As the planet warms, animals are leaving their usual homes and venturing closer to the poles or higher into the mountains to escape the heat. Normally, these areas are protected from many insects because becoming too cold leads to a coma as their central nervous system stops working. Although the insects can survive these comas for a limited amount of time, being unable to move leaves them at risk of becoming an easy meal for predators. With this in mind, Mads Andersen and Heath MacMillan of Carleton University, Canada, working with Quentin Willot of Aarhus University, Denmark, wanted to know if this cold-induced coma – and the events happening in the nervous system that cause it – are common among all insects. Perhaps surprisingly, they turned to several species of tropical butterfly to help them answer this intriguing question.
Andersen and colleagues began the difficult task of recording the electrical activity in the brains of 12 different species of tropical butterflies while slowly cooling them down. As they got colder, every butterfly eventually lost brain function, suggesting they would fall into a coma. Surprisingly, the butterflies did so within a narrow range of temperatures between ∼3.0°C and ∼5.3°C, depending on the species. The researchers point out that cold coma has been linked to the temperature at which other insects, such as fruit flies and locusts, stop being able to coordinate their movements. In most insects, the coma is caused by an imbalance in the normal amounts of sodium and potassium ions inside and outside the nerve cells of the brain. Usually, there is more potassium inside the cells than outside, but the cold temperatures cause a rush of potassium to leave the cells, shutting them down until they can restore the usual balance of these ions. Although this happens slightly differently in butterflies, the team believe that existence of the cold-induced coma in such distantly related species suggests that this is what causes the loss of movement in all insects when the temperatures keep dropping. But can the researchers predict at what temperature the coma will start depending on the species?
Andersen and colleagues constructed an evolutionary tree showing that related species of butterflies didn't necessarily fall into the coma at similar temperatures. However, when the researchers looked at where the butterflies originated, a pattern started to emerge. The temperature at which the coma would begin depended on how cold the coldest month was where the butterflies lived and the elevation they lived at. This means that the butterflies from the coldest climates or highest altitudes also had cold-induced comas beginning at the lowest temperatures. The researchers are quick to point out that the temperatures that cause these comas are 11.5°C below the average temperature of the coldest month that they would experience in the wild.
While the team didn't find the differences that they were expecting, they suggest that the phenomenon of falling into a coma because you are too cold may exist in all insects since the butterflies in this study aren't closely related to either fruit flies or locusts. This also suggested that the temperature that causes the coma could predict where certain insects are able to survive. As the world gets warmer, animals are going to be found in new and unexpected places, but it might be the cold that stops them from going any further.