As environmental temperatures rise, animals face a tough decision: seek out cooler areas or brave the heat. Both options come with significant risks. Venturing into the unknown could expose them to even hotter temperatures or other hazards, such as predators. In contrast, staying put forces them to endure temperatures that could make it more difficult for them to mate, move around and survive. As a result, predicting what animals will do when confronted with changing temperatures is incredibly difficult. This is especially important for tiny animals like insects, where temperature can vary drastically within the small areas they often live in. To better understand how animals overcome these challenges, Noah Leith and colleagues from Saint Luis University, USA, investigated the ability of insects called treehoppers (Enchenopa binotata) to move around their plant environment to avoid harmful temperatures.
Earlier work by Leith showed that treehoppers mate less at temperatures above 29.6°C. Building from this, the team wanted to find out whether the insects encounter temperatures higher than 29.6°C in the wild, and whether they avoid the extreme heat to ensure successful mating. So, the scientists created outdoor treehopper houses, consisting of a cage surrounding a 30 cm tall plant. Then, they placed fake 3D-printed treehoppers in the cages and used thermal imaging cameras to measure the temperatures of the fake insects on the plant. The results were startling. Even though air temperature differed by only 10.6°C throughout the study, temperature on a single plant at any given time could vary from 23.4 to 42.1°C and frequently exceeded 36°C – a temperature that can kill treehoppers. Additionally, when the air temperature rose above 28°C, the entire plant became hotter than the highest mating temperature of 29.6°C, meaning that rising environmental temperatures could have disastrous effects on treehopper populations.
Once the team knew the range of temperatures the treehoppers faced, they needed to figure out which temperatures the insects spent their time in. Leith and colleagues ran a similar test as before but used real treehoppers instead of the fakes. Although the treehoppers avoided the lethal temperatures above 36°C, they often settled in spots hotter than the maximum mating temperature of 29.6°C, even though the experiments happened during the mating season. The team also measured whether the treehoppers jumped off their home plant during the trials to rapidly escape from extreme heat. They found that once the maximum temperature in the cage exceeded 36°C, treehoppers started leaping off their home plant. While the treehoppers did not seek out temperatures to optimize mating, they readily hopped away from temperatures that could kill them.
To confirm their findings, the team ran a second test in the lab, where the insects were placed on a long wooden dowel that was cold (20°C) on one end and hot (45°C) on the other. The dowel was surrounded by mesh, making it impossible for the treehoppers to jump off. Any heat avoidance would have to come from the insects actively looking for the best temperature on the dowel. The team allowed the treehoppers plenty of time to search for a spot on the dowel before measuring the temperature where the insects ultimately settled. While most hung out at cooler non-lethal temperatures, many of the treehoppers were found at temperatures over 29.6°C and 13% were even found above 36°C. When they couldn't jump away, the insects did not distinguish between safe and harmful temperatures. This suggests that treehoppers rely solely on their ability to quickly flee from an area to avoid extreme heat as opposed to actively searching for a better temperature. This teaches us an important lesson about how animals respond to environmental change: even if better conditions are near, animals might not be able to find them.
