Feeding ecology and diet

The combination of small size and high metabolic rates means that bats consume enormous quantities of food. The heart of a flying little brown bat beats about 1,200 times a minute, reflecting the rate at which it burns energy. The same bat, having landed, has a heartbeat rate of less than 300 per minute. During seasons when they are active, little brown bats (like other bats) eat about half their weight in food every night. Nightly, fruit-eating bats may handle three times their weight in food. Lactating females have higher energy demands and nightly eat more than their weight in food.

Consumption of large amounts of food often means that bats eat a variety of prey species, whether insects or fruit. While some insectivorous bats may eat more soft (e.g., moths, flies) than hard (beetles, bugs) prey, there is little evidence of specialization by prey species. Insectivorous bats should not be thought of as consumers of mosquitoes. However, some smaller species, for example, Bodenheimer's pipistrelle (Pip-istrellus bodenheimeri) from the Middle East, are known to regularly eat mosquitoes.

Bats normally do not eat food they find distasteful. The list of such prey includes at least arrowhead frogs and insects (like tiger moths) protected by unpleasant chemicals. While insectivorous and frugivorous bats learn to avoid food that has made them sick, this taste aversion does not occur in vampire bats. Whatever their food, bats have ways of avoiding ingesting indigestible materials. From insects, they typically bite off and drop wings and legs, from bats and birds, the wings, or cellulose fibers from leaves and fruit. Insects pass quickly through the digestive tracts of bats, 20-30 minutes for little brown bats. Weight reduction translates into lower costs of flight. There is no evidence that bat populations are limited in size by the availability of food.

Insect-eating is a recurring lifestyle in bats, from tropical South America to Alaska, northern Scandinavia to tropical Africa, Malaysia to Tasmania. While most species of insectivorous bats hunt flying insects and use echolocation to detect, track, and assess their targets, others (gleaners) take prey from surfaces such as foliage or the ground. Some bats such as New Zealand lesser short-tailed bats (Mystacina tuberculata) show great mobility on the ground where they search for food. Their menu includes animals as well as nectar and pollen.

Gleaning bats eat more than flying insects, consuming a wider range of prey, including walking insects and arthropods that do not fly. Larger gleaning bats take a wider range of prey by size than smaller ones. Gleaners such as pallid bats (Antrozous pallidus) from western North America eat large scorpions and centipedes, Jerusalem crickets, and even small pocket mice (genus Perognathus). Larger gleaning bats like the Australian false vampire bat (Macroderma gigas), spectral bat (Vampyrum spectrum), and large slit-faced bats (Nycteris grandis) also eat birds and even other bats.

Aerial-feeding bats tend to take smaller prey than gleaning bats. Among bats, it is rare to find aerial-feeding species

A Wahlberg's epauletted fruit bat (Epomophorus wahlbergi) feeding from a baobab flower (Adansonia rubrostipa). (Photo by Merlin D. Tut-tle/Bat Conservation International/Photo Researchers, Inc. Reproduced by permission.)

taking vertebrate prey. The only known exception is the greater noctule (Nyctalus lasiopterus) from southern Europe. For at least part of the year, this 1.7 oz (50 g) bat preys on migrating birds.

Many species of animal-eating bats hunt along the water's surface. Called "trawlers," these bats (often in the family Ves-pertilionidae, the plain-nosed bats) have enlarged hind feet with which they gaff small fish. Mexican fishing bats (Myotis vivesi) from Baja California, and Rickett's big-footed bat (My-otis rickettii) from southern China are two examples. The best-known fishing bat is the greater bulldog bat (Noctilio leporinus, family Noctilionidae) from Central America, South America, and the West Indies. Other trawling bats do not have such large hind feet, but still take the occasional fish and even mosquito larvae. This list includes Daubenton's bat, pond bats (Myotis dasycneme), long-fingered bats (Myotis capaccinii), and large-footed myotis (Myotis adversus). Fish eating has been documented in two other bats, large slit-faced bats from Africa

A spotted bat (Euderma maculatum) echolocating in Utah, USA. (Photo by B. G. Thomson/Photo Researchers, Inc. Reproduced by permission.)

and greater false vampire bats (Megaderma lyra) from India and Southeast Asia. These bats do not have enlarged hind feet and are thought to catch their fish directly in their mouths— but to date, they have never been observed fishing.

Other bats eat frogs. Most is known about the fringe-lipped bat (Trachops cirrhosus, family Phyllostomidae) of the New World tropics. This bat listens for the songs male frogs use when courting females, and uses them to find prey. Fringe-lipped bats grab singing frogs directly in their mouths and eat all of them, starting from the head. In south-central Africa, large slit-faced bats prey heavily on frogs, but there is no indication of their using the frogs' songs to find their prey. The same is true of heart-nosed bats (Cardioderma cor) that occur further north in Africa, or of greater false vampire bats in India. Large slit-faced bats also eat frogs from the head down, invariably leaving one leg from the ankle, and the toes of the other foot.

Throughout the tropics, some species of bats get food from plants. Included on the menu are fruits, seeds, leaves, nectar, and pollen. In the Neotropics, the plant-visiting bats belong to the family Phyllostomidae. In the Old World tropics, the bats are in the family Pteropodidae. The two families show remarkable convergence in structure and behavior.

When eating fruit or leaves, bats typically chew their food thoroughly, all the while using their tongues to rub mashed food against prominent ridges on the roofs of their mouths (palates). During this process, the bats suck vigorously, removing the digestible parts of fruit and leaves before spitting out pellets of indigestible fibers.

Flower-visiting bats obtain sugars from nectar and proteins from pollen. Some Neotropical bat flowers have ultrasonic reflectors that guide nectar-feeding bats to nectar and pollen. By drinking their own urine, nectar-feeding bats create acidic conditions in their stomachs, ideal for digesting pollen. Some flower bats also obtain protein from insects.

The most infamous of bats are the blood-feeding vampires. There are three species: vampire bats, hairy-legged vampires (Diphylla ecaudata), and white-winged vampires (Diaemus youngii), all in the family Phyllostomidae. These bats only eat blood they obtain by making shallow bites on a prey's skin. The bites do not penetrate large blood vessels such as arteries or veins. Vampire bats use razor-sharp upper incisor teeth to remove a 0.2-in (5-mm) diameter divot of skin, creating a wound that bleeds readily. The bats enhance bleeding by the actions of their tongues and saliva. The saliva of vampire bats contains chemicals that inhibit the body's defenses against bleeding, including anticoagulants, anti-agglutinants, and chemicals that inhibit local vasoconstriction. Each vampire species appears to be a "one-stop shopper," getting each blood meal from one prey. The bats ingest about 2 tablespoonfuls (25 ml) of blood. Blood represents less than 10% of the mass of a bird or mammal, meaning that only victims larger than 4.4 lb (2 kg) are suitable hosts for vampire bats. Within two minutes of beginning to feed, vampire bats start to urinate. The urine consists mainly of the plasma from the current blood meal; therefore, it is very dilute. This is the bat's way of ridding itself of indigestible material.

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