Does a Dog maintain a constant body temperature?

Kinda-sorta, but not really.

Reptiles can't do much to regulate their temperature internally, but will move from sunny spots to shaded spots, or even into water to maintain a body temperature within a tolerable span. Below that, some will hibernate to deal with colder times.

Mammals have some other strategies available. Some, like humans, can sweat to cool off. Others, like dogs, can shed heat through panting. And we can generate heat by shivering or moving about.

Even fish will do a bit of adjusting by swimming between warmer and colder layers.