Black-and-white tegu lizards inhabit the plains east of the Andes Mountains. During the autumn and winter, tegus hibernate in their burrows.
Come spring they breed. A female lays a clutch of eggs in a nest insulated with moist grass, twigs, and other cozy litter. The mother remains with her eggs in the nest, keeping them warm.
During incubation, the tegu lizard endothermically raises her body temperature 10 °C above ambient. She does not eat while tending her nest, so metabolism does not generate the warmth.
The tegu is an exception to squamates being entirely ectothermic. Diamond python females also construct insulated nests which they keep warm. How these reptiles generate retentive body heat is not known.
Many bird and mammal females have upgraded thermogenesis during reproduction. These instances of improved endothermy for enhancing offspring survival independently evolved.