Whether it's birdwatching or catching Pokémon, everybody knows that different species have different geographical ranges. Some species like Canada geese, have far-flung populations spread across an entire continent, while others like the Christmas Island frigatebird are found only on a single tiny island in the middle of the ocean. While explanations for why some species have broader ranges than others vary, for diving beetles it might come down to their ability to perform under stress.
Diving beetles in the genus Deronectes are found throughout Europe; some species are found over areas covering thousands of kilometres, while others only cover an area with a 100 km radius. Rebekah Cioffi and her colleagues at Plymouth University, UK, along with Andrés Millán from the University of Murcia, Spain, thought each species’ distribution might be linked to their physiological plasticity in response to thermal stress. Since the climate of regions distributed towards the poles tends to be more variable than those in more equatorial regions and the species with the most widespread ranges tend to occur in more polar areas, the researchers thought that perhaps only those species that are able to rapidly adjust how their bodies function can become geographically widespread.
To test this idea, the researchers set out on a trans-European mission to collect five closely related species of Deronectes beetles, then brought them back to the lab to test their performance under heat stress. They warmed up the beetles to 15, 20, 25, 30 or 35°C, then held them at that temperature for 24 h before placing them in the deep freeze until they could later measure the beetles’ metabolic stores, such as carbohydrates (glycogen and glucose) and fats. The researchers also measured the beetles’ ability to mount an immune response and defined their plasticity as the difference between the highest and lowest values for each measurement.
Cioffa and colleagues found that the value that best explained the breadth of distribution of a diving beetle species included both the degree of plasticity (how much the measure of lipids, glycogen and glucose changed after changing temperatures) as well as how strongly the insect's immune system responded to pathogens. In particular, diving beetles that had more northerly and widely distributed ranges also seemed to have a greater ability to burn fat under thermal stress, which makes sense because fats are a more dense form of energy storage than carbohydrates and the fat body in insects is also involved in the production of important immune molecules such as antimicrobial peptides.
While the ability to survive harsher conditions closer to the poles is still important for an insect species to spread across vast geographical ranges, the researchers suggest that for diving beetles at least, having physiological systems that can bend with changing temperatures can be another important trait that allows them to spread far and wide.
