Common vampire bat

Appearance

The common vampire bat is short-haired, with silver-gray fur on its undersides, sharply demarcated from the darker fur on its back. It has small, somewhat rounded ears, a deeply grooved lower lip, and a flat, leaf-shaped nose. A well-developed, clawed thumb on each wing is used to climb onto prey and to assist the animal in take-off. The bat averages about 9 cm long with a wingspan of 18 cm . It commonly weighs about 57 grams , but its weight can double after a single feeding. The braincase is relatively large, but the snout is reduced to accommodate large incisors and canines. It has the fewest teeth among bats. The upper incisors lack enamel, which keeps them razor-sharp. Common vampire bats exhibit sexual dimorphism in which females are bigger than males.

While most other bats have almost lost the ability to maneuver on land, vampire bats are an exception. They can run using a unique, bounding gait in which the forelimbs are used instead of the hindlimbs to propel forward, as the wings are much more powerful than the legs. This ability likely evolved independently within the bat lineage. Three pads under the thumb function like a sole. It is also capable of leaping in various directions, magnitudes and temporal sequences. When making a jump, the bat pushes up with its pectoral limbs. The hindlimbs keep the body over the pectoral limbs which are stabilized by the thumbs.

Common vampire bats have good eyesight. They are able to distinguish different optical patterns and may use vision for long-range orientation. These bats also have well-developed senses of smell and hearing: the cochlea is highly sensitive to low-frequency acoustics, and the nasal passages are relatively large. They emit echolocation signals orally, and thus fly with their mouths open for navigation. They can identify a metal strip 1 centimetre wide at a distance of 50 centimetres , which is moderate compared to other bats.

Habitat

The common vampire bat is found in parts of Mexico, Central America, and South America. They can be found as far north as 280 kilometres south of the Mexico–United States border. Fossils of this species have been found in Florida and states bordering Mexico. The common vampire is the most common bat species in southeastern Brazil. The southern extent of its range is Uruguay, northern Argentina, and central Chile. In the West Indies, the bat is only found on Trinidad. It prefers warm and humid climates, and uses tropical and subtropical woodlands and open grasslands for foraging. Bats roost in trees, caves, abandoned buildings, old wells, and mines. Vampire bats will roost with nine other bat species, and tend to be the most dominant at roosting sites. They occupy the darkest and highest places in the roosts; when they leave, other bat species move in to take over these vacated spots.

Reproduction

A male and his females compose a harem. Harems may contain multiple males; in these groups, the males have a dominance hierarchy, and the dominant one mates preferentially with the females of the roost and sires about 50% of the offspring; the next-dominant male fathers the second-most offspring, and so on. Male vampire bats maintain roosting sites that attract or contain females, and harem males are usually the only ones that mate with their females. Bachelor males try to mate with harem females when possible, but the females usually refuse them. In multiple-male harems, a female may reject mating attempts by the dominant male, possibly to avoid inbreeding.

During estrus, a female releases one egg. Mating usually lasts three to four minutes; the male bat mounts the female from the posterior end, grasps her back with his teeth, holds down her folded wings, and inseminates her. Vampire bats are reproductively active year around, although the number of conceptions and births peak in the rainy season. Females give birth to one offspring per pregnancy, following a gestation period of about seven months. The young are raised primarily by the females. Mothers leave their young to hunt, and call their young to feed upon returning. They are given their mother’s milk exclusively for the first three months, and are subsequently fed mixtures of milk and regurgitated blood. The young accompany their mothers to hunt at six months, but are not fully weaned until nine months. Female offspring usually remain in their natal groups into adulthood, unless their mothers die or move. The occasional movements of unrelated females between groups leads to the formation of multiple matrilines within groups. Females are reluctant to join new groups, as a bat's survival rate depends on long-lasting social bonds—a female which enters a new group may not be fully accepted. Male offspring tend to live in their natal groups until they are one to two years old, sometimes being forced out by the resident adult males.

Food

The common vampire bat feeds primarily on mammalian blood, particularly that of livestock such as cattle and horses. Vampire bats feed on wild prey like the tapir, but seem to prefer domesticated animals, and favor horses over cattle when given the choice. Female animals, particularly those in estrus, are more often targeted than males. This could be because of the hormones.

Vampire bats hunt at night, using echolocation and olfaction to track down prey. They feed in a distance of 5 to 8 km from their roosts, and leave in an orderly fashion; bachelor males are the first to depart, followed by the females, and finally the harem males. When a bat selects a target, it lands on it, or jumps up onto it from the ground, usually targeting the rump, flank, or neck of its prey; heat sensors in the nose help it to detect blood vessels near the surface of the skin. It pierces the animal's skin with its teeth, peels away a small flap, and laps up the blood with its tongue, which has lateral grooves adapted to this purpose. The blood is kept from clotting by an anticoagulant in the saliva. When feeding, the blood is stored in the cardiac notch. Bats feed for 30 minutes and become so swollen with blood, they can barely fly. They must then hide themselves and wait for the blood to digest and some of the water to be excreted before taking off.

Vampire bats commonly return to the same host on consecutive nights, after marking the animal with urine. They are protective of their host and will fend off other bats while feeding. It is uncommon for two or more bats to feed on the same host, with the exception of mothers and their offspring.