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Photos: The world's longest animal migrations

Gray whale – 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.

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Photos: The world's longest animal migrations

Humpback whale – 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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Photos: The world's longest animal migrations

Blue wildebeest – 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.

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Photos: The world's longest animal migrations

Plains zebra – 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.

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