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. They're sick. They have parasites. We never used to see that," said Ecuadorian marine biologist Cristina Castro as she scanned the horizon for more humpback whales, the species she has studied for the past 18 years.
These whales swim thousands of kilometres each year from Antarctica to the waters around the equator to have their young, which measure 3-4.5m at birth and can weigh up to one tonne.
But as ocean temperatures rise, whales are migrating earlier and travelling further.
Warmer waters are killing off the supply of krill, the small crustaceans that are whales' main food source in their Arctic feeding grounds. The whales eat several tonnes a day to fatten up for their journeys.
Rising temperatures also trick the whales' biological clocks into thinking it is time to migrate.
"They are changing their migration cycles. They used to arrive here in July. Now we see them in May," said Castro.
Whales are also continuing north beyond the equator, as far as Costa Rica – a behaviour never seen before, she told AFP.
The International Whaling Commission estimates there were 8000 to 10,000 humpback whales this year in the Pacific breeding grounds, which stretch from Peru to Costa Rica.
Roger Payne, the American scientist who brought humpback whales' songs to world attention in the 1970s, said whales are also threatened by the acidification of the oceans caused by rising concentrations of carbon dioxide in the water.
Forty-five years of research off Argentina have shown that this and other effects of climate change are killing off whales' food, he said.
"The females will give birth only when the conditions to feed their young are favourable," Payne told AFP.
"Nothing is nearly as important as the threat that we get from that effect."
When there are less krill in Antarctica, birth rates drop at the equator, and calves tend to have a worse survival rate.
"Everything is linked," said Payne's Argentine colleague Mariano Sironi, a specialist in southern right whales.
In the latest alarming news, researchers said on Tuesday (local time) at least 337 dead whales had been found washed up in a remote inlet in Patagonia in southern Chile – one of the largest die-offs on record.
"It was an apocalyptic sight," said Vreni Haussermann, one of the scientists who made the discovery on a flyover in June.
It is not known what killed the whales, or if the event was linked to climate change.
The cyclical warming of the central Pacific – the El Nino phenomenon – is making matters worse and is a harbinger of the dangers to come, researchers said.