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1. Introduction

With a few exceptions — sessile animals like barnacles, sea anemones, and corals — moving from one place to another is a key part of what it means to be an animal. Throughout the day, animals move in search of food, shelter, and mates; and to avoid danger from predators or exposure to the elements. During migration, a population of animals interrupts its daily movement pattern to travel in a persistent direction to a new habitat. Often, these movements are seasonal and involve moving toward a breeding site.

Migratory behavior is widely dispersed among the members of the animal kingdom and is found in many species from groups as diverse as insects, crabs, birds, hoofed mammals, bats, whales, and fish.

2. Arctic Terns

The Arctic tern might be the Earth’s champion migrator. Every year, this tern migrates from the Arctic to Antarctica, flying from pole to pole to catch the long days of summer in each hemisphere. Each trip South or North is at least 19,000 kilometers (12,000 miles) each way, but these birds often follow the coastlines of the continents, meaning that a bird’s actual yearly flying distance can be much longer.

How do we know? Traditionally, the activity of individual birds has been followed by placing rings on their legs, releasing them, and then recapturing them. More recently, scientists have used lightweight, solar-powered tracking devices (such as the one shown on the right). One group of terns that were tracked flew 90,000 kilometers in one year (56,000 miles): more than twice the Earth’s circumference.

3. Learning to Navigate by the Stars

Indigo buntings are songbirds that migrate at night, completing a migratory journey that starts east of the North American continental divide, crosses the Gulf of Mexico, and ends in southern Mexico and Central America. How, while flying at night, do they know where to go?

If you were out at night, and you knew where to look, you could navigate by orienting yourself relative to the North Star. The North Star, unlike the other stars in the night sky, is fixed in position. All the other stars rotate around it, as you can see in this time-lapse photo (on your left).

Experiments in the 1950s indicated that buntings did, indeed, use the stars for navigation during their migrations. But not, as was initially believed, through a genetically encoded map of the stars. Rather, as was established by Cornell scientist Stephen Emlen, young buntings learned the position of the North Star and used that position to orient themselves to fly south.

How did Emlen figure this out? (sources: All About Birds (Cornell University) and Scientific American). First, Emlen used a funnel cage (shown below) to determine the bird’s directional preference. The bird could see the sky above through the top of the cage. When it attempted to fly, its feet, stained by ink, would make a mark on blotter paper.

Based on the pattern on the blotter paper, Emlen could create vector diagrams.

To test the hypothesis that buntings learned the position of the North Star, Emlen raised young buntings in a planetarium where he could control the appearance of the night sky. In one of his experiments, he divided the buntings into three groups, as shown below.

• Group A was raised under a sky with diffuse light.
• Group B was exposed to a planetarium sky that simulated the normal, northern hemispheric night sky, with the stars rotating around the North star.
• Group C was exposed to a planetarium sky in which the stars rotated around a Betelgeuse, a red-giant star in the constellation of Orion.

During the fall migration season (when the buntings migrate south), the birds were tested in funnel cages. Match the vector diagram below with the groups described above.

4. Loggerhead Sea Turtle Migrations

Loggerhead sea turtles (Caretta caretta) have shells that are about 90 cm long. Adults can weigh 135 kg (298 lbs), with the largest specimens weighing up to 450 kg (1000 lbs). These animals have a lifespan between 47 and 67 years of age and reach sexual maturity when they’re about 17 years old. They are omnivorous, eating bottom-dwelling invertebrates when living close to shore, and consuming jellyfish, floating mollusks, floating egg clusters, and any fish they can catch while out in the open sea (source: Wikipedia).

Loggerhead turtles that live in the Atlantic ocean migrate from the Atlantic coast of North and South America to the Western Coast of Africa, and then return, often many years later. Their return (as we’ll see below) is often to the beach where they were born.

Loggerheads face three navigational challenges during their lives:

1. FINDING THE OCEAN (SEAFINDING): Female loggerheads lay their eggs on sandy beaches. To do this, the females need to exit the water, drag themselves above the high tide line (so that the eggs don’t drown), and then use their hind flippers to excavate a pit into which they lay a clutch of over 100 eggs. Eighty days later, the eggs hatch. The hatchlings, which are only about 4.6 cm long and weigh about 20 grams, have to dig up through the sand to get to the surface. Once they’re at the surface, they’re in danger from predation and quickly have to find their way to the ocean.
2. FINDING THEIR WAY TO THE SARGASSO SEA: Once they’re in the open sea, Atlantic turtles use a combination of swimming and floating to find their way to the warm waters of the Sargasso sea (in the middle of the Atlantic ocean). Once there, the turtles forage for years as they grow to adult size and reproductive maturity.
3. FINDING THEIR WAY BACK TO THE BEACH WHERE THEY WERE BORN. Recent genetic studies indicate that female turtles return to the beaches where they were born to lay their eggs.

Let’s look at each of these challenges, focusing on how biologists have identified what stimuli are driving sea turtle behavior, and how the turtles respond.

4a. Seafinding

Hatchlings emerge from their nests onto the sand, usually during the night. There are a variety of stimuli that they can use to find their way to the ocean. The visual stimuli include the elevated landward horizon, usually darkened by vegetation, and the lower and usually brighter view toward the sea. At the same time, the beach slopes down toward the ocean.

When scientists observe turtle hatchlings in the wild, all of these variables are at play at once. To tease them apart, a team led by Michael Salmon constructed a circular arena (shown at left) where they were able to control variables such as the amount and direction of light, the slope, and the position of the silhouettes (the dark outlines of the vegetation-covered landward slope).

Here’s how the testing arena worked: f_t and f_b are fluorescent light tubes; P is the platform, the angle of which could be adjusted by the jack (J). S is a screen that could be used to create silhouettes.

The hatchling would be “gently placed on the center of the platform, and then released.” A short tether was attached to the turtle’s carapace (shell). Once released, the turtles would crawl until they reached the end of the tether, after which they would continue to crawl in place. Their chosen direction was measured after one minute.

The circles shown in the quiz below represent a top-down view of the arena. Each of the four setups is simulating, to varying degrees, the visual experience of hatchling turtles in nature, in which the seaward side is more brightly lit than the landward side. In all four experiments, there was illumination from the front (which represents the sea), but illumination behind varied (as indicated in the image and described in the text below each circle.

 

Further experiments by Salmon et al. looked at the effects of silhouettes and slopes on the hatchlings’ ability to find the ocean. For your purposes as a student in an AP or introductory college course, the key thing to note is how the researchers were able to isolate the variables that most strongly drive these animals’ behavior. I recommend that you read their article (available here for free download at ResearchGate.net).

4b. Using magnetoreception to orient while out at sea

Once they’re away from the shore, how does a sea turtle know where to go? Sea turtles born on the Atlantic coast of North, Central, or South America need to head east to reach the Sargasso sea (see map above) where they’ll mature. Then, decades later, they need to be able to swim west back to the beaches where they hatched. How do they know which way is east or west?

While humans lack this ability, organisms as diverse as bacteria, salmon, bees, ants, termites, frogs, and birds can sense and orient themselves using the Earth’s magnetic field. This ability is called magnetoreception. To test whether sea turtles used magnetoreception, Kenneth and Catherine Lohmann created this device:

Here’s how it worked. A sea turtle hatchling was tethered to a rotatable lever-arm, in a basin filled with seawater. The hatchling could swim in any desired direction (and it would continue swimming in place once it reached the end of its tether). The parallel lines surrounding the basin represent a Rubens coil system that could reverse the Earth’s magnetic field within the test chamber.

A turtle’s ability to orient itself using magnetic fields requires some learning that looks a lot like the imprinting discussed in the previous tutorial. Based on the turtles’ natural history, it was thought that turtles combine their experience of light coming from a particular direction with their perception of the direction of a magnetic field. For example, a turtle that hatches in the Americas will be following the light of the sea, and swimming away from the darker land behind. If they were heading directly east, then north would be to their left. Once that perception was imprinted into the turtle’s nervous system, it would no longer need light to orient itself and could orient itself by its internal magnetic compass.

To test this in an experimental setting, sea turtle hatchlings were permitted to establish a course swimming toward a dim light in the east. The result? This imprinted their internal compass. We know that because, after that initial exposure to light and a magnetic field, the hatchlings could orient toward the east even in total darkness. Moreover, when the Rubens coil system was used to flip the orientation of the magnetic field that the turtles were experiencing, they changed direction.

To make sure you’re getting it, answer the following questions.

4c. Using Geomagnetic Signatures to Find their Way Home

The hatchlings’ ability to detect magnetic fields helps them to find the Gulf stream, which brings the turtles to the Sargasso Sea, where they grow to adulthood. After decades at sea, adult sea turtles can find their way back to their natal beaches (the beaches where they were born). That raises two questions. First, how do we know that the turtles do return to the beaches where they were born? Secondly, what’s the mechanism by which they find their way back to their natal beaches, after decades out at sea?

The evidence for turtles’ returning to their natal beaches isn’t from capture, mark, release, and recapture studies. That’s because the turtles start life as very small animals, with high mortality rates, and they don’t return to their nesting beaches for decades. Any marked hatchling would have to remain marked for years, and any one individual has a low chance of returning (because it’s more likely to be part of the 99% or more of hatchlings that don’t survive to adulthood). In addition, any researcher who bothered to mark hatchlings for subsequent recapture as an adult would have to wait decades to get any data — not a recipe for success in academia.

Rather, the evidence for natal homing is genetic and comes through analysis of mitochondrial DNA (mtDNA), which is only passed down through the maternal line ( the males, once they make it to sea, never come ashore). A study by Meylan, Bowen, and Avise (source: Science. 1990 May 11;248(4956):724-7) of mtDNA in green turtle colonies (which are a distinct species from the loggerheads we’ve looked at so far) shows that the nesting colonies consist of genetically similar individuals. That’s what you’d expect to see if most of the females nesting on any beach are sisters or cousins (with the same mitochondrial DNA). If that’s not clear, just imagine the alternative: if females didn’t return to their natal beaches, then any colony would be full of non-related individuals, and there’d be a high degree of genetic difference observable on any beach.

So, what are turtles sensing that brings them home?

In a 2018 study by Brothers and Lohmann, mitochondrial DNA was obtained from 834 nesting sea turtles at 20 different locations. Using the mitochondrial DNA, the researchers were able to calculate how much the populations in different colonies were related to one another.

Before looking at their results, let’s think about the relatedness between nesting populations. Here’s a map of Florida. The white circles indicate nesting sites, with six sites lettered.

The degree of genetic differentiation between these different populations can be quantified as what’s called an F_ST value. Populations that have a lot of genetic differences (many differences in allele frequencies) between them have an F_ST  of 1.0. Populations that are identical to one another would have an F_ST  of 0.0.

The graphs above show genetic differentiation (on the Y-axis) as a function of geographic distance (Graph 2) and environmental distance (graph 3). Geographic distance is what it sounds like: how far away one population lives from one another. While there are many cases where populations that live close to one another are closely related (see “C”), there are other cases (see “E”) where distant populations are closely related to one another. Why would that be?

Graph 3 looks at genetic distance as a function of environmental distance. Environmental distance is a measure, based on a checklist of variables, of how similar two environments are to one another. Again, there are some cases where there is a close correlation between environmental distance and genetic distance (note the populations at “F” and “G”), but there are also cases where there’s a lot of environmental distance, but a close genetic relationship (see “H”).

It was already established that hatchling turtles navigate out to sea using magnetoreception. Could it be possible that these turtles, as adults, were finding their way back to the beaches where they were born through the same sense? Here’s a map of Florida showing its geomagnetic profile. The earth’s magnetic field, at any particular location on our planet, has a quantifiable intensity and inclination. The field’s intensity is simply how strong the field is; the inclination is the angle made with the horizontal by the magnetic field lines.

See if you can predict where this is going by completing the questions below.

For a 2018 summary of how sea turtles return to their natal beaches, read this article in the NY Times.

5. Migration: Checking Understanding

In our study of migration, we’ve covered a lot of distance. Let’s consolidate our learning through the quiz below.

6. Shorter-term group movement: schooling, flocking, herding, and swarming

In what’s above, we’ve focused on understanding the long-range and long-duration animal movements involved in migration. Let’s finish by looking at another example of group behavior: schooling, flocking, herding, and swarming.

While schools, flocks, herds, and swarms have different names, they’re unified by the fact that they all involve collective motion by animals that propel themselves with tight coordination. As the animals move, they follow a few simple rules (which have been successfully modeled in many online web-based computer simulations, including this one (which lets you see the code) and this one (both open in separate browser tabs, and both are addictive).

The rules are

• Alignment: move in the same direction as your closest neighbors
• Proximity: remain close to your closest neighbors
• Separation: don’t crash into your closest neighbors.

Why do animals move together in schools, flocks, swarms, or herds? Here are a few benefits:

• Increased foraging success, with more efficiency at finding food sources.
• Ease in finding mates
• Increased efficiency in moving through air or water (not applicable to herds).
• Predator avoidance (because it’s easier for members of a school to spot predators, and more difficult for predators to hone in on individuals).

7. What’s next?

Proceed to Topic 8.1, Part 4: Living in Groups (the next tutorial in AP Bio Unit 8)