Vampire bats sense infrared radiation to locate places where blood flows close to their prey's skin. At a molecular level, this ability is underpinned by the intricate redesign of an ion channel on facial nerves. See Letter p.88
The ability to sense heat or cold is vital: it allows animals to detect, and so avoid, debilitating or fatal temperatures. But there is more to thermosensation than keeping one's cool (or heat). On page 88 of this issue, Gracheva and colleagues1 report on modifications to the facial nerves that allow the common vampire bat (Desmodus rotundus) to detect infrared thermal radiation associated with hot spots — areas where blood flows close to the skin of the bats' prey — so that they can efficiently access the blood they eat.
This paper complements earlier work2 on infrared sensors in snakes, including some boas, pythons and pit vipers, which also use radiation in this range to locate food. The endings of somatosensory trigeminal nerves in the pits on the pit viper's face detect infrared radiation. Specifically, pit vipers detect infrared radiation through TRPA1 — a cell-surface ion channel that is usually heat insensitive — on these nerves. The heat sensors of pit vipers are more sensitive than those of pythons or boas, indicating independent evolution of thermosensation.
Gracheva et al.1 show that vampire bats sense infrared radiation through a different ion channel, TRPV1, which is used by other mammals to detect noxious heat (> 43 °C). In vampire bats, however, the TRPV1 activation threshold is lowered to about 30 °C through alternative splicing of its gene transcripts, which results in truncation of the channel's carboxy-terminal domain. The truncated version of the protein is not expressed in the bats' dorsal root ganglia, which innervate the spinal cord. Instead, it occurs only in their facial (trigeminal) nerves, where ion channels in both vipers and bats usually respond to similar temperatures. Trigeminal nerves also receive sensory input from the head and face in bats and snakes. But in bats, they innervate detectors not in pits1 but on the upper lip and modified noseleaf3 (Fig. 1).
How do these differences affect the animals' hunting behaviour? Pit vipers often prey on small mammals and can better detect hot spots at night. A good example of the importance of thermal cues to these snakes is the defensive behaviour of California ground squirrels4. When confronted by a pit viper such as a Pacific rattlesnake, ground squirrels flag their tails to distract the predator. This display has a thermal component that is missing when the predator is a gopher snake — a species that lacks thermosensation. Like pit vipers, some species of python and boa also use infrared sensors in facial pits to detect warm-blooded prey and to guide their strikes even in the absence of visual cues5.
Vampire bats can detect a heat source from about 20 cm (ref. 3), and probably use this proximal cue to find hot spots on their prey — often areas that are not covered with fur or feathers. Repeated attacks on the same cattle suggest that, together, the vampire's memory and the prey's breathing sounds6 provide distal cues that allow the bats to locate a sleeping target7. As yet, there are no data to suggest defences against thermoperception by vampire bats similar to those of California ground squirrels against vipers. However, it is important to note that the bats take only about two tablespoonfuls of blood (25 ml), and so, unlike those of pit vipers, vampire attacks are not fatal. Nonetheless, there is evidence that some prey develop antibodies to the anticoagulants in the vampire's saliva8, directly affecting the bats' feeding time.
In vampire bats, the organization of the Trpv1 gene seems to be characteristic of the Laurasiatheria, the superorder of mammals that includes bats (order Chiroptera), as well as several other orders. Chiroptera is further divided into two suborders, and Gracheva et al.1 show that Trpv1 occurs in both. The authors call these suborders microbats (Microchiroptera) and megabats (Megachiroptera). However, the two bat suborders are now called Yinpterochiroptera and Yangochiroptera9, and the split of species between the two is not the same as for microbats and megabats. Gracheva and co-workers' analysis of the Trpv1 gene in bats does not alter the current view9 of bat phylogeny.
Gracheva et al.1,3 have placed infrared detection high on the list of astonishing discoveries about the perceptual abilities of animals: it seems to have evolved in parallel within two snake lineages, and converged with the appearance in vampire bats. Their data, together with other recent findings, also enrich our knowledge of the sensory world of bats. Previous work suggested, for instance, that bats' wing membranes are as sensitive to touch as our fingertips10. And tactile receptors associated with sensory hairs on the bat wings are known to monitor flight speed and air flow11. The perceptual world of bats undoubtedly has many more intriguing secrets yet to be discovered.
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