken care of and fed milk by their mothers for a few weeks until they can fly and forage on their own. As is typical of most other mammals, the body of a bat, except for its wings, is covered by hair. Due to the way bats look at first glance, in some languages they are called 'flying mice'. However, bats are not related to mice or rodents at all, but belong to a large group (order) of mammals called CHIROPTERA. The order Chiroptera consists of two subgroups (suborders): Microchiroptera and Megachiroptera. Microchiroptera have a global distribution and include about 825 species worldwide. In all of Panama, we find about 120 species of Microchiroptera. All microchiroptera have a well developed sonar system ("echolocation") that allows them to orient in the dark and to a varying degree to find food. They emit high-frequency sounds, most of them too high for humans to hear, that bounce off objects and allow bats to build up an acoustic image of their surroundings. Megachiroptera do not occur in the New World. They are limited in their distribution to the tropics and subtropics of the Old World (i.e., Africa, Asia, Australia). They consist of a single family containing about 175 species. Megachiroptera have fox-like faces with a long snout and large eyes. Not surprisingly, they are also known by the vernacular name 'flying fox'. With one exception, flying foxes do not use sonar. They rely mostly on their keen senses of vision and smell to find their way, mostly in twilight or above the canopy at night and to locate food.

Bat Wings

Chiroptera means 'having wings' and characterizes the unique way bats fly. Bats literally 'fly with their hands'. Imagine a hand with very long fingers; then picture a thin, elastic skin (membrane), that stretches between the elongated fingers and the arm, reaches down to the leg and attaches to the side of the bat's body (Figure 1). This membrane forms the wing of a bat. However, not all five fingers of the bats hand are part of the actual wing. The first finger, the thumb, is very small and ends in a little claw that the bats use for crawling and to help manipulate food. In many bats a membrane stretches between the legs to include part or all of the tail (Figure 1). This tail membrane is used as a kind of pouch by insect-eating bats to trap prey in flight. Although the tail and wing membranes of bats are very thin and seem to be easily injured, they are actually highly elastic and tougher than a rubber glove. Sometimes, however, a bat accidentally rips a hole in a membrane or punctures it. Since the membranes have excellent healing capacities, those wounds usually pose no threat to the bat.

Bat Eyes

Bats are not blind. All bats have eyes, and a number of species, particularly Flying foxes, have very large eyes and excellent eyesight which helps them find their way and their food in twilight and above the canopy at night. In contrast, bats which rely almost exclusively on sonar for finding their way in the dark and for foraging, have mostly tiny eyes and rather poor eyesight. This may have led to the widespread myth that bats are blind.

Roosting

Most bats live in groups, and group size varies dramatically among species. Some form huge aggregations: several thousands or even several million bats may share a roost. Many bats, however, live in smaller groups, ranging from a pair to a few hundred. Roosts of bats can be found in many situations (Figure 2). Under natural conditions, bats roost in caves, under fallen logs, in tree- and earthholes, in hollowed-out, arboreal termite nests, between buttresses of large trees, in rocky crevices, and in the vegetation. Roosts of bats include also manmade structures such as attics, roofs, and eaves of houses as well as places under bridges, in tunnels, and in culverts.

A very interesting roosting behavior occurs in some Neotropical fruit-eating bats (e.g., small Artibeus, Uroderma bilobatum). They select a leaf of a particular plant, such as those of palms and wild bananas and modify it the way that the leaf forms a small tent-like shelter. This tent is accessible from below and is inhabited for a period of time by a small group of those 'tent-making' bats. The tents serve as an excellent shelter against rain and predators. Bats hanging in those tents are very difficult to spot unless one stands right underneath a modified leaf looking straight up. However, although these bats do not appear to be disturbed when one just looks at them, they leave the tent immediately at the slightest vibration of the leave.

Another fascinating roosting behavior has evolved in the Neotropical Disc- Winged Bats (Thyroptera sp.: murcielago de ventosas). These bats have a suction cup on each foot and on each thumb. Anatomy and function of the suction cups resemble those found for instance in an octopus. The disc-winged bats select leaves of wild bananas which are not yet unfurled. They crawl inside the tube-like leaf and use their suction cups to hold onto the leaf's slippery surface. These are the only bats which roost 'head up'. All other bats roost in the characteristic 'head down' position.

Most places where bats roost appear dirty and foul-smelling because of the bat guano that piles up beneath them. By the way, the guano is an excellent fertilizer for garden plants! The bats themseves are very clean animals. They groom themselves many times during the night and after each meal. They use their hind feet as comb and their tongue for grooming, carefully cleaning the fur, the delicate wings, and the tail membrane.

Group Composition

Not only does number of bats per group vary, but also composition of groups of bats spans a wide range. Many bats form maternity colonies at certain times of the year which are composed almost exclusively of females. The females raise their young in these colonies, which give protection to the individual bats. Males of most species are solitary or stay in small groups for most of the year. Other species form harems in which a male and several females live together. Harem males may defend their roosts from other males.

Reproduction

In the temperate zone, bats reproduce only once per year. Usually, they mate in autumn before hibernation starts. Over the winter, females store the males' sperm, a feature unique among mammals. Fertilization of the egg usually occurs in early spring. Young are born at the end of spring and the beginning of summer. Tropical bats often reproduce twice a year, following seasonal patterns in food availability. Long-term studies have shown that females of the Common Fruit Bat (Artibeus jamaicensis) synchronize births of young within groups.

Bats give birth usually to one young. There are only very few exceptions where bats regularly give birth to two or more young. At first, the young bats cannot fly because their wings are not fully developed. For two to six weeks, the female takes care of the baby, keeping it warm, grooming it, and feeding it milk. During that time, a strong bonds exist between mother and young. Mothers recognize their own young even when they have to identify them within huge aggregations of hundreds or even thousands of other baby bats. The mother is guided by scent and vocal communication to its own young. After the young has grown up and learned to fly, it begins to forage on its own, and the mother weans it.

Bat activity and diet. Bats are active at night and rest during the day. At dusk or shortly after nightfall, bats leave their roosts and head straight to their feeding areas to begin foraging. Many bats spend the whole night outside of their day roost and do not return until shortly before dawn. Usually bats have a number of other places, called night roosts, near their foraging area, for shelter and rest during the night. Neotropical fruit-eating bats and bats that take large insects or small vertebrates from the vegetation or the ground use their night roosts also as 'dining' roosts. Fruit-eating bats carry fruit from the fruiting tree to their temporary dining roost and process it there. Hence, seeds and other remains of the fruits accumulate beneath the roost and along the route between the fruit trees and dining roost. Since seeds distributed away from the parent tree have a much better chance to survive and to develop into seedlings and later into trees, bats are very beneficial for forest regeneration. Bats that eat large insects routinely carry them to a dining roost. While processing their prey, these bats discard and drop wings, legs, and other inedible parts of the insect. Examining the prey remains beneath dining roost gives a very good idea of kind and amount of food the bats eat. Overall, the variety of diets of bats ranges from fruit, nectar, pollen, and leaves to a great variety of insects, spiders, crustaceans, small vertebrates (including fish, birds, lizards, rodents, and other bats), and blood of birds and large mammals.

Bat Predators

Numerous anecdotal observations have revealed a wide range of predators for bats. Raptors and owls approach bats and often catch them in flight. Opossums and snakes may wait close to roost sites or attack fruit-eating bats when they approach trees loaded with ripe fruits. Presumably to lessen the risk of being caught by predators such as owls, some bats are known to drastically reduce their activity level during bright nights around full moon (lunarphobia).

Echolocation

All microchiropteran bats use a kind of sonar system called ECHOLOCATION to find their way in the dark and to find and capture their food. While searching for prey, bats emit high-pitched sound in a regular pattern. Most of the sounds are so high in frequency that they are not audible to humans. The echolocation signals strike all objects around the bats and echoes travel back. The bats process the echoes and extract information about distance, shape, size, texture, and movement of potential prey and surroundings. The bats use this information to avoid obstacles like branches, trees, and walls, and to find food. The resolution of the echolocation signals is so accurate that bats can detect and avoid obstacles as thin and as fine as a human hair!

Perhaps the most impressive 'perfection' in echolocation and foraging performance is found in bats that capture insects in the air (Figure 3). As soon as bats have detected potential insect prey with echolocation, they orient toward it and begin their pursuit. When very close to the insect, the bat reaches out with a wing to capture the insect. Then it shovels it into its tail membrane which forms a pouch. After a successful capture, the bat bends its head into the tail pouch, retrieves the insect, and eats it in flight. Some bats catch and eat the insect directly out of the wing or tail pouch alone.

Why, then, are there stories of bats flying into women's hair and holding onto it and sometimes touching people with their wings? Are these bats not using their sonar system to avoid obstacles or are they deliberately attacking people? Both answers are incorrect. People are most likely to come into contact with a flying bat either in a room or other confined space or when they walk into the flyway of bats, such as narrow trails in the forest. In the first case, bats in confined spaces are simply confused and trying to escape. In their confusion, and if they are exhausted from being chased by people, bats may seek refuge on a convenient prominence, which might be a person's head. This is NOT a deliberate attack of the bat! The easiest way to get rid of a bat in the room is to leave the bat alone, turn off the lights, close the door, and open a window. Bats will soon escape through the open window. In the second case, bats flying along well-known and traditional flyways in the forest may not pay much attention to their surroundings since they know the way 'by heart'. Thus, a person walking in the trail might just be an unexpected obstacle in the flightpath of the bat. By trying to get around the obstacle at the last instant, bats may accidently touch a person with their wings. In neither case does this behavior of the bats represent an attack on people.
© Elisabeth K. V. Kalko, 1999