Explain how the adaptations of a desert rodent, such as the kangaroo rat (Dipodomys), allow it to maintain water balance without access to liquid water.

IB DP Biology Higher Level (2023 syllabus) — B4.1 Adaptation to environment · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Metabolic water produced in the rodent’s aerobic respiration of carbohydrates and fats supplies the bulk of its water needs.
The kidneys concentrate urine to a very high osmolarity, minimising water loss in the urine.
Water is reabsorbed in the large intestine and colon, so faeces are very dry.
Behavioural and anatomical adaptations, such as nocturnal activity and a nasal counter‑current heat exchanger, reduce evaporative water loss by condensing water vapour before exhalation.

Examiner tips

  • Use the word ‘metabolic water’ to show understanding of internal water production.
  • Explain how concentrated urine and dry faeces reduce water loss.
  • Mention nocturnal behaviour or nasal counter‑current heat exchange as a key adaptation.
  • Keep the answer concise – 4 marks can be earned with 4 short points.

Common mistakes

  • Confusing metabolic water with water from food; students sometimes say ‘water from food’ instead of ‘metabolic water’.
  • Failing to mention the highly concentrated urine or dry faeces, giving only one adaptation.
  • Using vague terms like ‘good adaptations’ without specifying how they reduce water loss.

Mark scheme (4 marks)

  1. Metabolic (oxidative) water produced during aerobic respiration of carbohydrates/fats provides the rodent's primary water source
  2. The kidneys produce highly concentrated urine, reducing urinary water loss
  3. Dry faeces / minimal water loss in excretion because water is reabsorbed in the large intestine / colon
  4. Behavioural/anatomical adaptation reduces evaporative water loss — e.g. nocturnal activity avoids daytime heat, or nasal countercurrent heat exchange condenses water vapour before exhalation

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