A new study finds that anacondas have been giants for millions of years.
The average body size of huge snakes has remained constant since they first appeared in the fossil record about 12.4 million years ago, during the Middle Miocene (16-11.6 million years ago), researchers showed in a new study published Monday (December 1) in the journal Journal of Vertebrate Paleontology.
“Other species, such as giant crocodiles and giant tortoises, have gone extinct since the Miocene, likely due to global cooling and habitat loss,” study co-author. Andres Alfonso-Rojasa vertebrate paleontologist from the University of Cambridge said in his report statement. “But the giant anacondas survived—they are super-hardy.”
Anacondas form a group of constrictor snakes that today includes heaviest snake species in the world. Modern anacondas average 13 to 16 feet (4 to 5 meters) in length, although the largest can reach 23 feet (7 m). Scientists weren't sure whether anacondas were even larger in the Miocene or whether they were the same size and retained their enormous size to the present day.
To estimate how large 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 method called ancestral state reconstruction to predict the body length of ancient anacondas based on the characteristics of related snakes.
Based on these calculations, the team found that anacondas averaged about 17 feet (5.2 m) in length 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 that ancient anacondas were seven or eight meters tall. [23 to 26 feet] long,” Alfonso-Rojas said in a statement. “But we have no evidence of a larger snake from the Miocene, when global temperatures were higher.”
It is still unclear why anacondas have not become smaller over time.
While warm weather and abundant wetlands may have allowed anacondas to reach gigantic sizes early in their evolutionary history, cooler temperatures and shrinking ranges have not forced the snakes to become smaller in order to adapt. This may suggest that these were not the main factors that maintained the size of snakes over the intervening millennia, the researchers wrote in the study.
Predator-prey interactions also likely did not play a major role in maintaining the snakes' body size, the researchers said. The lack of competition for food may have helped the snakes grow larger. But they didn't get smaller as other predators moved into South America during the Pliocene (5.3–2.6 million years ago). Pleistocene (2.6 million to 11,700 years ago), suggesting that food availability is not a major factor in anacondas' gigantic size.
