Those 40 hour heat packs would be great just in case (they can get too hot though, so I wouldnt let the animal come in direct contact with it). Depending on what you keep as long as the house remains above 40F many species will tolerate it fine too. If you have acess to hot water that can be used too. As an ultra last resort, you can bag or contain them insome way and put them under the covers with you.