Developmental responses to temperature vary across populations from a latitudinal cline in a temperate butterfly

Global warming is causing temperatures to rise, but how do insects that originate from different areas cope with this? Researchers from the University of Helsinki found different strategies between Glanville fritillary butterflies from the north and south.

Global temperatures are rising because of climate change, which can have large effects on animals. One important question these days is therefore, how do different organisms respond to heat? This is especially relevant in insects, because their body temperature is often directly dependent on the outside temperature. “Insects are a large and diverse group of animals that play important roles in ecosystems and human lives, which make them even more interesting to study in this context”, says Nadja Verspagen, PhD researcher and lead author of the paper.

One way to study how insects respond to different temperatures is by using latitudinal clines. Ambient temperature varies with latitude: generally, the climate is warmer and more stable near the equator and gets colder and more variable towards the poles. By taking populations at different places across this cline and rearing them under the same temperature treatments, we can test whether they respond differently to temperature.

Using the well-known Glanville fritillary model system

For this study, we reared caterpillars of the Glanville fritillary butterfly from a latitudinal cline that ranges from Finland to Spain at four different temperatures in the laboratory. The Glanville fritillary is a well-known model species with a long history in Helsinki, that has contributed significantly to our understanding of various ecological and evolutionary processes. Like many other insects, they go through different discrete life-stages to complete their lifecycle and they use diapause, similar to hibernation, to survive winter. Timing of these life-stages is of crucial importance for survival.

Clear differences between northern and southern caterpillars

We found clear differences in how caterpillars from different regions responded to temperature. While all caterpillars grew faster and larger when the temperature increased, those from the north adopted a different strategy than those from the south. This was especially the case at low temperatures. Caterpillars from the north developed fast even when the temperature was low, but because of this they did not manage to grow big at low temperatures. Southern larvae, on the other hand, developed slowly but manage to reach a larger body size when temperatures were low.

This can be explained by the differences in climate across latitude. While the suitable growing season (spring and summer) is long in the south, it is much shorter in the north. This results in pressure to grow fast for northern larvae, whereas this pressure is less intense for southern caterpillars. “This is an exciting example of how different populations of the same species can respond differently to the environmental conditions they encounter, depending on the location they originate from”, says Nadja. “It also highlights the need to assess risk to climate change below the species level”.

Original paper:

Verspagen, N., Ikonen, S., Maes, D., Stefanescu, C., DiLeo, M. F., & Saastamoinen, M. (2023). Thermal plasticity in development and diapause strategy in a temperate butterfly across a latitudinal gradient. Functional Ecology, 00, 1–13.