Infrared-spectrum image of an ornamented dragonfly from the genus Tramea. Lighter colours indicate hotter temperatures, ranging from 27°C to 35°C across the image. Image credit: Noah Leith. 

Researchers have found that male dragonflies with darker wings have evolved to tolerate higher temperatures and attract partners.

Temperature determines where species can live and if they are threatened by a warming climate. So, for a long time, biologists studied how heat tolerance affects survival. Yet, less is known about how thermal traits influence reproduction, which is directly linked to extinction risk.

Now, researchers in the US have examined if males of dragonfly species that produce sexual signals in the form of dark colouration on their wings are more resistant to heat.

Dr Noah Leith, a biologist at the University of Pittsburgh, said: “We show that dragonfly species that have evolved dark breeding colouration on their wings have also evolved the ability to tolerate high temperatures.

“This finding paves the way for a whole new field of research exploring interactions between thermal traits and sexual signals.”

Visible-spectrum image of a dragonfly with dark wing colouration form the genus Tramea. Image credit: Noah Leith.

Dark spots, hot dragonflies

In dragonflies – same as in many animals – sexual signals can help them effectively locate mates, identify the correct species to mate with, and decide when to back out of mating contests.

Producing extensive dark wing colouration, though, comes at a cost: dark ornaments absorb extra heat, increasing dragonflies’ body temperature. This can cause physiological stress or lead to males abandoning reproductive territories.

Leith said: “We see time and time again that animals will put their lives on the line to reproduce, even if it means encountering potentially lethal temperatures.”

The researchers examined the wing colouration of 14 dragonfly species living in tropical climates and five species living in temperate climates.

They found that species that possess dark, heat-absorbing wing colouration have evolved to be able to withstand higher heat stress before reaching critical thermal maxima.

Leith explained: “This enhanced ability to tolerate high body temperatures is likely crucial for shaping how dragonflies may respond to the changing climates of the future.”

Beat the heat

Dark wing ornaments cause additional heating of 1°C to 2°C, which roughly equals the increased thermal maxima of ornamented species.

Of the species studied, the arch-tipped glider (Tauriphila argo), a tropical species with very dark wing colour patches near their core body, could tolerate the highest temperatures.

Generally, this pattern of co-evolution was even stronger in tropical species.

Previous research showed that due to rising temperatures worldwide, some ornamented dragonfly species are evolving reduced wing colouration.

The present results, however, suggest that even if those species lose their colouration, they will still have a leg up on adaptation to climate change because they’ve already evolved to tolerate hotter temperatures, the researchers said.

Preventing extinction

The study is one of the first to test whether thermal tolerance co-evolves with reproductive traits.

Leith advised: “Our finding is particularly exciting because dark sexual colouration has evolved over and over across the tree of life and causes a wide variety of other animals to absorb extra heat too – from reptiles, to lions, and fruit flies.”

In a rapidly warming world, being able to predict which species are vulnerable to extinction is essential to preserving biodiversity, the researchers said.

Leith said: “Looking at vulnerability in only one aspect of animals’ lives is insufficient.

“We need a more nuanced understanding of how animals respond to changing environments as whole, complex organisms, in which their reproductive traits might influence their chances of surviving a heat wave, and vice versa.”

While the researchers noted that looking at 19 species was plenty for their analysis, they said that there are thousands of dragonfly species.

Future research should examine if similar patterns exist in other species, as well as in different types of animals.

Leith concluded: “It would be fantastic to someday test if heat tolerance co-evolves with sexual traits across life on Earth.”

The research is published in Frontiers in Ethology.