Anacondas became massive 12 million years ago — and it worked so well, they haven't changed size since

Anacondas have been giant for millions of years, a new study finds.

The enormous snakes' average body size has remained constant since they first appeared in the fossil record about 12.4 million years ago, during the Middle Miocene (16 million to 11.6 million years ago), researchers revealed in a new study published Monday (Dec. 1) in the Journal of Vertebrate Paleontology.

During the Middle and Upper Miocene (12.4 million to 5.3 million years ago), warm temperatures, expansive wetlands and abundant food enabled many animal species to grow much larger than their modern relatives. But few of these giant animals have survived to the present day.

"Other species like giant crocodiles and giant turtles have gone extinct since the Miocene, probably due to cooling global temperatures and shrinking habitats," study co-author Andrés Alfonso-Rojas, a vertebrate paleontologist at the University of Cambridge, said in a statement. "But the giant anacondas have survived — they are super-resilient."

Anacondas make up a group of constricting snakes that today includes the heaviest snake species in the world. Modern anacondas average 13 to 16 feet (4 to 5 meters) in length, though the largest can reach up to 23 feet (7 m). Scientists weren't sure whether anacondas had been even larger during the Miocene, or whether they had been the same size and retained their massive size into the present day.

To estimate how big ancient anacondas might have been, Alfonso-Rojas and his colleagues measured 183 fossilized anaconda vertebrae from at least 32 individual snakes collected in Venezuela. They also used a technique called ancestral state reconstruction to predict the body lengths of ancient anacondas from characteristics of related snakes.

Based on these calculations, the team found that anacondas averaged about 17 feet (5.2 m) long when they first appeared during the Miocene 12 million years ago — roughly the same length as modern anacondas.

"This is a surprising result because we expected to find the ancient anacondas were seven or eight meters [23 to 26 feet] long," Alfonso-Rojas said in the statement. "But we don't have any evidence of a larger snake from the Miocene when global temperatures were warmer."

It's still unclear why anacondas have not become smaller over time.

Although warm weather and abundant wetlands may have enabled anacondas to reach their giant size early in their evolutionary history, cooler temperatures and shrinking ranges haven't forced the snakes to get smaller to adapt. That could suggest that these weren't the primary factors keeping the snakes large in the intervening millennia, the researchers wrote in the study.

Predator-prey interactions likely didn't play a major role in maintaining the snakes' body size, either, the researchers said. A lack of competition for food could have helped the snakes grow large in the first place. But they didn't get smaller as other predators moved into South America during the Pliocene (5.3 million to 2.6 million years ago) and the Pleistocene (2.6 million to 11,700 years ago), suggesting that food availability isn't a big factor in anacondas' giant size.

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