Mr. Yuan ventured to the Izu Islands with a tripod and a makeshift circuit on his back. After Mr. Yuan caught lizards on Kozu, an island with snakes, as well as the snake-free and weasel-free Hachijo-Kojima, he recorded the lizards as they ran along the route with a camera in order to calculate their speed at different body temperatures.

Lizards and snakes are both ectotherms, which means that their body temperatures are dependent on their surrounding thermal environment. Body temperature in turn influences their ability to move. Immobile in cold temperatures, their speed of movement increases as they warm up before they plateau and quickly decrease when it gets too hot.

The researchers found that the lizards on the islands with snakes ran faster at higher temperatures, suggesting they are better adapted to warmer body temperatures, Yuan said. At these higher body temperatures, the lizards sprinted faster than the snakes could crawl.

The presence of predators also had evolutionary effects on the lizards’ bodies; Lizards on islands with snakes had longer hind legs.

“We essentially associate the presence of the snakes with the lizards that run faster, those lizards with longer legs, and those lizards that search at warmer body temperatures,” said Yuan, lead author of the study.

The data collected for the study also highlighted another incidental, albeit worrying, finding. The Izu Islands have only got warmer since Dr. Hasegawa entered it for the first time, as did the lizards. From 1981 to 2019, the average body temperature of lizards on all islands rose by 2.3 degrees Fahrenheit, which correlated with global warming.

Rising temperatures can threaten ectotherms, which make up the majority of animal species, and alter the dynamics of prey-predator relationships, said Shane Campbell-Staton, an evolutionary biologist at the University of California at Los Angeles who was not involved in the study.