Natures wonders: heat sensing in snakes and bats (Introduction)

by David Turell , Wednesday, September 28, 2016, 15:22 (2976 days ago) @ David Turell

These animals have special sensors for heat. Human sensors are different:-http://www.the-scientist.com/?articles.view/articleNo/46824/title/Sensory-Biology-Around-the-Animal-Kingdom/#-"Many animals are able to sense heat in the environment, but vampire bats and several types of snakes are the only vertebrates known to have highly specialized systems for doing so. Humans and other mammals sense external temperature with heat-sensitive nerve fibers, but pit vipers, boa constrictors, and pythons have evolved organs in their faces that the animals use to detect infrared (IR) energy emitted by prey and to select ecological niches. And vampire bats have IR receptors on their noses that let them home in on the most blood-laden veins in their prey.-"'Infrared sense is basically a souped-up [version] of thermoreception in humans,” says David Julius, a professor and chair of the physiology department at the University of California, San Francisco (UCSF), who studies this sense in snakes. The difference is, snakes and vampire bats “have a very specialized anatomical apparatus to measure heat,” he says.-"These IR-sensing apparatuses, known as pit organs, have evolved at least twice in the snake world—once in the ancient family that includes pythons and boas (family Boidae) and once in the pit vipers (subfamily Crotalinae), which includes rattlesnakes. Pythons and boas have three or more simple pits between scales on their upper and sometimes lower lips; each pit consists of a membrane that is lined with heat-sensitive receptors innervated by the trigeminal nerve. Pit vipers, by contrast, typically have one large, deep pit on either side of their heads, and the structure is more complex, lined with a richly vascularized membrane covering an air-filled chamber that directs heat onto the IR-sensitive tissue. This geometry maximizes heat absorption, Julius notes, and also ensures efficient cooling of the pit, which reduces thermal afterimages.-***-"In 2010, Julius and Elena Gracheva, now at Yale University, identified the heat-sensitive ion channel TRPA1 (transient receptor potential cation channel A1) that triggers the trigeminal nerve signal in both groups of snakes. The same channels in humans are activated by chemical irritants such as mustard oil or by acid, and the resulting signal is similar to those produced by wounds on the skin, Gracheva says. In snakes, these channels have mutated to become sensitive to heat as well.-"Vampire bats—which, true to their name, feed on the blood of other creatures—are the only mammals known to have a highly developed infrared sense. Like snakes, the bats have an innervated epithelial pit, which is located in a membrane on the bats' noses. In 2011, Julius, Gracheva, and their colleagues identified the key heat-sensitive ion channel in vampire bats as TRPV1. In humans, this channel is normally triggered by temperatures above 43 °C, but in the bats, it is activated at 30 °C, the researchers found.-"More than 30 years ago biologists Peter Hartline, now of New England Biolabs in Ipswich, Massachusetts, and Eric Newman, now at the University of Minnesota, found that information from the snake pit organ activates a brain region called the optic tectum (known in mammals as the superior colliculus), which is known to process visual input. The pit organ appears to act like a pinhole camera for infrared light, producing an IR image, Newman says. However, it's impossible to know whether snakes actually “see” in infrared.-“'Unfortunately we don't have a sensory map [of the brain] in snakes or vampire bats,” Gracheva agrees. “I don't think we have enough data to say [these animals] can superimpose a sensory picture onto the visual picture, though it definitely would make sense.'”-Comment: These organs help hunting warm-blooded prey, and the complexity of the receptors again suggests saltation.