Temperature is a survival mechanic. Each biome has its own temperature, and this temperature may vary. For a creature to be "comfortable" and not negative effects, they must have equal or higher temperature resistance to the current temperature.
There are 2 types of temperature, and therefore 2 types of resistances: "Heat" and "Cold".
Cold resistance is given by Big Body (+2), Medium Body (+1) and Medium Tail (+1). You can also gain temporary cold resistance by standing in a hot spring (+3), or have 2 adjacent creatures (+2). Cold resistance is taken away from by Toxic Body (-1), Lean Body (-1) and Big Ears (-1).
Heat resistance is given by Lean Body (+2), Medium Body (+1) and Big Ears (+1). Heat resistance can be taken away from by Water Body (-1) and Big Body (-1). There is no way currently to gain temporary heat resistance.
If a creature is too hot or too cold, they will suffer -1 action point. Under certain circumstances, a cold creature may freeze, making them take damage, and they are unable to move. This happens when a creature is too far away from (not adjacent to) the pack for 1-2 days. However, pack-mates can manually warm up frozen creatures by means of licking them, which will remove the status.
Heat Waves and Cold Waves
Randomly, at the beginning of each turn, a Heat or Cold wave may blow in; this will for that turn increase or decrease the temperature by 1.