Which animal migrates the furest by land ? And why ?

Answer 1

Caribou in Canada make the longest known terrestrial migration, 500 kilometers (~300 miles) each way. These and other animals follow the seasons north into summer grazing to give birth. By expanding their territory they reduce competitive pressure for resources.
Migrating ungulates may follow seasonally abundant resources, or seasonal resources of exceptional quality, and/or avoid of non-migratory predators.
http://books.google.com/books?id=bUSLWcOZZksC&pg=PA220&lpg=PA220&dq=longest+terrestrial+migration+caribou&source=web&ots=QS0DqIzc0J&sig=I5p6yNfeN0YuwB9Qy_oifkoKXBU
http://arctic.fws.gov/carcon.htm
Satellite telemetry tracking in the Yukon & Alaska has shown some individuals had minimal annual movements of 5,055 km (Fancy et al, 1989) The majority travel make seasonal movements of 450 km (Miller, Russel, Gunn 1977)
http://books.google.com/books?id=7W-DGRILSBoC&pg=PA1129&lpg=PA1129&dq=longest+terrestrial+mammal+migration&source=web&ots=WHH3oQQyQH&sig=s7a8TRBZEOwtTkUJAZcgDKboxLU

Pronghorn antelope make the second longest going only half the distance of the caribou.
http://www.wcs.org/international/northamerica/yellowstone/pathofthepronghorn
In Africa the longest migrations are accomplished by the Kob at 700 km & Chiru at 600km round-trip.
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Answer 2

Probably the caribou. Some herds travel 3,000 miles (5,000 km) in a year. In the spring, cows leave forests heading north to the Arctic shore to give birth. The bulls follow, feeding on snow covered tundra. In fall, they head back to the forest again. When spring comes late, cows drop their calves on the way. The calves can run within hours of being born and keep up with the herd within a day or so. Usually, though, the herd remains at the birthing spot for a couple of weeks.

Answer 3

I don't know which, but the answer is easy. Food.

Answer 4

Cause they can't swim.