Final answer:
Allen's rule states that bodies with increased mass to surface area are optimal for cold climates in order to reduce heat loss.
Step-by-step explanation:
The rule of Allen's rule, which is an ecogeographic rule, states that animals living in colder climates generally have shorter extremities than closely related animals living in hotter climates. This is due to the fact that bodies with increased mass to surface area are optimal for cold climates in order to reduce heat loss. Having a smaller surface area relative to their mass helps organisms in cold climates retain heat more effectively.