Which of the following animals can exist solely on the water they get from food and metabolic reactions?

Analyze the question: The question asks which animals can survive without drinking water, getting all they need from the food they eat and from metabolic reactions.

Concept: Water is gained by an animal in three ways:

1. Drinking
2. Water contained in food
3. Metabolic water, produced during cellular respiration (e.g. oxidation of carbohydrates and fats)

Respiration equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

Animals in habitats where free water is scarce have adaptations that let them live on this water alone. A classic example is the kangaroo rat, which never drinks. It:

• Produces concentrated urine
• Passes very dry faeces
• Stays in burrows by day to reduce water loss through evaporation
• Is active at night when it is cooler

Evaluate the options:

• A. Forest arboreal dwellers: Live in trees where humidity is high and water (rain, dew, moist fruits) is available. They have no special need to conserve water.
• B. Desert dwellers: Water is very scarce, so they have evolved to depend on metabolic water and the moisture in their food. ✔
• C. Forest-ground dwellers: Have plenty of water from streams, puddles and damp soil.
• D. Rainforest dwellers: Water is abundant, so there is no pressure to evolve such adaptations.

Answer: Option B, desert dwellers.

Exam tip: When you see "survive solely on metabolic water", think desert animals such as the kangaroo rat or camel. Their adaptations focus on conserving water, such as concentrated urine, dry faeces, nocturnal habits and burrowing.