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Difficulty: MediumEvaluating the Impact of New Evidence

### Avian Navigation Mechanisms

Migratory birds are known to use Earth's magnetic field to navigate during long-distance flights. Two hypotheses propose different mechanisms for how birds detect this magnetic field.

Hypothesis 1
Birds detect magnetic fields using iron oxide (magnetite) crystals located in sensory cells in their upper beak. The magnetic torque on these crystals opens ion channels, sending signals to the brain through the ophthalmic branch of the trigeminal nerve. This system functions independently of ambient light, allowing birds to navigate in total darkness.

Hypothesis 2
Birds detect magnetic fields using light-sensitive proteins called cryptochromes (specifically Cry4) located in the retina of their eyes. When cryptochromes absorb blue light, they undergo a chemical reaction that creates a pair of radicals sensitive to the orientation of Earth’s magnetic field. This visual pattern is processed by the brain's visual center (Cluster N). Consequently, this mechanism requires the presence of short-wavelength light (such as blue light) to function.

Researchers conducted an experiment on a species of migratory bird in an orientation chamber. They observed the orientation behavior of two groups of birds under different conditions:

GroupTrigeminal NerveCluster N PathwayLight ConditionOrientation Ability
Group ASeveredIntactBlue lightNormal orientation
Group ASeveredIntactDarknessDisoriented
Group BIntactSeveredBlue lightDisoriented
Group BIntactSeveredDarknessDisoriented

Based on this experimental evidence, which of the following statements best describes the impact of the results on Hypothesis 1 and Hypothesis 2?

  1. A
    The results support Hypothesis 1 because birds with a severed trigeminal nerve oriented normally under blue light, and weaken Hypothesis 2 because birds with a severed visual pathway were disoriented under blue light.
  2. B
    The results weaken Hypothesis 1 because birds were disoriented in darkness, and weaken Hypothesis 2 because birds with a severed trigeminal nerve oriented normally under blue light.
  3. The results weaken Hypothesis 1 because birds with a severed trigeminal nerve oriented normally under blue light, and support Hypothesis 2 because birds with a severed visual pathway were disoriented under blue light.Answer
  4. D
    The results support both Hypothesis 1 and Hypothesis 2 because orientation was normal in Group A under blue light and disoriented in Group B under darkness.

Answer

The results weaken Hypothesis 1 because birds with a severed trigeminal nerve oriented normally under blue light, and support Hypothesis 2 because birds with a severed visual pathway were disoriented under blue light.
The correct answer correctly identifies that the experimental evidence weakens Hypothesis 1 and supports Hypothesis 2. Hypothesis 1 states that the trigeminal nerve is necessary for magnetic navigation; however, Group A oriented normally even when this nerve was severed, disproving its necessity and weakening Hypothesis 1. Hypothesis 2 states that the visual pathway (Cluster N) and ambient light are necessary; Group B was disoriented when the visual pathway was severed, and Group A was disoriented in the dark, which supports Hypothesis 2.

Step-by-Step Solution

1
Evaluate the impact of the Group A results on Hypothesis 1.
Hypothesis 1 claims the beak-to-trigeminal nerve pathway is required for magnetic navigation. Group A birds had their trigeminal nerves severed but still oriented normally under blue light. Since the nerve is not necessary for normal orientation, this result weakens Hypothesis 1.
To test whether the data supports or contradicts the structural pathway proposed in Hypothesis 1.
2
Evaluate the impact of the Group B results on Hypothesis 2.
Hypothesis 2 claims that the visual pathway to Cluster N is required for orientation. Group B birds had their Cluster N pathways severed and were disoriented under blue light, showing that this visual pathway is indeed necessary. This result supports Hypothesis 2.
To test whether the data supports or contradicts the visual pathway proposed in Hypothesis 2.
3
Evaluate the light conditions against both hypotheses.
Hypothesis 1 states navigation is independent of light, but Group A was disoriented in darkness. Hypothesis 2 states navigation requires blue light, and indeed orientation only occurred under blue light. This confirms that the evidence weakens Hypothesis 1 and supports Hypothesis 2.
To ensure that all independent variables in the experiment align consistently with the chosen option.

Key Concept

Evaluating the Impact of New Evidence
Estimated Time:1m 30s
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Topics

Evaluating the Impact of New Evidence

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