Kleptothermy

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. Its most common form is huddling.

Huddling
Some species of ectotherms including lizards and snakes, such as boa constrictors and Tiger snakes, increase their effective mass by clustering tightly together. This allows the individuals to increase their thermal inertia (as with gigantothermy) and so reduce heat loss. 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 at least one case this is not reciprocal, and might be accurately described as heat-stealing. Some male Canadian red sided garter snakes engage in female mimicry by producing 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. This allows those males that mimic females to become more quickly revitalized after hibernation (which depends upon raising their body temperature), giving them an advantage in their own attempts to mate.

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 in their mounds high and regulated temperatures and this is exploited by some species of lizards, snakes and crocodiles.

Research has shown such kleptothermy can be advantageous: the Blue-lipped sea krait, when it occupies a burrow of a pair of Wedge-tailed Shearwater incubating their chick, raises its body temperature to 37.5 C compared to 31.7 C when in other habitats. Its body temperature is also more stable. Burrows without birds did not provide this heat being only 28 C.