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Difficulty: MediumCommand of Evidence: Textual

Dr. Aris Thorne and colleagues investigated the foraging behavior of the desert harvester ant (*Pogonomyrmex salinus*). Because foraging in arid conditions risks desiccation, it has been hypothesized that *P. salinus* workers rely on external environmental cues—specifically, the humidity level near the colony entrance—to decide when to begin foraging, rather than depending solely on internal physiological signals of hydration. To test this hypothesis, the researchers monitored colony foraging rates while experimentally manipulating both the humidity at the nest entrance and the hydration levels of the ants inside the nest. Which finding from the experiment, if true, would most directly support the researchers' hypothesis?

  1. A
    Colonies exposed to drier entrance conditions over several weeks showed a gradual decline in the total number of offspring produced.
  2. Foraging rates rose significantly when humidity at the nest entrance was artificially increased, even for workers with depleted internal hydration levels.Answer
  3. C
    Workers with high internal hydration levels were found to have a greater tolerance for high temperatures than workers with low hydration levels.
  4. D
    When internal hydration levels were high, workers began foraging at high rates regardless of whether the humidity at the nest entrance was high or low.

Answer

Foraging rates rose significantly when humidity at the nest entrance was artificially increased, even for workers with depleted internal hydration levels.
The hypothesis states that desert harvester ants (*P. salinus*) rely on the external cue of humidity at the nest entrance to initiate foraging, rather than depending solely on internal hydration levels. The finding that foraging rates increased when entrance humidity was artificially raised, even among ants with depleted internal hydration, directly supports this hypothesis by showing that the external cue can stimulate foraging behavior independent of the ants' internal physiological state.

Step-by-Step Solution

1
Identify the core hypothesis being tested in the passage.
The hypothesis is that the desert harvester ants rely on an external environmental cue (humidity at the nest entrance) to decide when to forage, rather than depending solely on internal signals of hydration.
Understanding the precise claim is necessary to determine what evidence would support it.
2
Analyze what kind of experimental outcome would isolate and support this external cue over the internal state.
An outcome where changing the external humidity alters foraging behavior regardless of the ants' internal hydration state would show the primacy or independent effect of the external cue.
This isolates the variable in question (nest entrance humidity) and shows it functions independently of the baseline internal signal.
3
Evaluate the choices to find the one matching this experimental outcome.
The finding that foraging increased when entrance humidity was artificially raised, even for dehydrated workers, shows that the external cue drives the behavior even when internal signals would normally suppress it.
This directly matches the prediction and supports the claim that the decision does not depend solely on internal hydration.

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Command of Evidence: Textual
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