Like many birds and mammals today, ancient plant-eating dinosaurs migrated hundreds of miles each year as seasons changed, according to a study published online Wednesday by the journal Nature.

Scientists have long suspected that camarasaurus - a 50-foot-long, 20-ton dinosaur that lived 145 million years ago during the late Jurassic Period - migrated.

But what really surprised scientists was how far these big lizards walked: a six-month, 186-mile trek from lowlands to the mountains and then back again.

"That's a lot of  walking to do over the course of a year," said the study's lead scientist, Henry Fricke of Colorado College.

The research touches on key questions among dinosaur experts: How did these giant beasts behave, and why were they so big?

Fossilized teeth and chemicals called oxygen isotopes may have unlocked a few clues.

Fricke and his team spent four years analyzing oxygen isotopes in fossilized camarasaurus teeth found in Wyoming and Utah.

Here's a basic idea of how it worked. Water across the ancient landscape contained specific ratios of two isotopes: oxygen 18, which has eight protons and 10 neutrons in its nucleus, and oxygen 16, which has eight protons and eight neutron in its nucleus.  Researchers were able to track locations where the dinosaurs drank their water by examining the isotopes built up in the fossilized tooth enamel, like a "tiny tape recorder of what animals were drinking," Fricke said. From this data, scientists tracked the dinosaurs from lowlands in what is now Wyoming and Utah to mountainous regions to the west.
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