Thermoception
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Thermoception or thermoreception is the sense by which an organism perceives temperature. In larger animals, most thermoception is done by the skin. The details of how temperature receptors work is still being investigated. Mammals have at least two types of sensor: those that detect heat (i.e. temperatures above body temperature) and those that detect cold (i.e. temperatures below body temperature).
A particularly specialized form of thermoception is used by some snakes, such as the pit vipers, which can effectively see the infrared radiation emitted by hot objects. The snake's face has a pair of holes, or pits, lined with temperature sensors. The sensors indirectly detect infrared radiation by its heating effect on the skin inside the pit. They can work out which part of the pit is hottest, and therefore the direction of the heat source, which could be a warm-blooded prey animal. By combining information from both pits, the snake can also estimate the distance of the object.