Anacondas Have Been Giants for 12.4 Million Years — And They Never Shrunk

For as long as people have encountered them, anacondas have been among the most imposing predators of South America’s wetlands. The massive snakes typically measure four to five meters long, with some individuals exceeding six meters. New fossil research shows that anacondas reached and have retained this enormous size since the Miocene — roughly 12.4 million years ago.

During the Miocene, warm temperatures, expansive wetlands, and abundant prey supported many giant animals. Freshwater turtles the size of small cars and caimans up to 12 meters once roamed South America; most of those giants later went extinct as climates cooled and environments changed. Anacondas, however, appear to be an exception.

To test whether ancient anacondas were larger than today’s snakes, researchers examined 183 fossilized vertebrae from 32 individual snakes recovered in northern Venezuela and dated to the Middle and Upper Miocene. By comparing vertebral size and shape with modern specimens, the team estimated total body length for each fossil specimen.

The study found the average Miocene anaconda measured about 5.2 meters long — essentially the same average size observed in modern anacondas.

“We expected to find the ancient anacondas were seven or eight meters long,” said study co-author Andres Alfonso-Rojas. “But we don’t have any evidence of a larger snake from the Miocene when global temperatures were warmer.”

The result surprised many scientists because the prevailing expectation was that animals living during warmer, richer eras would often be larger than their modern descendants. Instead, anacondas appear to have reached an optimal large size early in their evolutionary history and retained it through major environmental shifts.

“Species like giant crocodiles and giant turtles have gone extinct since the Miocene... but the giant anacondas have survived,” Alfonso-Rojas added. “They are super-resilient.”

Why anacondas have not shrunk over time is still uncertain. Hypotheses include relatively low competition for large prey, life-history traits that favor large size, and constraints on how climate influences body size in large ectotherms. The findings challenge simple assumptions that climate warming always leads to larger cold-blooded animals and highlight the value of the fossil record for testing evolutionary hypotheses.

Significance: The study shows that anacondas reached modern-scale gigantism at least 12.4 million years ago and retained that size despite the extinction of many other contemporaneous giants, offering new insight into ecological resilience and the drivers of body size evolution in reptiles.