I doubt that a polar bear or a gray seal would enjoy a tropical environment, nor an elephant an Arctic one. None of these species are reptiles.
One thing is that Earth-based animal taxonomy may only be Earth-based. Mammals as we know them on Earth may not have evolved exactly on other worlds. For all we know, Cardassians may have a reptilian physiology, but give birth in an exact manner to mammals. As a tangential point, Klingons appear to be descended distantly to carnivorous crustacean-like animals, and may even have retained crustacean traits despite them having hair and being (in that respect) mammalian.