Puerto López (Ecuador) (AFP) - The sight of thousands of whales surfacing, jumping and playing off the coast of South America as they migrate toward their breeding grounds is one of nature's most majestic displays.But global warming is killing off their food and changing their age-old migratory routes.To the tourists watching a humpback whale frolic with her newborn calf in the tropical waters off Ecuador's coast near Puerto Lopez, the sight of enormous fins surfacing, tails flipping and blowholes spouting is breathtaking.The same scenes can be found up and down the South American coast, from Puerto Piramides in Argentina to Cabo Blanco in Peru and Bahia Malaga in Colombia.But to marine biologists, these huge mammals are not as carefree and healthy as they appear.They are skinny, covered in parasites and exhausted from the increasingly long journeys they are making to reproduce."You can see their bones.