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

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

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

The average size of the enormous snakes 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 found in a new study published Monday (Dec. 1) in the journal. Journal of Vertebrate Paleontology.

“Other species like giant crocodiles and giant tortoises have gone extinct since the Miocene, likely due to cooling global temperatures and shrinking habitats,” co-author of the study. Andrés Alfonso-Rojasvertebrate paleontologist at the University of Cambridge, said in a statement. “But the giant anacondas survived: they are extremely resistant.”

Anacondas constitute a group of constrictor snakes which today includes the heaviest species of snake in the world. Modern anacondas average 13 to 16 feet (4 to 5 meters) in length, although the largest can reach up to 23 feet (7 m). Scientists were unsure whether anacondas were even larger in the Miocene, or whether they had been the same size and retained their massive size to the present day.

To estimate the size of ancient anacondas, 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 length of ancient anacondas from the characteristics of related snakes.

Five fossil vertebrae of an ancient anaconda on an aqua background

Researchers measured the fossilized anaconda vertebrae to determine the body length of ancient snakes. (Image credit: Jorge Carrillo-Briceño)

Based on these calculations, the team found that anacondas were on average about 5.2 m long when they first appeared in the Miocene, 12 million years ago, about the same length as modern anacondas.

“This is a surprising result because we expected to find the ancient anacondas measuring seven or eight meters [23 to 26 feet] “But we have no evidence of a larger snake from the Miocene, when global temperatures were warmer.”

It’s still unclear why anacondas haven’t gotten smaller over time.

Although warm weather and abundant wetlands may have allowed anacondas to reach their giant size early in their evolutionary history, cooler temperatures and shrinking ranges did not force the snakes to become smaller to adapt. This could suggest that these were not the main factors that maintained snakes’ sizes over the ensuing millennia, the researchers wrote in the study.

Predator-prey interactions probably also didn’t play a major role in maintaining the snakes’ body sizes, the researchers said. A lack of competition for food could have helped the snakes grow in the first place. But they did not decline as other predators moved into South America during the Pliocene (5.3 to 2.6 million years ago) and Pleistocene (2.6 million to 11,700 years ago), suggesting that food availability is not a significant factor in the giant size of anacondas.