In biology, kleptothermy is any form of thermoregulation by which an animal shares in the metabolic thermogenesis of another animal. It may or may not be reciprocal, and occurs in both endotherms and ectotherms. One of its forms is huddling. However, kleptothermy can happen between different species that share the same habitat, and can also happen in pre-hatching life where embryos are able to detect thermal changes in the environment. This process requires two major conditions: the thermal heterogeneity created by the presence of a warm organism in a cool environment in addition to the use of that heterogeneity by another animal to maintain body temperatures at higher (and more stable) levels than would be possible elsewhere in the local area. The purpose of this behaviour is to enable these groups to increase its thermal inertia, retard heat loss and/or reduce the per capita metabolic expenditure needed to maintain stable body temperatures. Kleptothermy is seen in cases where ectotherms regulate their own temperatures and exploit the high and constant body temperatures exhibited by endothermic species. In this case, the endotherms involved are not only mammals and birds; they can be termites that maintain high and constant temperatures within their mounds where they provide thermal regimes that are exploited by a wide array of lizards, snakes and crocodilians. However, many cases of kleptothermy involve ectotherms sheltering inside the burrows used by endotherms to help maintain a high constant body temperature.
Huddling
Huddling confers higher and more constant body temperatures than solitary resting. Some species of ectotherms including lizards and snakes, such as boa constrictors and tiger snakes, increase their effective mass by clustering tightly together. It is also widespread amongst gregarious endotherms such as bats and birds (such as the mousebird and emperor penguin) where it allows the sharing of body heat, particularly among juveniles. In white-backed mousebirds (Colius colius), individuals maintain rest-phase body temperature above 32 °C despite air temperatures as low as -3.4 °C. This rest-phase body temperature was synchronized among individuals that cluster. Sometimes, kleptothermy is not reciprocal and might be accurately described as heat-stealing. For example, some male Canadian red sided garter snakes engage in female mimicry in which they produce fake pheromones after emerging from hibernation. This causes rival males to cover them in a mistaken attempt to mate, and so transfer heat to them. In turn, those males that mimic females become rapidly revitalized after hibernation (which depends upon raising their body temperature), giving them an advantage in their own attempts to mate.
On the other hand, huddling allows emperor penguins (Aptenodytes forsteri) to save energy, maintain a high body temperature and sustain their breeding fast during the Antarctic winter. This huddling behaviour raises the ambient temperature that these penguins are exposed to above 0 °C (at average external temperatures of -17 °C). As a consequence of tight huddles, ambient temperatures can be above 20 °C and can increase up to 37.5 °C, close to birds' body temperature. Therefore, this complex social behaviour is what enables all breeders to get an equal and normal access to an environment which allows them to save energy and successfully incubate their eggs during the Antarctic winter.
Habitat sharing Many ectotherms exploit the heat produced by endotherms by sharing their nests and burrows. For example, mammal burrows are used by geckos and seabird burrows by Australian tiger snakes and New Zealand tuatara. Termites create high and regulated temperatures in their mounds, and this is exploited by some species of lizards, snakes and crocodiles. Research has shown such kleptothermy can be advantageous in cases such as the blue-lipped sea krait (Laticauda laticaudata), where these reptiles occupy a burrow of a pair of wedge-tailed shearwater incubating their chick. This in turn, raises its body temperature to 37.5 °C (99.5 °F), compared to 31.7 °C (89.1 °F) when present in other habitats. Its body temperature is also observed to be more stable. On the other hand, burrows without birds did not provide this heat, being only 28 °C (82 °F). Another example would be the case of the fairy prion (Pachyptila turtur) that forms a close association with a medium-sized reptile, the tuatara (Sphenodon punctatus). These reptiles share the burrows made by the birds, and often stay when the birds are present which helps maintain a higher body temperature. Research has shown that fairy prions enable tuatara to maintain a higher body temperature through the night for several months of the year, October to January (austral spring to summer). During the night, tuatara sharing a burrow with a bird had the most thermal benefits and helped maintain their body temperature up to 15 hours the next day.
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