Jan. 7 (UPI) -- Crocodiles are a hardy group -- they've been around for more than 200 million years -- but the reptiles aren't particularly prolific, evolutionarily speaking.
Modern crocodiles look much the same as their ancestors that lived alongside dinosaurs, and research published Thursday in the journal Nature Communications Biology suggests they haven't changed much because they haven't really had a reason.
"Crocodiles are interesting because they have changed very little in the time that they have been on the earth, and they have only a few species," lead study author Max Stockdale told UPI in an email.
"There are 24 species of crocodile, compared to over 10,000 species of birds, for example," said Stockdale, a teaching assistant at the University of Bristol.
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In addition to being intriguing, from an evolutionary perspective, crocodiles are also relatively abundant in the fossil record.
"Crocodiles are excellent subjects for studying palaeontology," Stockdale said. "Since many of them live in water their remains are very likely to be buried; many forms also have tough armor and thick skulls, so they are well-represented in the fossil record since they are resistant to decay."
To quantify the crocodile's sluggish evolutionary pace, Stockdale and his research partner used a unique machine learning algorithm called a Markov chain.
"This algorithm starts with an initial assumption and then improves it iteratively, until the best-fitting solution is found and cannot be improved any further," he said.
Stockdale and study co-author Michael Benton, a professor of vertebrate paleontology at Bristol, used body size to measure the rate of evolution among crocodiles. Their analysis showed crocodiles are characterized by an extremely low evolutionary rate.
"Low evolutionary rate means that the amount of morphological change, in this case body size, is small per unit of time," Stockdale told UPI. "This means the crocodiles had gone into a state of equilibrium, which they maintained until the environment changed and forced them to adapt."
The research suggests crocodiles don't dabble in physiological experimentation unless they really have to -- they stick with what works. The reason for this unique evolutionary strategy remain unknown.
"It isn't clear why evolution in some animals is driven by the environment and in others it is not," Stockdaile said. "It could be to do with how well they control their body temperature -- crocodiles are reptiles and cannot control their body temperature, so perhaps they are more likely to suffer if their environment changes."
In followup studies, Stockdale and Benton plan to investigate whether attributes other than body size follow a similar evolutionary pattern.
They also hope to figure out which environmental factors -- temperature, rainfall, sea level -- were most important in triggering evolutionary changes.
