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07 Aug 2026, 12:25
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Biologists studying a species of harvester ant observed that workers regularly transport small pebbles from the soil surface into the nest interior. These ants subsist primarily on seeds, but their mandibles are too weak to crack the shells of the hardest seed varieties available in their habitat. Thus, the pebbles may function as a grinding tool, helping workers process seeds that their mandibles cannot crack. Alternatively, the pebbles may serve to reinforce the walls of the nest's tunnels, which are prone to collapse in the loose sandy soil the colony inhabits.
In order to help decide between the two hypotheses for the pebbles' function, which of the following would it be most useful to know?
A. Whether pebbles recovered from the nest interior show greater surface wear than pebbles of similar composition collected from the surrounding habitat surface
B. Whether other harvester ant species that are known to collect pebbles for structural reinforcement also inhabit regions where hard-shelled seeds make up a large proportion of the local food supply
C. Whether the majority of the pebbles are deposited in the brood-rearing chambers rather than in the granaries or along the load-bearing sections of the tunnels
D. Whether the hard-shelled seeds that this species cannot crack contain significantly more nutrients than the soft-shelled seeds their mandibles can process
E. Whether colonies of this species that nest in compact, stable soil - where the risk of tunnel collapse is not a concern - also transport pebbles into the nest interior
In order to help decide between the two hypotheses for the pebbles' function, which of the following would it be most useful to know?
A. Whether pebbles recovered from the nest interior show greater surface wear than pebbles of similar composition collected from the surrounding habitat surface
B. Whether other harvester ant species that are known to collect pebbles for structural reinforcement also inhabit regions where hard-shelled seeds make up a large proportion of the local food supply
C. Whether the majority of the pebbles are deposited in the brood-rearing chambers rather than in the granaries or along the load-bearing sections of the tunnels
D. Whether the hard-shelled seeds that this species cannot crack contain significantly more nutrients than the soft-shelled seeds their mandibles can process
E. Whether colonies of this species that nest in compact, stable soil - where the risk of tunnel collapse is not a concern - also transport pebbles into the nest interior
07 Aug 2026, 12:25
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Official Solution:
Biologists studying a species of harvester ant observed that workers regularly transport small pebbles from the soil surface into the nest interior. These ants subsist primarily on seeds, but their mandibles are too weak to crack the shells of the hardest seed varieties available in their habitat. Thus, the pebbles may function as a grinding tool, helping workers process seeds that their mandibles cannot crack. Alternatively, the pebbles may serve to reinforce the walls of the nest's tunnels, which are prone to collapse in the loose sandy soil the colony inhabits.
In order to help decide between the two hypotheses for the pebbles' function, which of the following would it be most useful to know?
A. Whether pebbles recovered from the nest interior show greater surface wear than pebbles of similar composition collected from the surrounding habitat surface
B. Whether other harvester ant species that are known to collect pebbles for structural reinforcement also inhabit regions where hard-shelled seeds make up a large proportion of the local food supply
C. Whether the majority of the pebbles are deposited in the brood-rearing chambers rather than in the granaries or along the load-bearing sections of the tunnels
D. Whether the hard-shelled seeds that this species cannot crack contain significantly more nutrients than the soft-shelled seeds their mandibles can process
E. Whether colonies of this species that nest in compact, stable soil - where the risk of tunnel collapse is not a concern - also transport pebbles into the nest interior
The argument presents two hypotheses for why harvester ants transport pebbles into the nest. The first is that pebbles serve as a grinding tool for seeds the ants' mandibles cannot crack. The second is that pebbles reinforce tunnel walls that are prone to collapse in loose sandy soil. Both hypotheses are motivated by the same observation - pebbles being carried into the nest despite providing no obvious nutritional value. The correct answer must present a question whose YES answer supports one hypothesis and whose NO answer supports the other.
(A) Incorrect. This is the key trap. If nest pebbles show greater surface wear than pebbles from the surrounding habitat, this seems to support the grinding hypothesis - grinding would cause wear. However, the argument establishes that the nest is built in loose sandy soil and that pebbles are transported repeatedly through the tunnels. Pebbles carried through sandy tunnels accumulate surface wear from friction with soil particles and tunnel walls during transport and routine nest activity, regardless of whether they are used for grinding. Greater wear in nest pebbles is therefore equally consistent with a structural function, and the answer to this question cannot help decide between the two hypotheses.
(B) Incorrect. This tells us whether the environmental conditions that would motivate both hypotheses - structural need and grinding need - coexist in the same habitat for other species. Even if YES, all this establishes is that both functions are plausible under the same conditions in other species. It says nothing about which function this species' pebbles actually serve. Evidence about other species does not determine the function of pebbles in this species.
(C) Incorrect. Location within the nest might seem to reveal function - pebbles in granaries suggesting grinding, pebbles along load-bearing sections suggesting structural use. However, this choice asks specifically about brood-rearing chambers, a location neither hypothesis predicts as the primary site. A YES answer - majority in brood-rearing chambers - does not support either hypothesis. A NO answer tells us the pebbles are not primarily in brood-rearing chambers, but does not reveal whether they are concentrated in granaries or along load-bearing sections, leaving the two hypotheses equally unresolved.
(D) Incorrect. Even if hard-shelled seeds are significantly more nutritious than soft-shelled ones, this only establishes that grinding would be nutritionally valuable - it does not establish that the pebbles are actually used for that purpose. The motivational case for grinding tells us nothing about whether grinding or structural reinforcement is the actual function the pebbles serve.
(E) Correct. If colonies of this species that nest in compact, stable soil - where tunnel collapse is not a concern - also transport pebbles, then the structural need cannot be what drives pebble collection. This undermines the structural reinforcement hypothesis and supports the grinding hypothesis. If colonies in stable soil do not transport pebbles, then pebble collection is tied to the structural need, which undermines the grinding hypothesis and supports structural reinforcement. The answer is informative in both directions and directly tests whether the structural need is the driver of the behavior.
Answer: E
Biologists studying a species of harvester ant observed that workers regularly transport small pebbles from the soil surface into the nest interior. These ants subsist primarily on seeds, but their mandibles are too weak to crack the shells of the hardest seed varieties available in their habitat. Thus, the pebbles may function as a grinding tool, helping workers process seeds that their mandibles cannot crack. Alternatively, the pebbles may serve to reinforce the walls of the nest's tunnels, which are prone to collapse in the loose sandy soil the colony inhabits.
In order to help decide between the two hypotheses for the pebbles' function, which of the following would it be most useful to know?
A. Whether pebbles recovered from the nest interior show greater surface wear than pebbles of similar composition collected from the surrounding habitat surface
B. Whether other harvester ant species that are known to collect pebbles for structural reinforcement also inhabit regions where hard-shelled seeds make up a large proportion of the local food supply
C. Whether the majority of the pebbles are deposited in the brood-rearing chambers rather than in the granaries or along the load-bearing sections of the tunnels
D. Whether the hard-shelled seeds that this species cannot crack contain significantly more nutrients than the soft-shelled seeds their mandibles can process
E. Whether colonies of this species that nest in compact, stable soil - where the risk of tunnel collapse is not a concern - also transport pebbles into the nest interior
The argument presents two hypotheses for why harvester ants transport pebbles into the nest. The first is that pebbles serve as a grinding tool for seeds the ants' mandibles cannot crack. The second is that pebbles reinforce tunnel walls that are prone to collapse in loose sandy soil. Both hypotheses are motivated by the same observation - pebbles being carried into the nest despite providing no obvious nutritional value. The correct answer must present a question whose YES answer supports one hypothesis and whose NO answer supports the other.
(A) Incorrect. This is the key trap. If nest pebbles show greater surface wear than pebbles from the surrounding habitat, this seems to support the grinding hypothesis - grinding would cause wear. However, the argument establishes that the nest is built in loose sandy soil and that pebbles are transported repeatedly through the tunnels. Pebbles carried through sandy tunnels accumulate surface wear from friction with soil particles and tunnel walls during transport and routine nest activity, regardless of whether they are used for grinding. Greater wear in nest pebbles is therefore equally consistent with a structural function, and the answer to this question cannot help decide between the two hypotheses.
(B) Incorrect. This tells us whether the environmental conditions that would motivate both hypotheses - structural need and grinding need - coexist in the same habitat for other species. Even if YES, all this establishes is that both functions are plausible under the same conditions in other species. It says nothing about which function this species' pebbles actually serve. Evidence about other species does not determine the function of pebbles in this species.
(C) Incorrect. Location within the nest might seem to reveal function - pebbles in granaries suggesting grinding, pebbles along load-bearing sections suggesting structural use. However, this choice asks specifically about brood-rearing chambers, a location neither hypothesis predicts as the primary site. A YES answer - majority in brood-rearing chambers - does not support either hypothesis. A NO answer tells us the pebbles are not primarily in brood-rearing chambers, but does not reveal whether they are concentrated in granaries or along load-bearing sections, leaving the two hypotheses equally unresolved.
(D) Incorrect. Even if hard-shelled seeds are significantly more nutritious than soft-shelled ones, this only establishes that grinding would be nutritionally valuable - it does not establish that the pebbles are actually used for that purpose. The motivational case for grinding tells us nothing about whether grinding or structural reinforcement is the actual function the pebbles serve.
(E) Correct. If colonies of this species that nest in compact, stable soil - where tunnel collapse is not a concern - also transport pebbles, then the structural need cannot be what drives pebble collection. This undermines the structural reinforcement hypothesis and supports the grinding hypothesis. If colonies in stable soil do not transport pebbles, then pebble collection is tied to the structural need, which undermines the grinding hypothesis and supports structural reinforcement. The answer is informative in both directions and directly tests whether the structural need is the driver of the behavior.
Answer: E
