Argentine Tegu

Not fully, but it shows a genuine, striking exception to the usual reptilian pattern. Research monitoring wild tegus found that during the roughly two months of the spring reproductive season, individuals raised their body temperature substantially, by as much as around 10 degrees Celsius above the temperature of their burrow and the surrounding air, particularly overnight, through heat generated internally by their own metabolism rather than absorbed from the environment. Outside the breeding season the tegu reverts to typical cold-blooded reptile physiology, so it is best described as showing a seasonal, partial, facultative form of endothermy, and its discovery gave scientists a possible living model for how full warm-bloodedness might have first evolved in the ancestors of mammals and birds.

It is widely regarded as one of the most cognitively capable lizards, showing behaviours uncommon in reptiles such as apparent problem-solving, the ability to learn and remember routes and routines, recognition of individual familiar people, and responsiveness to gentle handling and training. Keepers frequently describe well-socialised captive tegus as curious, interactive, and even affectionate in a way more often associated with mammals, seeking out human contact and learning to respond to their names, though these descriptions are anecdotal and the underlying cognition is still being formally studied.

Argentine tegus kept as pets have repeatedly escaped or been deliberately released in Florida, and because the state's warm, humid climate closely suits the species, they have established self-sustaining and expanding wild populations, particularly in the south and centre of the state. As large, adaptable omnivores with a strong appetite for eggs, they raid the nests of native ground-nesting birds, gopher tortoises, sea turtles, and American alligators, and they also eat native small animals and fruit, making them a significant threat to local wildlife. Wildlife agencies now run trapping and removal programmes and encourage the public to report sightings.

In the cooler months of its native range, and in colder parts of its introduced range, the tegu retreats into a deep burrow that it digs itself and enters a period of dormancy called brumation, the reptile equivalent of hibernation, during which its metabolism slows dramatically and it remains inactive underground for weeks or months without feeding. It emerges in spring as temperatures rise, and it is shortly after this emergence that the breeding season, and the tegu's remarkable seasonal spike in self-generated body heat, begins.