Which law states that endothermic animals from cold climates have smaller extremities or appendages than closely related species from warm climates?

Correct — A. Allen's rule, named for the American zoologist Joel Asaph Allen, is exactly the statement in the stem.

Among endotherms, populations from cold climates tend to have shorter ears, limbs, tails and snouts than related forms from warm climates.

The mechanism is heat loss. Short, stubby extremities give a body a smaller surface-area-to-volume ratio, and heat leaves a body across its surface — so the cold climate form conserves heat and the warm climate form sheds it.

Why the others are wrong

• (b)
  Reich's rule — Reich is not attached to any of the ecogeographical rules. The recognised set in this area is Allen's rule, Bergmann's rule and Gloger's rule.
• (c)
  Harper’s rule — Harper likewise names no rule about body form and climate. The stem's content, extremity size against temperature, is Allen's rule.
• (d)
  Moller’s rule — Moller is another plausible-sounding surname in the option set. The rule that matches the stem, extremity size against climate, is Allen's.

Ecogeographical rules describe regular trends in body form along a climate gradient, and two of them are about heat.

Bergmann's rule works on overall size: within a broadly related group, animals in colder regions tend to be larger, because a bigger body has less surface per unit of volume.

Allen's rule works on shape: extremities shrink in the cold and lengthen in the heat.

The standard illustration is the fox — the Arctic fox with its small rounded ears against the desert fennec with enormous ones, which act as radiators.

Key facts

• Allen's rule: endotherms from cold climates have shorter extremities than related forms from warm climates.
• Bergmann's rule: within a related group, body size tends to be larger in colder climates.
• Gloger's rule: animals tend to be more darkly pigmented in warm, humid regions.
• Both Allen's and Bergmann's rules work through the surface-area-to-volume ratio and heat exchange.

Common traps

• Inverting the rule and expecting longer ears in the cold.
• Applying it to cold-blooded animals, when the rule is stated for endotherms.

No directly related past PYQ was found.