PrepTest 46, Section 4, Question 16
The pronghorn, an antelope-like mammal that lives on the western plains of North America, is the continent's fastest land animal, capable of running 90 kilometers per hour and of doing so for several kilometers. Because no North American predator is nearly fast enough to chase it down, biologists have had difficulty explaining why the pronghorn developed its running prowess. One biologist, however, has recently claimed that pronghorns run as fast as they do because of adaptation to predators known from fossil records to have been extinct for 10,000 years, such as American cheetahs and long-legged hyenas, either of which, it is believed, were fast enough to run down the pronghorn.
Like all explanations that posit what is called a relict behavior—a behavior that persists though its only evolutionary impetus comes from long-extinct environmental conditions—this one is likely to meet with skepticism. Most biologists distrust explanations positing relict behaviors, in part because testing these hypotheses is so difficult due to the extinction of a principal component. They typically consider such historical explanations only when a lack of alternatives forces them to do so. But present-day observations sometimes yield evidence that supports relict behavior hypotheses.
In the case of the pronghorn, researchers have identified much supporting evidence, as several aspects of pronghorn behavior appear to have been shaped by enemies that no longer exist. For example, pronghorns—like many other grazing animals—roam in herds, which allows more eyes to watch for predators and diminishes the chances of any particular animal being attacked but can also result in overcrowding and increased competition for food. But, since pronghorns have nothing to fear from present-day carnivores and thus have nothing to gain from herding, their herding behavior appears to be another adaptation to extinct threats. Similarly, if speed and endurance were once essential to survival, researchers would expect pronghorns to choose mates based on these athletic abilities, which they do—with female pronghorns, for example, choosing the victor after male pronghorns challenge each other in sprints and chases.
Relict behaviors appear to occur in other animals as well, increasing the general plausibility of such a theory. For example, one study reports relict behavior in stickleback fish belonging to populations that have long been free of a dangerous predator, the sculpin. In the study, when presented with sculpin, these stickleback fish immediately engaged in stereotypical antisculpin behavior, avoiding its mouth and swimming behind to bite it. Another study found that ground squirrels from populations that have been free from snakes for 70,000 to 300,000 years still clearly recognize rattlesnakes, displaying stereotypical antirattlesnake behavior in the presence of the snake. Such fear, however, apparently does not persist interminably. Arctic ground squirrels, free of snakes for about 3 million years, appear to be unable to recognize the threat of a rattlesnake, exhibiting only disorganized caution even after being bitten repeatedly.
The pronghorn, an antelope-like mammal that lives on the western plains of North America, is the continent's fastest land animal, capable of running 90 kilometers per hour and of doing so for several kilometers. Because no North American predator is nearly fast enough to chase it down, biologists have had difficulty explaining why the pronghorn developed its running prowess. One biologist, however, has recently claimed that pronghorns run as fast as they do because of adaptation to predators known from fossil records to have been extinct for 10,000 years, such as American cheetahs and long-legged hyenas, either of which, it is believed, were fast enough to run down the pronghorn.
Like all explanations that posit what is called a relict behavior—a behavior that persists though its only evolutionary impetus comes from long-extinct environmental conditions—this one is likely to meet with skepticism. Most biologists distrust explanations positing relict behaviors, in part because testing these hypotheses is so difficult due to the extinction of a principal component. They typically consider such historical explanations only when a lack of alternatives forces them to do so. But present-day observations sometimes yield evidence that supports relict behavior hypotheses.
In the case of the pronghorn, researchers have identified much supporting evidence, as several aspects of pronghorn behavior appear to have been shaped by enemies that no longer exist. For example, pronghorns—like many other grazing animals—roam in herds, which allows more eyes to watch for predators and diminishes the chances of any particular animal being attacked but can also result in overcrowding and increased competition for food. But, since pronghorns have nothing to fear from present-day carnivores and thus have nothing to gain from herding, their herding behavior appears to be another adaptation to extinct threats. Similarly, if speed and endurance were once essential to survival, researchers would expect pronghorns to choose mates based on these athletic abilities, which they do—with female pronghorns, for example, choosing the victor after male pronghorns challenge each other in sprints and chases.
Relict behaviors appear to occur in other animals as well, increasing the general plausibility of such a theory. For example, one study reports relict behavior in stickleback fish belonging to populations that have long been free of a dangerous predator, the sculpin. In the study, when presented with sculpin, these stickleback fish immediately engaged in stereotypical antisculpin behavior, avoiding its mouth and swimming behind to bite it. Another study found that ground squirrels from populations that have been free from snakes for 70,000 to 300,000 years still clearly recognize rattlesnakes, displaying stereotypical antirattlesnake behavior in the presence of the snake. Such fear, however, apparently does not persist interminably. Arctic ground squirrels, free of snakes for about 3 million years, appear to be unable to recognize the threat of a rattlesnake, exhibiting only disorganized caution even after being bitten repeatedly.
The pronghorn, an antelope-like mammal that lives on the western plains of North America, is the continent's fastest land animal, capable of running 90 kilometers per hour and of doing so for several kilometers. Because no North American predator is nearly fast enough to chase it down, biologists have had difficulty explaining why the pronghorn developed its running prowess. One biologist, however, has recently claimed that pronghorns run as fast as they do because of adaptation to predators known from fossil records to have been extinct for 10,000 years, such as American cheetahs and long-legged hyenas, either of which, it is believed, were fast enough to run down the pronghorn.
Like all explanations that posit what is called a relict behavior—a behavior that persists though its only evolutionary impetus comes from long-extinct environmental conditions—this one is likely to meet with skepticism. Most biologists distrust explanations positing relict behaviors, in part because testing these hypotheses is so difficult due to the extinction of a principal component. They typically consider such historical explanations only when a lack of alternatives forces them to do so. But present-day observations sometimes yield evidence that supports relict behavior hypotheses.
In the case of the pronghorn, researchers have identified much supporting evidence, as several aspects of pronghorn behavior appear to have been shaped by enemies that no longer exist. For example, pronghorns—like many other grazing animals—roam in herds, which allows more eyes to watch for predators and diminishes the chances of any particular animal being attacked but can also result in overcrowding and increased competition for food. But, since pronghorns have nothing to fear from present-day carnivores and thus have nothing to gain from herding, their herding behavior appears to be another adaptation to extinct threats. Similarly, if speed and endurance were once essential to survival, researchers would expect pronghorns to choose mates based on these athletic abilities, which they do—with female pronghorns, for example, choosing the victor after male pronghorns challenge each other in sprints and chases.
Relict behaviors appear to occur in other animals as well, increasing the general plausibility of such a theory. For example, one study reports relict behavior in stickleback fish belonging to populations that have long been free of a dangerous predator, the sculpin. In the study, when presented with sculpin, these stickleback fish immediately engaged in stereotypical antisculpin behavior, avoiding its mouth and swimming behind to bite it. Another study found that ground squirrels from populations that have been free from snakes for 70,000 to 300,000 years still clearly recognize rattlesnakes, displaying stereotypical antirattlesnake behavior in the presence of the snake. Such fear, however, apparently does not persist interminably. Arctic ground squirrels, free of snakes for about 3 million years, appear to be unable to recognize the threat of a rattlesnake, exhibiting only disorganized caution even after being bitten repeatedly.
The pronghorn, an antelope-like mammal that lives on the western plains of North America, is the continent's fastest land animal, capable of running 90 kilometers per hour and of doing so for several kilometers. Because no North American predator is nearly fast enough to chase it down, biologists have had difficulty explaining why the pronghorn developed its running prowess. One biologist, however, has recently claimed that pronghorns run as fast as they do because of adaptation to predators known from fossil records to have been extinct for 10,000 years, such as American cheetahs and long-legged hyenas, either of which, it is believed, were fast enough to run down the pronghorn.
Like all explanations that posit what is called a relict behavior—a behavior that persists though its only evolutionary impetus comes from long-extinct environmental conditions—this one is likely to meet with skepticism. Most biologists distrust explanations positing relict behaviors, in part because testing these hypotheses is so difficult due to the extinction of a principal component. They typically consider such historical explanations only when a lack of alternatives forces them to do so. But present-day observations sometimes yield evidence that supports relict behavior hypotheses.
In the case of the pronghorn, researchers have identified much supporting evidence, as several aspects of pronghorn behavior appear to have been shaped by enemies that no longer exist. For example, pronghorns—like many other grazing animals—roam in herds, which allows more eyes to watch for predators and diminishes the chances of any particular animal being attacked but can also result in overcrowding and increased competition for food. But, since pronghorns have nothing to fear from present-day carnivores and thus have nothing to gain from herding, their herding behavior appears to be another adaptation to extinct threats. Similarly, if speed and endurance were once essential to survival, researchers would expect pronghorns to choose mates based on these athletic abilities, which they do—with female pronghorns, for example, choosing the victor after male pronghorns challenge each other in sprints and chases.
Relict behaviors appear to occur in other animals as well, increasing the general plausibility of such a theory. For example, one study reports relict behavior in stickleback fish belonging to populations that have long been free of a dangerous predator, the sculpin. In the study, when presented with sculpin, these stickleback fish immediately engaged in stereotypical antisculpin behavior, avoiding its mouth and swimming behind to bite it. Another study found that ground squirrels from populations that have been free from snakes for 70,000 to 300,000 years still clearly recognize rattlesnakes, displaying stereotypical antirattlesnake behavior in the presence of the snake. Such fear, however, apparently does not persist interminably. Arctic ground squirrels, free of snakes for about 3 million years, appear to be unable to recognize the threat of a rattlesnake, exhibiting only disorganized caution even after being bitten repeatedly.
Which one of the following most accurately states the main point of the passage?
Evidence from present-day animal behaviors, together with the fossil record, supports the hypothesis that the pronghorn's ability to far outrun any predator currently on the North American continent is an adaptation to predators long extinct.
Although some biologists believe that certain animal characteristics, such as the speed of the pronghorn, are explained by environmental conditions that have not existed for many years, recent data concerning arctic ground squirrels make this hypothesis doubtful.
Research into animal behavior, particularly into that of the pronghorn, provides strong evidence that most present-day characteristics of animals are explained by environmental conditions that have not existed for many years.
Even in those cases in which an animal species displays characteristics clearly explained by long-vanished environmental conditions, evidence concerning arctic ground squirrels suggests that those characteristics will eventually disappear.
Although biologists are suspicious of hypotheses that are difficult to test, there is now widespread agreement among biologists that many types of animal characteristics are best explained as adaptations to long-extinct predators.
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