An eastern gray whale set a record in 2015 for the longest recorded migration by a mammal -- a journey of nearly 14,000 miles from Russian waters to Mexico and back. Here's a look at some other arduous animal migrations -- many of which are facing human-made challenges. OMAR TORRES/AFP/Getty Images
The gray whale's odyssey broke the previous migration record, a 11,706-mile round trip by a humpback whale in 2011. North Pacific humpback whales migrate to cool Alaskan waters in the summer. At a sedate speed of about one mile per hour, humpbacks return from Hawaii with their new calves to feed, mostly on krill and small fish, having lived off their fat reserves for months.
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At the end of the rainy season East African blue wildebeest head towards pastures new on a circular migration of between 500-,1000 miles around the Serengeti. Herds can stretch to 25 miles long as they cross the plains, traversing cliffs and rivers, employing "swarm intelligence" to overcome obstacles as a collective. But their migration is threatened by poaching and habitat fragmentation -- when land is fenced off or cut up by roads. ROBERTO SCHMIDT/AFP/Getty Images Wildebeest may wander on a huge circular journey, but according to a 2014 World Wildlife Foundation study the plains zebra undertakes the longest linear land-mammal migration in Africa. The journey between Namibia and Botswana is a round trip of over 300 miles. Along the way zebras face considerable disruption from man-made obstacles such as roads and fences, and farming causing habitat fragmentation. MARCO LONGARI/AFP/Getty Images In spring, Porcupine caribou (named after the Porcupine River that runs through their range) migrate north from the boreal forests of Canada and Alaska to escape the heat and satisfy their large appetites. Annual migrations can reach 1,500 miles along well-trodden routes. Their migration has been under the shadow of possible oil exploration in the Arctic National Wildlife Refuge (ANWR), but plans to designate 12 million acres of the ANWR for conservation may yet protect them. courtesy U.S. Fish and Wildlife Service
The longest known shark migration was recorded in 2003, by scientists tracking a great white between the coasts of South Africa and Australia. The journey, some 6,900 miles, took the shark a mere 99 days. The GPS tracker fell off the shark once it arrived, but nine months later the same shark reappeared in South African waters, identified by its dorsal fin. It is estimated the great white traveled at least 12,400 miles.
During their epic journeys great whites are often accidentally caught by trawlers and threatened by illegal fishing for their jaws and fins.
Leatherback turtles migrate vast distances to beaches where they lay their eggs. Leatherbacks typically migrate about 6,000 miles, but some have gone much further. Over 647 days one leatherback traveled 12,700 miles between Papua, Indonesia and North West America, before losing its GPS receiver halfway through the return trip. The journey was one of the longest ocean migrations ever recorded. But rising sea levels and beach erosion are forcing turtles to travel to new and unfamiliar sites to create their nests. Many populations are critically endangered. Peter Richardson/Getty Images Adelie penguins migrate in an 8,000-mile circle around the Ross Sea off Antarctica, on "fast ice" that develops during the winter. Doing so means they stay in sunlight, the clockwise migration taking them back to land and their breeding colonies for the summer months.
A 2013 census suggested ice retreat was -- contrary to other polar species -- actually boosting penguin numbers at a colony on Beaufort Island. Scientists recorded a population growth of 53% across 20 years; it is believed the increase is due to the diminishing journey between land and the sea where Adélie penguins feed. Getty Images
The migration of the monarch butterfly takes four generations on a 5,500-mile journey. Starting in Central Mexico and California, the first generation fly to Texas and Oklahoma where it lays its eggs and dies. The next generation takes off further north before doing the same. The third generation reaches America's Great Lakes and succumbs before the fourth generation flies all the way back south -- up to 3,000 miles -- to lay its eggs. The current population has declined sharply from around one billion in the mid-1990s to a mere 35 million last year, according to the U.S. Center for Biological Diversity. The use of herbicide by farmers is being blamed for reducing the availability of milkweed, their primary food source. Climate change also could disrupt the butterfly's migration pattern. MARIO VAZQUEZ/AFP/Getty Images Biologist Charles Anderson believes that certain dragonfly species travel between Southern India and Southern Africa every year, stopping in the Maldives and skirting the East African coast along the way. If he is correct, the migration, a round trip of 8,000-11,000 miles, is the longest by any insect and all the more remarkable considering the lack of fresh water available. JOEL SAGET/AFP/GettyImages
The Alaskan bar-tailed godwit holds the record for the longest nonstop journey through the air. Its annual migration is approximately 15,500 miles, but its journey between Alaska and New Zealand -- a journey of 7,000 miles -- is done without food or rest in a staggering eight days.
Scientists have concerns that due to habitat loss, future generations will struggle to accrue the necessary fat reserves required for the journey, but it is thought that the godwit is resilient enough to cope with climate change for at least the near future. Dan Kitwood/Getty Images
Arctic terns hold the record for the longest annual migration recorded by any animal. Moving between Greenland and Antarctica in a zig-zag route, the bird covers 44,000 miles a year. With an average lifespan of 30 years, this means the arctic tern would cover around 1.3 million miles in its lifetime -- the equivalent to going to the moon and back three times.
But climate change may be forcing Arctic terns further north to breed. Due to fatigue, many more birds are dying along the way. Dan Kitwood/Getty Images