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Paleontologist: Sifrhippus, a miniature horselike animal, lived about 56 million years ago. The average weight of Sifrhippus adults declined from 5.4 to 3.9 kilograms during a period of climatic warming that lasted about 175,000 years, then rose as the climate cooled again. The most likely explanation is that smaller mammals can shed heat more easily than larger ones—Sifrhippus must have adapted to the hot climate by shrinking because larger individuals would more often have died from overheating.
Which of the following would, if true, most strongly support the paleontologist's hypothesis?
| Text from Passage | Analysis |
| Sifrhippus, a miniature horselike animal, lived about 56 million years ago. |
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| The average weight of Sifrhippus adults declined from 5.4 to 3.9 kilograms during a period of climatic warming that lasted about 175,000 years, then rose as the climate cooled again. |
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| The most likely explanation is that smaller mammals can shed heat more easily than larger ones—Sifrhippus must have adapted to the hot climate by shrinking because larger individuals would more often have died from overheating. |
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The argument starts with background info about Sifrhippus, then presents the key evidence (weight changes matching climate changes), and finally offers an evolutionary explanation for why this happened.
Sifrhippus adapted to hot climate by evolving smaller body size because smaller animals can shed heat more easily and survive better in warm conditions.
The paleontologist uses the correlation between climate warming and size reduction as evidence, then applies the biological principle that smaller mammals handle heat better to explain why this adaptation would help survival during hot periods.
Strengthen - We need to find information that would make the paleontologist's heat-shedding hypothesis more believable
The argument makes specific claims about weight changes (5.4kg to 3.9kg over 175,000 years), connects these to climate warming/cooling, and proposes heat-shedding as the mechanism. We need to be precise about supporting this heat-based explanation rather than alternative explanations
To strengthen this argument, we want evidence that directly supports the heat-shedding mechanism. We should look for: (1) evidence that heat regulation was indeed the driving factor, (2) evidence that rules out other possible explanations for size changes, or (3) evidence that smaller animals actually do better in hot climates for heat-related reasons. We need to avoid anything that suggests alternative explanations like food scarcity, predation, or other environmental factors
This suggests that habitat shrinkage caused size reduction rather than heat regulation. While this could explain why animals got smaller, it actually provides an alternative explanation that competes with the paleontologist's heat-shedding hypothesis. We want to strengthen the heat-regulation theory, not introduce competing explanations.
This tells us that Sifrhippus changed size multiple times before the warming period without climate changes being involved. This actually weakens the argument by suggesting that size changes in Sifrhippus might have causes other than climate adaptation, making the heat-regulation explanation less compelling.
This describes what happened to other mammal species during the warming period, but it doesn't specifically support the heat-shedding mechanism for Sifrhippus. In fact, if small mammals went extinct while large ones thrived, this contradicts the idea that smaller size was advantageous during warming periods.
Population changes during and after the warming period don't directly support the heat-shedding explanation. While it's interesting that populations increased during warming, this doesn't tell us anything about whether smaller individuals had survival advantages due to better heat regulation.
This is the strongest support because it shows that within the same warming period, Sifrhippus populations in cooler habitats remained larger while those in hotter areas became smaller. This directly demonstrates that local temperature was the driving factor behind size changes, perfectly supporting the heat-regulation hypothesis while ruling out other explanations that would affect all populations equally.