Homing In For A Blood-thirsty Meal

Vampire bats (Figure 1) are arguably one of the most feared and despised mammals. Their stealthy, nocturnal habits and their penchant for feeding on blood have made them Hollywood B movie stars. Yet humans have little to fear from these small animals and much to learn.

Figure 1. A South American vampire bat (Desmodus rotundus). (Photo by Dr. Pascual Soriano)

In South America where vampire bats are common, vampires approach their prey on the ground, galloping quickly and quietly as they sneak up on, bite, and drink the blood from sleeping cows, goats and birds.

Vampire bats are the only mammal that survives solely on blood, and they need to drink it pretty much every day to survive. They support this need through a number of evolutionary adaptations.

Like other bats, they feed only at night, and they have excellent eyesight enhanced by acute hearing and an ability to emit high-pitched sounds that help them navigate. Their teeth lack enamel, which keeps them constantly razor sharp and allows them to delicately tear through the hide of a sleeping animal without waking it. Grooves in their tongue draw up the blood seeping through the open wound through capillary action, and they have anticoagulation chemicals inside their saliva to keep it flowing.

Within minutes of sinking its teeth in an animal’s flesh, an adult vampire bat can drink half its body weight in blood. But first they must find a vein.

Scientists have known for years that vampire bats seek out the best spots to tear through an animal’s skin with their razor-sharp teeth – where a precise bite will strike a vein and spill forth nourishing blood. But nobody knew exactly how bats knew where to bite.

By investigating wild vampire bats in South America, researchers at the University of California, San Francisco and Instituto Venezolano de Investigaciones Científicas in Caracas, Venezuela have discovered their secret (
Gracheva et al., 2011). Its called TRPV1, and it is a sensitive, heat-detecting molecule covering nerve endings on their noses.

“Vampire bats feed on blood, and it’s useful for them to have an infrared detector to be able to find the circulation,” said David Julius, the Morris Herzstein Chair in Molecular Biology & Medicine at UCSF, who led the research.

Similar TRPV1 molecules can be found on pain sensing nerve fibers in human tongue, skin or eyes. They allow people to detect the chemical capsaicin in chili peppers and experience the burning tinge of spicy food.

Described this week in the journal
Nature, the discovery highlights how small changes to genes in the genome of a species can contribute to major evolutionary adaptations over time – in this case, allowing the vampire bat to detect infrared heat from their prey, helping them efficiently find and feed on blood.

TRPV1 belongs to a large family of similar molecules common to many types of animals but they differ slightly from animal to animal – both in terms of their DNA and in terms of where they appear in the body.

Subtle changes to these molecules contribute to highly specialized physiologies for sensing the world. Many animals have highly specialized adaptations allowing them to see, feel, hear or taste in special ways. The heat-sensing molecules within vampire bat’s nose is one example.

Researchers have known for years that pits on vampire bats’ noses allow them to detect blood vessels because vessel radiate heat (Figure 2). But no one knew exactly how this occurred.

Figure 2. Face of a vampire bat. Red arrowheads mark pit organs surrounding the vampire bat’s nose. (From Gracheva et al., 2011)

Working with three researchers in South America, Julius’ postdoctoral fellows Elena Gracheva, PhD and Julio Cordero-Morales, PhD, together with UCSF colleagues Nicholas Ingolia, PhD and Jonathan Weissman, PhD, sequenced genes from samples of nose tissue from wild vampire bats in Venezuela, determining that TRPV1 is the molecule responsible for their ability to detect heat.

They also determined that it was not just TRPV1 but an evolutionary genetic variation of it that allows vampire bats to detect low temperature heat. Through a mechanism known as “alternative splicing” a special form of the molecule emerged in the noses of the bats, becoming a sensitive detector for finding the hottest spots.

Story Source:
This story is adapted from materials provided by the University of California - San Francisco.

Gracheva, E., Cordero-Morales, J., González-Carcacía, J., Ingolia, N., Manno, C., Aranguren, C., Weissman, J., & Julius, D. (2011). Ganglion-specific splicing of TRPV1 underlies infrared sensation in vampire bats Nature, 476 (7358), 88-91 DOI: 10.1038/nature10245