avoid them. for example, humans avoid touching a sharp needle, or hot object, or extending an arm beyond a safe limit because it is dangerous, and thus hurts. without pain, people could do many dangerous things without being aware of the dangers. other internal sensations and perceptions an internal sensation and perception also known as interoception is "any sense that is normally stimulated from within the body". these involve numerous sensory receptors in internal organs. interoception is thought to be atypical in clinical conditions such as alexithymia. specific receptors include: hunger is governed by a set of brain structures (e.g., the hypothalamus) that are responsible for energy homeostasis. pulmonary stretch receptors are found in the lungs and control the respiratory rate. peripheral chemoreceptors in the brain monitor the carbon dioxide and oxygen levels in the brain to give a perception of suffocation if carbon dioxide levels get too high. the chemoreceptor trigger zone is an area of the medulla in the brain that receives inputs from blood-borne drugs or hormones, and communicates with the vomiting center. chemoreceptors in the circulatory system also measure salt levels and prompt thirst if they get too high; they can also respond to high blood sugar levels in diabetics. cutaneous receptors in the skin not only respond to touch, pressure, temperature and vibration, but also respond to vasodilation in the skin such as blushing. stretch receptors in the gastrointestinal tract sense gas distension that may result in colic pain. stimulation of sensory receptors in the esophagus result in sensations felt in the throat when swallowing, vomiting, or during acid reflux. sensory receptors in pharynx mucosa, similar to touch receptors in the skin, sense foreign objects such as mucous and food that may result in a gag reflex and corresponding gagging sensation. stimulation of sensory receptors in the urinary bladder and rectum may result in perceptions of fullness. stimulation of stretch sensors that sense dilation of various blood vessels may result in pain, for example headache caused by vasodilation of brain arteries. cardioception refers to the perception of the activity of the heart. opsins and direct dna damage in melanocytes and keratinocytes can sense ultraviolet radiation, which plays a role in pigmentation and sunburn. baroreceptors relay blood pressure information to the brain and maintain proper homeostatic blood pressure. the perception of time is also sometimes called a sense, though not tied to a specific receptor. nonhuman animal sensation and perception human analogues other living organisms have receptors to sense the world around them, including many of the senses listed above for humans. however, the mechanisms and capabilities vary widely. smell an example of smell in non-mammals is that of sharks, which combine their keen sense of smell with timing to determine the direction of a smell. they follow the nostril that first detected the smell. insects have olfactory receptors on their antennae. although it is unknown to the degree and magnitude which non-human mammals can smell better than humans, humans are known to have far fewer olfactory receptors than mice, and humans have also accumulated more genetic mutations in their olfactory receptors than other primates. vomeronasal organ many animals (salamanders, reptiles, mammals) have a vomeronasal organ that is connected with the mouth cavity. in mammals it is mainly used to detect pheromones of marked territory, trails, and sexual state. reptiles like snakes and monitor lizards make extensive use of it as a smelling organ by transferring scent molecules to the vomeronasal organ with the tips of the forked tongue. in reptiles the vomeronasal organ is commonly referred to as jacobson's organ. in mammals, it is often associated with a special behavior called flehmen characterized by uplifting of the lips. the organ is vestigial in humans, because associated neurons have not been found that give any sensory input in humans. taste flies and butterflies have taste organs on their feet, allowing them to taste anything they land on. catfish have taste organs across their entire bodies, and can taste anything they touch, including chemicals in the water. vision cats have the ability to see in low light, which is due to muscles surrounding their irides–which contract and expand their pupils–as well as to the tapetum lucidum, a reflective membrane that optimizes the image. pit vipers, pythons and some boas have organs that allow them to detect infrared light, such that these snakes are able to sense the body heat of their prey. the common vampire bat may also have an infrared sensor on its nose. it has been found that birds and some other animals are tetrachromats and have the ability to see in the ultraviolet down to 300 nanometers. bees and dragonflies are also able to see in the ultraviolet. mantis shrimps can perceive both polarized light and multispectral images and have twelve distinct kinds of color receptors, unlike humans which have three kinds and most mammals which have two kinds. cephalopods have the ability to change color using chromatophores in their skin. researchers believe that opsins in the skin can sense different wavelengths of light and help the creatures choose a coloration that camouflages them, in addition to light input from the eyes. other researchers hypothesize that cephalopod eyes in species which only have a single photoreceptor protein may use chromatic aberration to turn monochromatic vision into color vision, explaining pupils shaped like the letter u, the letter w, or a dumbbell, as well as explaining the need for colorful mating displays. some cephalopods can distinguish the polarization of light. spatial orientation many invertebrates have a statocyst, which is a sensor for acceleration and orientation that works very differently from the mammalian's semi-circular canals. not human analogues in addition, some animals have senses that humans do not, including the following: magnetoception magnetoception (or magnetoreception) is the ability to detect the direction one is facing based on the earth's magnetic field. directional awareness is most commonly observed in birds, which rely on their magnetic sense to navigate during migration. it has also been observed in insects such as bees. cattle make use of magnetoception to align themselves in a north–south direction. magnetotactic bacteria build miniature magnets inside themselves and use them to determine their orientation relative to the earth's magnetic field. there has been some recent (tentative) research suggesting that the rhodopsin in the human eye, which responds particularly well to blue light, can facilitate magnetoception in humans. echolocation certain animals, including bats and cetaceans, have the ability to determine orientation to other objects through interpretation of reflected sound (like sonar). they most often use this to navigate through poor lighting conditions or to identify and track prey. there is currently an uncertainty whether this is simply an extremely developed post-sensory interpretation of auditory perceptions or it actually constitutes a separate sense. resolution of the issue will require brain scans of animals while they actually perform echolocation, a task that has proven difficult in practice. blind people report they are able to navigate and in some cases identify an object by interpreting reflected sounds (especially their own footsteps), a phenomenon known as human echolocation. electroreception electroreception (or electroception) is the ability to detect electric fields. several species of fish, sharks, and rays have the capacity to sense changes in electric fields in their immediate vicinity. for cartilaginous fish this occurs through a specialized organ called the ampullae of lorenzini. some fish passively sense changing nearby electric fields; some generate their own weak electric fields, and sense the pattern of field potentials over their body surface; and some use these electric field generating and sensing capacities for social communication. the mechanisms by which electroceptive fish construct a spatial representation from very small differences in field potentials involve comparisons of spike latencies from different parts of the fish's body. the only orders of mammals that are known to demonstrate electroception are the dolphin and monotreme orders. among these mammals, the platypus has the most acute sense of electroception. a dolphin can detect electric fields in water using electroreceptors in vibrissal crypts arrayed in pairs on its snout and which evolved from whisker motion sensors. these electroreceptors can detect electric fields as weak as 4.6 microvolts per centimeter, such as those generated by contracting muscles and pumping gills of potential prey. this permits the dolphin to locate prey from the seafloor where sediment limits visibility and echolocation. spiders have been shown to detect electric fields to determine a suitable time to extend web for 'ballooning'. body modification enthusiasts have experimented with magnetic implants to attempt to replicate this sense. however, in general humans (and it is presumed other mammals) can detect electric fields only indirectly by detecting the effect they have on hairs. an electrically charged balloon, for instance, will exert a force on human arm hairs, which can be felt through tactition and identified as coming from a static charge (and not from wind or the like). this is not electroreception, as it is a post-sensory cognitive action. hygroreception hygroreception is the ability to detect changes in the moisture content of the environment. infrared sensing the ability to sense infrared thermal radiation evolved independently in various families of snakes. essentially, it allows these reptiles to "see" radiant heat at wavelengths between 5 and 30 μm to a degree of accuracy such that a blind rattlesnake can target vulnerable body parts of the prey at which it strikes. it was previously thought that the organs evolved primarily as prey detectors, but it is now believed that it may also be used in thermoregulatory decision making. the facial pit underwent parallel evolution in pitvipers and some boas and pythons, having evolved once in pitvipers and multiple times in boas and pythons. the electrophysiology of the structure is similar between the two lineages, but they differ in gross structural anatomy. most superficially, pitvipers possess one large pit organ on either side of the head, between the eye and the nostril (loreal pit), while boas and pythons have three or more comparatively smaller pits lining the upper and sometimes the lower lip, in or between the scales. those of the pitvipers are the more advanced, having a suspended sensory membrane as opposed to a simple pit structure. within the family viperidae, the pit organ is seen only in the subfamily crotalinae: the pitvipers. the organ is used extensively to detect and target endothermic prey such as rodents and birds, and it was previously assumed that the organ evolved specifically for that purpose. however, recent evidence shows that the pit organ may also be used for thermoregulation. according to krochmal et al., pitvipers can use their pits for thermoregulatory decision-making while true vipers (vipers who do not contain heat-sensing pits) cannot. in spite of its detection of ir light, the pits' ir detection mechanism is not similar to photoreceptors – while photoreceptors detect light via photochemical reactions, the protein in the pits of snakes is in fact a temperature-sensitive ion channel. it senses infrared signals through a mechanism involving warming of the pit organ, rather than a chemical reaction to light. this is consistent with the thin pit membrane, which allows incoming ir radiation to quickly and precisely warm a given ion channel and trigger a nerve impulse, as well as vascularize the pit membrane in order to rapidly cool the ion channel back to its original "resting" or "inactive" temperature. other pressure detection uses the organ of weber, a system consisting of three appendages of vertebrae transferring changes in shape of the gas bladder to the middle ear. it can be used to regulate the buoyancy of the fish. fish like the weather fish and other loaches are also known to respond to low pressure areas but they lack a swim bladder. current detection is a detection system of water currents, consisting mostly of vortices, found in the lateral line of fish and aquatic forms of amphibians. the lateral line is also sensitive to low-frequency vibrations. the mechanoreceptors are hair cells, the same mechanoreceptors for vestibular sense and hearing. it is used primarily for navigation, hunting, and schooling. the receptors of the electrical sense are modified hair cells of the lateral line system. polarized light direction/detection is used by bees to orient themselves, especially on cloudy days. cuttlefish, some beetles, and mantis shrimp can also perceive the polarization of light.