Questions 9–14 are based on the following passage and supplementary material.
This passage is adapted from Ed Yong, “Turtles Use the Earth’s Magnetic Field as Global GPS.” ©2011 by Kalmbach Publishing Co.
In 1996, a loggerhead turtle called Adelita swam across 9,000 miles from Mexico to Japan, crossing the entire Pacific on her way. Wallace J. Nichols tracked this epic journey with a satellite tag. But Adelita herself had no such technology at her disposal. How did she steer a route across two oceans to find her destination?
Nathan Putman has the answer. By testing hatchling turtles in a special tank, he has found that they can use the Earth’s magnetic field as their own Global Positioning System (GPS). By sensing the field, they can work out both their latitude and longitude and head in the right direction.
Putman works in the lab of Ken Lohmann, who has been studying the magnetic abilities of loggerheads for over 20 years. In his lab at the University of North Carolina, Lohmann places hatchlings in a large water tank surrounded by a large grid of electromagnetic coils. In 1991, he found that the babies started
Later, Lohmann showed that they can also use the magnetic field to work out their position. For them, this is literally a matter of life or death. Hatchlings born off the sea coast of Florida spend their early lives in the North Atlantic gyre, a warm current that circles between North America and Africa. If they’re swept towards the cold waters outside the gyre, they die. Their magnetic sense keeps them safe.
Using his coil-surrounded tank, Lohmann could mimic the magnetic field at different parts of the Earth’s surface. If he simulated the field at the northern edge of the gyre, the hatchlings swam southwards. If he simulated the field at the gyre’s southern edge, the turtles swam west-northwest. These experiments showed that the turtles can use their magnetic sense to work out their latitude—their position on a north-south axis. Now, Putman has shown that they can also determine their longitude—their position on an east-west axis.
He tweaked his magnetic tanks to simulate the fields in two positions with the same latitude at opposite ends of the Atlantic. If the field simulated the west Atlantic near Puerto Rico, the turtles swam northeast. If the field matched that on the east Atlantic near the Cape Verde Islands, the turtles swam southwest. In the wild, both headings would keep them within the safe, warm embrace of the North Atlantic gyre.
Before now, we knew that several animal migrants, from loggerheads to reed warblers to sparrows, had some way of working out longitude, but no one knew how. By keeping the turtles in the same conditions, with only the magnetic fields around them changing, Putman clearly showed that they can use these fields to find their way. In the wild, they might well also use other landmarks like the position of the sea, sun and stars.
Putman thinks that the turtles work out their position using two features of the Earth’s magnetic field that change over its surface. They can sense the field’s inclination, or the angle at which it dips towards the surface. At the poles, this angle is roughly 90 degrees and at the equator, it’s roughly zero degrees. They can also sense its intensity, which is strongest near the poles and weakest near the Equator. Different parts of the world have unique combinations of these two variables. Neither corresponds directly to either latitude or longitude, but together, they provide a “magnetic signature” that tells the turtle where it is.
Orientation of Hatchling Loggerheads Tested in Magnetic Fields
Adapted from Nathan Putman, Courtney Endres, Catherine Lohmann, and Kenneth Lohmann, “Longitude Perception and Bicoordinate Magnetic Maps in Sea Turtles.” ©2011 by Elsevier Inc.
Orientation of hatchling loggerheads tested in a magnetic field that simulates a position at the west side of the Atlantic near Puerto Rico (left) and a position at the east side of the Atlantic near the Cape Verde Islands (right). The arrow in each circle indicates the mean direction that the group of hatchlings swam. Data are plotted relative to geographic north
Select an Answer
The passage most strongly suggests that Adelita used which of the following to navigate her 9,000-mile journey?
The current of the North Atlantic gyre
Cues from electromagnetic coils designed by Putman and Lohmann
The inclination and intensity of Earth’s magnetic field
A simulated “magnetic signature” configured by Lohmann
Choice C is the best answer. The first paragraph describes the 9,000-mile journey that Adelita made and raises the question, which the rest of the passage tries to answer, of how this loggerhead turtle was able to “steer a route across two oceans to find her destination” (line 5). The answer comes most directly in the last paragraph, which presents Putman’s belief that loggerhead turtles “work out their position using two features of the Earth’s magnetic field that change over its surface” (line 56–57): its inclination and its intensity. It is reasonable, therefore, to infer from the passage that this was the method that Adelita used.
Choice A is not the best answer because there is no evidence in the passage that Adelita used the current of the North Atlantic gyre to navigate her 9,000-mile journey. The passage does discuss the North Atlantic gyre but only as the place where loggerhead turtle hatchlings “born off the sea coast of Florida spend their early lives” (line 24).
Choice B is not the best answer because there is no evidence in the passage that Adelita navigated her 9,000-mile journey with the aid of cues from electromagnetic coils designed by Putman and Lohmann. The passage does say that Putman and Lohmann use electromagnetic coils as part of their research on loggerhead turtles, but the coils are part of tanks used in a laboratory to study loggerhead hatchlings (see line 13).
Choice D is not the best answer because there is no evidence in the passage that Adelita navigated her 9,000-mile journey with the aid of a simulated “magnetic signature” configured by Lohmann. The passage does describe how Lohmann and Putman manipulate magnetic fields as part of their research on loggerhead turtle hatchlings (see, for example, lines 15–20), but there is no indication that the two scientists used (or even could use) the kind of equipment necessary for this project outside of laboratory tanks or with Adelita in the wild.
Students must draw a reasonable inference from the text.