The snakes stayed large and thrived even when cooling temperatures and shrinking habitats killed off other giant reptiles millions of years ago.
Anacondas Have Stayed Giant for Over 12 Million Years, Study Finds
New research reveals that anacondas have been enormous for millions of years — and they haven’t gotten any smaller.
According to a study published in the Journal of Vertebrate Paleontology, the average body size of anacondas has remained remarkably stable since they first appeared in the fossil record around 12.4 million years ago, during the Middle Miocene epoch.
That finding surprised scientists because many animals from that same warm, ancient period were much larger than their modern relatives — and most of those giants eventually disappeared.
During the Miocene, global temperatures were warmer, wetlands were more widespread, and food was plentiful. This allowed numerous species to evolve massive sizes. However, as climates cooled and habitats shrank, giant crocodiles, turtles, and other large animals went extinct.
“But giant anacondas made it through,” said study co-author Andrés Alfonso-Rojas, a paleontologist at the University of Cambridge. “They’re incredibly resilient.”
Today’s anacondas are already the heaviest snakes on Earth. Most are around 13 to 16 feet (4 to 5 meters) long, with the largest reaching about 23 feet (7 meters). Until now, scientists weren’t sure if ancient anacondas were even bigger, or if modern snakes simply carried that size forward.
To find out, researchers examined 183 fossilized vertebrae from at least 32 ancient anacondas found in Venezuela. They also used a scientific method called ancestral state reconstruction, which estimates body size based on features of related species.
Their results showed that ancient anacondas averaged about 17 feet (5.2 meters) in length — essentially the same size as today’s snakes.
“That was unexpected,” Alfonso-Rojas said. “We thought they might have been much longer — possibly over 23 feet. But we found no evidence that they were bigger than modern anacondas.”
What remains a mystery is why anacondas haven’t shrunk over time.
Scientists also found that their size doesn’t seem to be controlled by predator-prey relationships or food competition. Even when new predators arrived in South America millions of years later, and ecosystems changed during the Pliocene and Pleistocene eras, anacondas stayed big.
The study suggests their giant size became established early in their evolutionary history — and whatever forces made them large in the first place weren’t strong enough to make them smaller later on.