What do we mean by Allen’s rule?

Correct — D. Allen's rule states that mammals from colder climates generally have shorter ears and shorter limbs, which reduces the body surface available for heat loss.

The option text stops mid-sentence at the word minimise, but the rule it names is complete enough to identify: smaller extremities where the climate is cold.

Hold a polar bear's small ears against a desert fox's large ones and the rule reads straight off the animals → option (d).

Why the others are wrong

• (a)
  The body compensates low oxygen availability by increasing red blood cell — That describes acclimatisation to high altitude — the body making more red blood cells where oxygen is thin. It is a physiological adjustment, not a rule about the shape of extremities.
• (b)
  Some plants have no leaves – they are reduced to spines – and the photosynthetic — That is the desert plant adaptation, in which leaves are reduced to spines and a flattened green stem takes over photosynthesis. Allen's rule is about mammals.
• (c)
  Desert lizards lack the physiological ability that mammals have to deal with the high — Desert lizards handle heat behaviourally — basking, burrowing, moving into shade — because they cannot regulate temperature the way mammals do. That is behavioural thermoregulation.

Two named rules tie body shape to climate, and both turn on the same physics.

Allen's rule is about extremities: ears, limbs and tails run shorter in colder climates, cutting the surface across which heat escapes.

Bergmann's rule is about body size: individuals tend to be larger in colder regions, because a bigger body has less surface area for each unit of volume.

The common thread is the surface-area-to-volume ratio, which decides how fast a warm body sheds heat into a cold environment.

Key facts

• Allen's rule: mammals from colder climates have shorter ears and limbs, which minimises heat loss.
• Bergmann's rule: body size tends to be larger in colder climates.
• Both rules operate through the surface-area-to-volume ratio of the body.
• Allen's rule applies to endothermic animals, which hold their body temperature by internal regulation.

Common traps

• Swapping Allen's rule with Bergmann's — extremities against overall body size.
• Rejecting the keyed option because its sentence is cut short — the wording is truncated, the rule is not.

No directly related past PYQ was found.