Vampire bats have developed a specialized system using infrared-sensitive receptors on their nose-leaf to prey on

homeotherm Homeothermy, homothermy or homoiothermy is thermoregulation that maintains a stable internal body temperature regardless of external influence. This internal body temperature is often, though not necessarily, higher than the immediate environmen ...

ic (warm-blooded) vertebrates.

Trigeminal nerve In neuroanatomy, the trigeminal nerve ( lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for sensation in the face and motor functions such as biting and che ...

fibers that innervate these IR-sensitive receptors may be involved in detection of infrared

thermal radiation Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) i ...

emitted by their prey. This may aid bats in locating blood-rich areas on their prey. In addition, neuroanatomical and molecular research has suggested possible similarities of IR-sensing mechanisms between vampire bats and IR-sensitive snakes. Infrared sensing in vampire bats has not yet been hypothesized to be image forming, as it was for IR-sensitive snakes. While the literature on IR-sensing in vampire bats is thin, progress continues to be made in this field to identify how vampire bats can sense and use infrared thermal radiation.

Vampire bats Vampire bats, species of the subfamily Desmodontinae, are leaf-nosed bats found in Central and South America. Their food source is blood of other animals, a dietary trait called hematophagy. Three extant bat species feed solely on blood: the c ...

are the only known mammals whose entire nutrition relies on blood from mammals or birds. In the family Phyllostomidae and the subfamily Desmodontinae, there are three known species of vampire bats: ''Desmodus rotundus'' (

common vampire bat The common vampire bat (''Desmodus rotundus'') is a small, leaf-nosed bat native to Latin America. It is one of three extant species of vampire bat, the other two being the hairy-legged and the white-winged vampire bats. The common vampire bat ...

), ''Diphylla ecaudata'' ( hairy-legged vampire bat), and ''Diaemus youngi'' (

white-winged vampire bat The white-winged vampire bat (''Diaemus youngi''), a species of vampire bat, is the only member of the genus ''Diaemus''. They are found from Mexico to northern Argentina and are present on the islands of Trinidad and Margarita. Etymology and ta ...

). Most of the referenced research on

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

sensing in vampire bats has been done on the common vampire bat because this is the most commonly found species.Mulheisen, M. and R. Anderson. 2001. Animal Diversity Web. "Desmodus rotundus." Accessed November 14, 2011 http://animaldiversity.ummz.umich.edu/site/accounts/information/Desmodus_rotundus.html.

Behavior

Vampire bats were not studied in their natural habitats until about 1935. During

predation Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...

, bats first spend a few minutes in the air circling the target prey, eventually landing on the back or neck crest of the animal, and sometimes the ground. They then proceed to search for a suitable spot to bite, which can take seconds to minutes, and usually will feed on the neck or flank. The same spot may be fed on multiple times by different bats. Kürten and Schmidt (1982) were the first to suggest that infrared perception in vampire bats is possibly used in detecting regions of maximal blood flow on targeted prey. Although warm receptors are also found in the facial regions of species such as mice, humans, and dogs, the extreme low-temperature sensitivity of these receptors on vampire bats suggest specialization for sensing infrared. (see section on Physiology). In 1982, Kürten and Schmidt performed behavioral studies to examine the ability of vampire bats to detect infrared radiation. Their study showed that when given a choice between a warm and cold object, vampire bats can be trained to choose the infrared emitting signal unit (SU). Two SUs made of a heating element with a copper plate backing were mounted on a wall. For each trial, one SU was warmed while the other was maintained at room temperature. The heating and warming of the SUs were interchanged at random between trials. If bats chose the warm SU correctly, they were rewarded with food through a feeding tube below each SU. In this study, olfactory and visual stimuli were minimized to ascertain that only thermal cues affected behavioral learning. Olfactory cues were eliminated by the full feeding tube attached behind the opening from which the bats received their rewards. Dim lighting minimized visual cues of the two SUs.

Anatomy

The central nose-leaf and a semicircular ring of pads construct the nasal structure of the vampire bat. There are also three depressions known as nasal or leaf pits, located between the nose-leaf and pads. Two lateral pits, one on each side of the nose-leaf, are situated at 45° angles towards the nose-leaf. The apical pit is slightly raised and directed upward and forward relative to the nose-leaf. The pits are about 1 millimeter wide and 1 millimeter deep, hairless, and glandless. A layer of dense connective-tissue with sparsely distributed blood vessels insulates the nasal structure. Based on structure alone, these pits were first suggested to house IR-receptors.

Neuroanatomy

Kürten and Schmidt (1982) first suggested that the location and orientation of each pit structure provided directional information in infrared radiation detection. From their initial studies, the nasal pits seemed to be ideal for purposes of IR-perception. Later in 1984, Kürten and collaborators made electrophysiological recordings from nerve fibers of temperature-sensitive infrared thermoreceptors located on the central nose-leaf and upper lip, but did not find such receptors in the nasal pits (see Physiology). This revoked their earlier hypothesis and established that the infrared-sensitive receptors are located on the central nose-leaf. Schäfer and collaborators confirmed this by recording impulses from thermoreceptors on the nose-leaf as well. Klüver-Barrera and Nissl staining of vampire bat brain sections uncovered a unique nucleus located lateral to the descending trigeminal tract (dv). The nucleus is composed of

neuropil Neuropil (or "neuropile") is any area in the nervous system composed of mostly unmyelinated axons, dendrites and glial cell processes that forms a synaptically dense region containing a relatively low number of cell bodies. The most prevalent an ...

s and medium-sized cells, which is very similar to the nucleus (DLV) in the lateral descending trigeminal system in IR-sensitive snakes. This special nucleus is found in all three species of vampire bats and no other bats, but does not necessarily indicate a direct connection with infrared sensing. More recent studies using

in situ hybridization ''In situ'' hybridization (ISH) is a type of hybridization that uses a labeled complementary DNA, RNA or modified nucleic acids strand (i.e., probe) to localize a specific DNA or RNA sequence in a portion or section of tissue (''in situ'') or ...

studies have located large diameter neurons in the

trigeminal ganglia A trigeminal ganglion (or Gasserian ganglion, or semilunar ganglion, or Gasser's ganglion) is the sensory ganglion at the base of each of the two trigeminal nerves (CN V), occupying a cavity ( Meckel's cave) in the dura mater, covering the trigem ...

(TG) that are unique to vampire bats and extremely similar to those found in IR-sensitive snakes. Although the morphological organization of neurons suggests convergent evolution with IR-sensing snakes lineages, it remains unclear what the exact neural pathway is for infrared sensing in vampire bats.

Physiology

Thermography Infrared thermography (IRT), thermal video and/or thermal imaging, is a process where a thermal camera captures and creates an image of an object by using infrared radiation emitted from the object in a process, which are examples of infrared i ...

, a method which produces pictures of the distribution of temperatures on an object, was used to investigate temperature variation across facial structures of the common vampire bat. The nasal structure has a temperature of 9 °C lower than the rest of the face. The

thermal insulation Thermal insulation is the reduction of heat transfer (i.e., the transfer of thermal energy between objects of differing temperature) between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with ...

of the nasal structure and maintained temperature difference may possibly prevent interference of self-emitted thermal radiation. Warm receptors located in the nose may then optimally detect outside sources of infrared radiation. Vampire bats are sensitive to power densities (a measure of emitted energy) greater than 50 µW/cm2 at distances between 13 and 16 cm (a power density of 1.8x10⁻⁴W/cm2 corresponds to 50 °C). This was first determined by quantifying the temperature at which vampire bats could not behaviorally distinguish between heat emitting and room temperature SUs. A positive linear relationship exists between the energy-threshold of heat detection and distance from stimuli. Through mathematical calculations, at a distance of 8 cm, vampire bats should be able to detect humans who emit radiation of 80 µW/cm2. Temperature threshold measurements were directly measured by stimulating

nerve fiber An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...

s of

thermoreceptor A thermoreceptor is a non-specialised sense receptor, or more accurately the receptive portion of a sensory neuron, that codes absolute and relative changes in temperature, primarily within the innocuous range. In the mammalian peripheral nervous s ...

s in the nose-leaf and upper lip with a water-circulated brass thermode and recording the impulses/second at every 5 °C shift in temperature from 10 to 40 °C. These receptors have a threshold of 28 °C and a maximum temperature response to 40 °C, beyond which there was either no firing or an irregular firing pattern. This threshold is 8 °C lower compared to those of warm receptor in other species of mammals, which implies extreme sensitivity to heat. After stimulation of these receptors, there is a transient increase in impulse activity which quickly decays due to

adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...

and thus strengthens temporal acuity.

Molecular mechanism of infrared detection

A family of TRP (transient receptor potential) channels, including

TRPV1 The transient receptor potential cation channel subfamily V member 1 (TrpV1), also known as the capsaicin receptor and the vanilloid receptor 1, is a protein that, in humans, is encoded by the ''TRPV1'' gene. It was the first isolated member of th ...

(transient receptor potential

vanilloid The vanilloids are compounds which possess a vanillyl group. They include vanillyl alcohol, vanillin, vanillic acid, acetovanillon, vanillylmandelic acid, homovanillic acid, capsaicin, etc. Isomers are the isovanilloids. : A number of vani ...

) and TRPA1 (transient receptor potential cation channel A1), is important in thermal and

pain Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, ...

detection. TRPV1 channels are activated by

capsaicin Capsaicin (8-methyl-''N''-vanillyl-6-nonenamide) ( or ) is an active component of chili peppers, which are plants belonging to the genus ''Capsicum''. It is a chemical irritant for mammals, including humans, and produces a sensation of burnin ...

(a chemical which can be extracted from chili peppers), noxious temperature ranges (>43 °C), membrane-derived

lipid Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids in ...

s, low pH, and

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...

changes.Rosenbaum, Tamara, and Sidney A. Simon. "TRPV1 Receptors and Signal Transduction." In TRP Ion Channel Function in Sensory Transduction and Cellular Signaling Cascades, edited by Wolfgang B. Liedtke and Stefan Heller. Boca Ranton: CRC Press, 2007. Available online. Activation of TRPV1 by capsaicin results in calcium and sodium influx, and functionally allows for detection of painful thermal stimuli. TRPV1 may also act as a molecular thermometer in response to temperatures greater than 43 °C. The result is an inward calcium and sodium current similar to capsaicin-evoked currents. TRPV1 channels may also have voltage-sensitive properties responsible for its activation.

Phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...

and

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, m ...

s, especially at the

C-terminus The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein i ...

(carboxylic acid end of primary

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

sequence), can alter the threshold temperature of heat-activation. The specific mechanism behind heat-activation of TRPV1 channels has yet to be deciphered.

TRPV1-S

TRPV1-S (TRPV1 short) is an

isoform A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some is ...

of the capsaicin receptor TRPV1 and comprises 35-46% of TRPV1 transcripts in the

of common vampire bats. Deep sequencing of complementary DNA of these receptor channels shows that this is not true for closely related fruit bats ''

Carollia brevicauda The silky short-tailed bat (''Carollia brevicauda'') is a bat species found in Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Panama, Peru, Suriname, Mexico and Venezuela Venezuela (; ), officially the Bolivarian Republic of ...

'' (< 6% TRPV1-S) and other fruit, nectar, or insect feeding bats (''Uroderma bilobatum'', ''Sturnira lilium'', and ''Anoura cultrata'') (<1% TRPV1-S). TRPA1 channels in pit-bearing snakes, such as the western diamondback rattlesnake (''

Crotalus atrox The western diamondback rattlesnake or Texas diamond-backWright AH, Wright AA. (1957). ''Handbook of Snakes''. Comstock Publishing Associates. (7th printing, 1985). . (''Crotalus atrox'') is a rattlesnake species and member of the viper family, ...

''), are sensitive to infrared thermal radiation as well. TRPA1 transcripts are mostly found in the TG of IR-sensitive snake linages as was for bats. TRPV1-S isoform results from

alternative splicing Alternative splicing, or alternative RNA splicing, or differential splicing, is an alternative splicing process during gene expression that allows a single gene to code for multiple proteins. In this process, particular exons of a gene may be i ...

during

post-transcriptional regulation Post-transcriptional regulation is the control of gene expression at the RNA level. It occurs once the RNA polymerase has been attached to the gene's promoter and is synthesizing the nucleotide sequence. Therefore, as the name indicates, it occ ...

, a variation of the TRPV1 C-terminus due to

insertion Insertion may refer to: * Insertion (anatomy), the point of a tendon or ligament onto the skeleton or other part of the body * Insertion (genetics), the addition of DNA into a genetic sequence *Insertion, several meanings in medicine, see ICD-10-PC ...

of 23-base-pair sequence, exon14a, that contains a

stop codon In molecular biology (specifically protein biosynthesis), a stop codon (or termination codon) is a codon (nucleotide triplet within messenger RNA) that signals the termination of the translation process of the current protein. Most codons in mess ...

. The inserted sequence is flanked by two

intron An intron is any Nucleic acid sequence, nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e. a region inside a gene."The notion of ...

s marked with G T/ AG donor-acceptor sites that are necessary for U2-dependent splicing. Consequently, incorporation or bypass of

exon An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term ''exon'' refers to both the DNA sequence within a gene and to the corresponding sequen ...

14a results in the short or long isoforms, respectively. It is suggested that efficient splicing of the exon14a segment necessitates the specialized environment of TG in vampire bats. Compared to the long isoform (threshold ~ 40 °C), the temperature threshold of channel activation is much lower for TRPV1-S. TRPV1-S channels expressed in HEK293 cells and ''Xenopus oocytes'' (cells commonly used for manipulation of expressing certain genes) have a threshold of 30 °C. This is highly consistent with ''

in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called " test-tube experiments", these studies in biology a ...

'' studies pertaining to temperature thresholds of IR-sensitive receptors in vampire bats (28 °C). Activation of TRPV1-S channels in the TG may then suggest a similar mechanism (as seen in IR-sensing snakes) for how infrared sensing may work in vampire bats. Trigeminal nerves which innervate specialized temperature sensitive receptors on the nose-leaf may in turn activate TRPV1-S channels in the TG in response to infrared thermal radiation.

Behavior

Anatomy

Neuroanatomy

Physiology

Molecular mechanism of infrared detection

TRPV1-S

See also

References

Further reading