Migrations occur in every major branch of the animal kingdom, from birds and crustaceans to insects and whales. But some of these journeys are amazing feats of endurance, spanning thousands of miles and deadly terrain without rest. Here's what science can tell us about the most extreme migrations, and how animals survive them.
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The word "migration" typically brings to mind the seasonal en masse movement of animals from colder climates to warmer ones, but there's no real consensus on the exact definition of what types of movements it includes. In a 2007 overview of the phenomenon — aptly titled "What Is Migration?" — experts Hugh Dingle and V. Alistair Drake note that definitions of animal migration in dictionaries and the scientific literature encompass many overlapping concepts.
Some definitions describe migration as the seasonal movement of animals between favorable and unfavorable regions, often for the purpose of breeding. But migration can also more generally refer to the relocation of an animal that involves longer journeys than those of its normal daily activities. Other definitions say migration is when animals move around single-mindedly, ignoring their usual food-gathering and home-building habits. Finally, migration can be movements that lead to "redistribution within a spatially extended population." In other words, it's when animals move around within a large territory occupied by their own species.
A few years ago, Dingle offered his own definition for migration. According to Dingle, migration has five specific characteristics that separate it from other forms of movement: It is prolonged and takes animals outside of their normal habitats; it is typically linear; it is undistractable, meaning, for example, that you can't get a migrating bird to stop migrating if you wave food in front of it; it involves special start and stop behaviors, such as overfeeding before the big trip; and it requires a lot of stored energy.
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But there are always exceptions.
Some animals, such as song sparrows (Melospiza melodia), engage in partial migration, where only some of the population migrates. Insects often perform one-way migration, in which they travel from their birthing grounds to another location to breed and produce the next generation before dying. Species living in mountains, including birds and bats, often conduct altitudinal migration — they move from one elevation to another, such as when spotted owls travel to lower elevations when snow covers the upper parts of the mountain.
The reasoning behind animal migration is simple: Survival, both for the individual and the species. Animals will typically migrate to find food, reproduce or escape bad climate.
For example, in the arctic tundra and other high latitude regions, migratory birds such as the bar-tailed godwit (Limosa lapponica) take advantage of the long days and high abundance of food in the early summer; after breeding, they leave the region to avoid low food availability, as well as winter's short days and low temperatures. Research has suggested that many other birds migrate because of the scarcity of food. And wildebeest are also known to be in a constant search of good food in the Serengeti.
Salmon (Oncorhynchus spp.) are a prime example of animals that migrate for reproductive reasons. The fish live out their adult lives in the ocean, but when it's time to spawn the next generation, they return to their natal freshwater streams (for a comprehensive account of the Pacific Salmon's lifecycle, see here).
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The migration of some baleen whales involves both feeding and breeding. Humpback whales, for example, gorge themselves on krill in high latitude waters during the summer. As winter approaches and the waters get ever colder, the cetaceans travel some 5,000 miles (8,000 kilometers) south to give birth in warm tropical waters, which are more suitable for their calves but lack high quantities of food.
North America's pronghorn antelope (Antilocapra Americana) also migrate to avoid the cold. The "Path of the Pronghorn" runs from Jackson Hole in Wyoming to the Upper Green River Valley in the state. In the summer, the ungulates give birth and raise their young in Jackson Hole, but as autumn approaches they gather to return south to the Upper Green River Valley. They know they wouldn't be able to survive the deep snows of the North, and they must get started on their journey before the snow blocks their path through the Gros Ventre Mountain
Last year, researchers discovered another potential reason for migration: To avoid predators. The common roach (Rutilus rutilus) is a freshwater fish that lives in the lakes and streams throughout Europe; during winter, cormorant birds (Phalacrocorax carbo) roost and breed near the roaches' lake homes. When the birds come along, some 80 percent of the fish migrate to the streams — where there is virtually no food — to avoid getting eaten by the birds, which pick off roaches in the lakes.
Though some animals stop to eat along their migratory paths, others do not. Instead, they prepare for their trip by essentially fattening themselves up, once their internal clocks tell them to do so. During this "fueling" stage, birds ingest and process food at or close to their metabolic capacity. What's more, organs involved with feeding, including the stomach, gut, gizzard, liver and kidneys, enlarge to support fueling. During takeoff and flight, these organs shrink, while organs that support flight, including the heart, flight muscles and skeletal muscles, grow.
Once they're on their way, migratory animals have several different mechanisms they use to find their destination and stay on the right path, most notably celestial cues and the Earth's magnetic field.
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Numerous animals, including fish, insects and possibly birds, can use the sun and the polarized light patterns in the sky or reflecting off bodies of water as compass cues during the day. At night, when the sun and its related polarization patterns are not visible, animals can use the stars as their guide — that is, the rotation center of the night sky (the Pole Star in the northern hemisphere) points them to the geographical north direction.
The Earth's magnetic field also plays a role in migratory navigation. Birds, turtles, fish and other animals seem to be able to detect the intensity of Earth's magnetic field, an ability that may have to do with the bodily presence of the magnetic material magnetite.
Interestingly, some animals can even smell their way back home. Research has suggested that juvenile Pacific salmon learn odors associated with their natal site as they swim out to sea; as adults returning to their birthing grounds, they continuously sample the seawater for chemical characteristics that match those odors (a recent study has suggested that the fish also use geomagnetism for migratory navigation).
Arctic terns (Sterna paradisaea) may be small, weighing in at less than 4.5 ounces (125 grams), but these birds have one of the most remarkable long-distance migrations. Scientists previously thought the birds migrated 25,000 miles (40,000 kilometers) from their Arctic breeding grounds to the Antarctic and back. But a few years ago, researchers fitted birds with miniature geolocators and discovered that some of them traveled more than 50,000 miles (80,000 km) in a year because of the meandering routes they took to exploit prevailing wind systems — the birds have the longest recorded annual migration of any animal.
The Arctic tern may travel the farthest each year, but other birds have longer non-stop migration routes. In 2009, satellite telemetry revealed that the bar-tailed godwits make their 8-day, 7,000-mile (11,000-km) trip from Alaska to New Zealand with no stops to rest or refuel. Other researchers suspect that this record will never be broken because "the physical limitations of the Earth do not offer any combination of ecologically feasible breeding and wintering areas more distantly apart that would require longer flights."
Last year, another avian migrator made headlines. Scientists discovered that the Northern wheatear (Oenanthe oenanthe), which at 0.9 ounces (25 g) is the smallest migrating bird, travels 9,000 miles (14,500 km) each way every year. As Alasdair puts it:
To put this migration into awesome, if admittedly rather ridiculous, perspective, we weigh about 3,000 times as much as these tiny songbirds. Proportionally speaking, we would have to travel roughly 50 million miles to cover the same sort of distance these wheatears do. In other words, for us to migrate on the same scale as these tiny Arctic birds, our entire species would need to travel to Mars and back every single year. (And yes, I'm aware that's not exactly a fair analogy - it's simply too awesome not to point out.)
Perhaps even more remarkable than the wheatears are the bar-headed geese (Anser indicus). From sea level these birds migrate over Himalayan mountain passes of up to 20,000 feet (6,000 meters) in just 7 or 8 hours, at speeds of about 40 miles per hour (64.5 km per hour). And they don't make this journey during the day when there's wind at their backs — they do it at night, when they're likely to face headwinds.
As for mammals, humpback whales (Megaptera novaeangliae) hold the record for the longest migratory voyage. In 2007, researchers recorded seven individuals, including a mother-calf pair, swim 5,200 miles (8,300 km) from their wintering area off the Pacific coast of Central America to their feeding grounds off Antarctica. On the other hand, the world's largest mammal migration (in terms of numbers of individuals) occurs every October through December, when up to 10 million straw-coloured fruit bats (Eidolon helvum) migrate from the Congo to Zambia's Kasanka National Park.
On land, North America's caribou (Rangifer tarandus) migrate farther than any other terrestrial mammal — 3,000 miles (5,000 km) in a year. But the most famous terrestrial mammal migration is, of course, the "Great Migration," which involves over 2 million animals, including 1.5 million wildebeest, 200,000 zebra and 500,000 Thomson's gazelle.
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Monarch butterflies are known for their crazy, multi-generational migrations, but they may pale in comparison to dragonflies. In 2009, biologist Charles Anderson discovered that various species of dragonflies (mostly the globe skimmer, Pantala flavescens) might migrate from southern India to the Maldives, a trip of up to 600 miles (1,000 km) over open ocean. From here, they appear to cross the western Indian Ocean to eastern and southern Africa. Their round trip from India to Africa would total some 8,700 miles (14,000) to 11,000 miles (18,000 km). If that's not enough, new research from Anderson and his colleagues suggests the migration may be even longer than that.
According to Guinness World Records, the leatherback sea turtle (Dermochelys coriacea) holds the record for the longest migration of any reptile. Astoundingly, a tagged turtle reportedly took 647 days to travel from its nesting site on the beaches of Papua, Indonesia, to its feeding grounds off the coast of Oregon in the United States.
And let's not forget about Christmas Island's red crab migration, which travel inland to the sea to breed. During their month-long, 5-mile (8-kilometer) journey, the millions of crustaceans must cross paved roads, suffer dehydration, battle evil yellow crazy ants and climb down a 40-foot (12-meter) vertical cliff.
The quest for sex has never been more dramatic.
Like nomadic humans, animals migrate because it's often difficult to survive if you remain in the same place all year long. By moving from one place to another, these animals also give their environments time to rebound as well. Food supplies are often more plentiful when the animals return after a long absence — and, of course, the population of the migratory animals has been culled on the long, arduous journey. Migrations may be one way that ecosystems keep themselves in balance.