Scientists compared flies from cool California forests with those from hot Southwestern deserts; a 2025 study found their brains responded differently to heat

For one species of fly, heat is a warning sign to escape. For another, the same warmth can be a signal to move closer. Scientists discovered this surprising difference after comparing flies that evolved in two very different environments: the cool forests of Northern California and the hot deserts of the American Southwest. Studies found that the brains of flies are able to react to heat signals in different manners.

In a 2025 study, scientists compared Drosophila persimilis, a fly species found in cooler California forests, with Drosophila mojavensis, a species adapted to the hot deserts of the Southwestern United States, to see how evolution affected their brains in response to heat. Results showed that forest flies were averse to heat while desert flies were drawn to it. Northwestern University’s report on the study states that the difference was linked to changes in the flies’ nervous systems, showing how evolution can reshape the brain’s response to the same environmental signal.

Two flies shaped by opposite climates


Although the two fly species share a common evolutionary history, they developed in environments with very different temperature conditions. Drosophila persimilis lives in cooler, higher-altitude areas of California, where the climate is relatively mild. This species interpreted extreme heat as a warning. By relocating from warmer regions, it can escape from conditions that may be detrimental to its survival. Drosophila mojavensis, however, has developed in desert environments where high temperatures are a way of life. This species learned to prefer warmer places that offer conditions similar to those found in its natural habitat rather than heat as a warning. Scientists wanted to understand how these different behaviors developed. The answer was not only found in the flies’ surroundings but also inside their brains.

The test that separates forest flies from desert flies

To compare the two species, researchers placed the flies in environments where they could move between different temperature zones. This allowed scientists to see how the insects chose to spend their time between cooler and warmer parts. The results showed a clear difference. The forest flies were pushed away from warmer temperatures, while the desert flies actively moved toward warmer temperatures. The shock of the difference was that both species were reacting to the same physical stimulus, heat. But their brains had given different interpretations to that signal. Warmth was a danger for one fly, while for the other it represented an appropriate habitat. This demonstrated to the scientists that adaptation to climate is not simply survival in extreme environments, but it can involve changes in how an animal senses and responds to the world around it.

The neural differences hidden behind heat behavior

Scientists investigated the biological systems responsible for these different behaviors. They found that the flies’ temperature reaction was linked to the way sensory information was processed. The scientists examined the temperature-sensing receptor Gr28b.d, which is used to sense heat. These differences in the system affected the response of the flies to the various temperatures to which they were exposed. The study also revealed changes in the lateral horn, an area in the insect brain that plays a role in the processing of sensory information and directing behavior. It was discovered that the same type of environmental signal could produce opposite behavioral responses because the brain circuits handling that information had evolved differently.

What fly brains reveal about climate adaptation

These choices are crucial for insects when it comes to food, breeding, and escaping harmful situations. Even small local temperature shifts can influence the distribution and range of populations. This is particularly significant as insects react rapidly to changes in the environment. As temperatures increase, their numbers can change, and this impacts ecosystems as they serve as pollinators, decomposers, and sources of food for other animals. The knowledge of how these decisions are made can be used to guide researchers in predicting the ability of a species to adapt to global warming, as well as those that will struggle if their environment becomes inhospitable.

How evolution creates different survival strategies

A forest fly and a desert fly are faced with the same problem but come up with opposite answers. The desert fly was not successful just because it was able to handle high temperatures. Instead, its nervous system has evolved to make it react to conditions in which it can survive. This difference shows that adaptation is not only written in an animal’s body size or physical features. It can also be hidden in the way its brain interprets information from the environment.

A blueprint hidden inside insect evolution

Findings also provide a way to study how small genetic changes can produce major differences in animal behavior. Because closely related fly species can develop opposite temperature preferences, scientists can compare their genes and neural circuits to identify the biological changes that drive adaptation. This approach provides an opportunity to explore other species experiencing seemingly shifting environments, from insects invading new areas to changing behavior in animals along with temperature change. The findings show that evolution can reshape not only an animal’s body but also how its brain responds to the environment.