The pronghorn, an antelope-like mammal that lives on the western plain

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
(5) 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
(10) 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.

(15) Like all explanations that posit what is called a
relict behavior—a behavior that persists though its only
evolutionary impetus comes from long-extinct environ-
mental conditions—this one is likely to meet with skep-
ticism. Most biologists distrust explanations positing relict
(20) 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
(25) 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,
(30) 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
(35) 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
(40) 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
(45) 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
(50) 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
(55) 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
(60) threat of a rattlesnake, exhibiting only disorganized
caution even after being bitten repeatedly.

1. Which one of the following most accurately states the main point of the passage?

(A) 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.
(B) 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.
(C) 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.
(D) 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.
(E) 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.


2. Based on the passage, the term “principal component” (line 21) most clearly refers to which one of the following?

(A) behavior that persists even though the conditions that provided its evolutionary impetus are extinct
(B) the original organism whose descendants’ behavior is being investigated as relict behavior
(C) the pronghorn’s ability to run 90 kilometers per hour over long distances
(D) the environmental conditions in response to which relict behaviors are thought to have developed
(E) an original behavior of an animal of which certain present-day behaviors are thought to be modifications


3. The last paragraph most strongly supports which one of the following statements?

(A) An absence of predators in an animal’s environment can constitute just as much of a threat to the well-being of that animal as the presence of predators.
(B) Relict behaviors are found in most wild animals living today.
(C) If a behavior is an adaptation to environmental conditions, it may eventually disappear in the absence of those or similar conditions.
(D) Behavior patterns that originated as a way of protecting an organism against predators will persist interminably if they are periodically reinforced.
(E) Behavior patterns invariably take longer to develop than they do to disappear.


4. Which one of the following describes a benefit mentioned in the passage that grazing animals derive from roaming in herds?

(A) The greater density of animals tends to intimidate potential predators.
(B) The larger number of adults in a herd makes protection of the younger animals from predators much easier.
(C) With many animals searching it is easier for the herd to find food and water.
(D) The likelihood that any given individual will be attacked by a predator decreases.
(E) The most defenseless animals can achieve greater safety by remaining in the center of the herd.


5. The passage mentions each of the following as support for the explanation of the pronghorn’s speed proposed by the biologist referred to in line 8 EXCEPT:

(A) fossils of extinct animals believed to have been able to run down a pronghorn
(B) the absence of carnivores in the pronghorn’s present-day environment
(C) the present-day preference of pronghorns for athletic mates
(D) the apparent need for a similar explanation to account for the herding behavior pronghorns now display
(E) the occurrence of relict behavior in other species


6. The third paragraph of the passage provides the most support for which one of the following inferences?

(A) Predators do not attack grazing animals that are assembled into herds.
(B) Pronghorns tend to graze in herds only when they sense a threat from predators close by.
(C) If animals do not graze for their food, they do not roam in herds.
(D) Female pronghorns mate only with the fastest male pronghorn in the herd.
(E) If pronghorns did not herd, they would not face significantly greater danger from presentday carnivores.