I'll keep this one brief. I have an alien from a planet which has extremely cold nights and extremely hot days (the day and night cycle may be short or long, I haven't decided).
When the alien gets too cold, it becomes comatose, and its heart beat slows right down, like a strangled alligator. When the sun comes up, it gets heated up, and it returns to normal. In the day, its body focuses on trying to prevent overheating, and the creature spends a lot of its time going between caves to not waste energy in the heat. These creatures are pretty large, by the way, they weigh about four hundred pounds (at 1G gravity), and are terribly strong.
The question is, what organs and biology are best for this temperature control? Giant ears won't work so well with the dust storms, and they can't sweat because water is too precious and scarce, so I wondered if they'd have five hearts or something.
Perhaps more details are necessary?