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Difficulty: MediumSuggesting Experiments to Resolve Viewpoints

### Water Absorption in Horned Lizards

Texas horned lizards (*Phrynosoma cornutum*) live in arid environments and are known for their ability to collect water from moist sand using their body surfaces. Two scientists discuss the mechanism by which this water enters the lizard's body.

Scientist 1
Horned lizards collect water through their skin via capillary action. The lizard's scales form a network of microscopic, hinge-like channels. When the lizard stands on damp sand or is rained on, capillary forces draw water along these channels toward the corners of the lizard's mouth. The lizard then actively gulps and swallows this water, importing it into the digestive tract for absorption. The skin itself is entirely impermeable to water, serving only as a physical transport network.

Scientist 2
Horned lizards absorb water directly through their skin cells (transdermal absorption) into their bloodstream, bypassing the mouth and digestive tract. The scale channels merely spread water evenly across the body to maximize the surface area available for absorption. The skin possesses specialized, moisture-sensitive micro-pores that open upon contact with liquid water, allowing direct diffusion into the subcutaneous capillaries. The lizard does not need to swallow to hydrate.

Which of the following experiments would best resolve the conflict between the two scientists' viewpoints?

  1. A
    Place lizards on dry sand under high-humidity conditions with their mouths unsealed, and measure the rate of water loss through evaporation.
  2. B
    Feed lizards dyed water directly into their mouths, and check for the presence of the dye in their skin cells.
  3. Place lizards on damp sand with their mouths temporarily sealed shut, and measure the change in their blood hydration levels.Answer
  4. D
    Measure the capillary flow rate of water through isolated, shed lizard skin in a vacuum chamber.

Answer

Placing lizards on damp sand with their mouths temporarily sealed shut, and measuring the change in their blood hydration levels.
The correct option is the experiment where lizards are placed on damp sand with their mouths temporarily sealed shut while measuring hydration levels. This isolates the variable of ingestion. Under Scientist 1's hypothesis, the lizard must swallow water to hydrate, so sealing the mouth would prevent hydration. Under Scientist 2's hypothesis, water is absorbed directly through the skin, so the lizard would still hydrate even with a sealed mouth. This difference in predicted outcomes allows the experiment to resolve the conflict.

Step-by-Step Solution

1
Identify the core point of disagreement between the two scientists' models.
Scientist 1 argues that water drawn by capillary channels must be swallowed at the mouth to be absorbed, while Scientist 2 argues that water is absorbed directly through skin cells without ingestion.
Resolving a conflict requires finding an experimental intervention that tests the specific point of difference between the models.
2
Identify an experimental control that isolates the disputed path of entry.
Temporarily sealing the lizard's mouth shut prevents swallowing (Scientist 1's proposed pathway) while leaving the skin surface exposed to moisture (Scientist 2's proposed pathway).
By blocking one pathway, we can observe if the target outcome (hydration) is still achieved.
3
Predict the outcomes for both viewpoints to confirm the experiment is decisive.
Under Scientist 1's view, a lizard with a sealed mouth cannot hydrate (no change in blood hydration). Under Scientist 2's view, it will still hydrate (increase in blood hydration). The outcomes are mutually exclusive and resolve the conflict.
An experiment only resolves a viewpoint if it produces different, identifiable results depending on which hypothesis is correct.

Key Concept

Suggesting an experiment to resolve conflicting viewpoints by isolating the disputed independent variable.
Estimated Time:1m 30s
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Suggesting Experiments to Resolve Viewpoints

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