Allen's rule

    It is an ecogeographical rule formulated by Joel Asaph Allen in 1877 stating that animals that are adapted to colder climates tend to have shorter limbs and body appendages in comparison to animals that are adapted to more warm climates. It predicts that the body surface area-to-volume ratio of endotherimc animals (mammals and birds) tends to vary with the average temperature in which they occur (lower ratios in colder climates and higher ratios in hotter climates).  
     One of the main explanations for the theory is that endothermic animals in colder climates need to save as much energy as possible in the form of heat. Thus, the decreasing the surface area helps to avoid dissipating heat. Shorter body appendages and limbs also help to avoid heat loss as body extremities are some of the main body parts where animals exchange heat with the environment mainly due to heat exchange with the blood vessels close to the skin. Thus, having shorter ones help to decrease heat loss.
     One of the most classical examples of the Allen's rule is the correlation between ear length and air temperature for hares (genus Lepus) and foxes. However, some studies have showed that this rule might oversimplify too much.  

- From right to left the climate tend to be increasingly colder.

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References
Feldhamer, G. A., Drickamer, L. C., Vessey, S. H., Merritt, J. F., & Krajewski, C. (2007). Mammalogy: adaptation, diversity, ecology. JHU press.