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 Crotalinae (pit viper) and Boidae (boa) snakes, 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.
The common vampire bat may also have specialized infrared sensors on its nose (see ). A nucleus has been found in the brain of vampire bats that has a similar position and has similar histology to the infrared nucleus of infrared sensitive snakes.
Other animals with specialized heat detectors are forest fire seeking beetles (Melanophilia acuminata), which lay their eggs in conifers freshly killed by forest fires. Darkly pigmented butterflies Pachliopta aristolochiae and Troides rhadamathus use specialized heat detectors to avoid damage while basking. Blood sucking bugs Triatoma infestans may also have a specialised thermoception organs.
Campbell A, Naik RR, Sowards L, Stone MO. (2002) Biological infrared imaging and sensing. Micron 33:211-225. pdf.