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Amongst the vast expanse of North America lives a beautiful little insect called the Blue Dasher. A dazzlingly bright Dragonfly, their wing colouration play’s an important role in mating success, and yet, it is also dependent on the temperature of their environment.
The Blue Dashers, also known by their rather amusing scientific name, Pachydiplax longipennis, are widespread across North America. They exhibit sexual dimorphism in body colour. This just means that the males look different to the females. Interestingly, the blue colouration of the male Blue Dasher is not a result of pigments, but is actually caused by the scattering of light across its body. In the same sort of way that the sky appears blue! However in this article, it’s the colour of the wings that are of most interest.
A group of scientists from Ohio, USA, showed that males of this species produce brown patches on their wings [1]. These patches are believed to intimidate rivals and to help them to gain territories. The dragonfly with the most territorial success, usually has the most mating success too. Meaning that they can pass on more of their genes to the next generation as they have a greater number of offspring. In zoological terms, this is also called having a greater fitness.
Over time, males have been sexually selected to produce darker wing pigmentations, as it is the darker individuals that are mating most successfully.
In that case, you would expect all males of the Blue Dasher to have dark wings, right?
Wrong. The story doesn’t end there.
It turns out that the extent of the brown wing colouration can vary depending on environmental conditions; specifically the temperature.
Why would temperature affect the amount of brown patches on a dragonfly’s wing? I hear you ask, well let me explain…
The wing colouration alters the body temperature of the male flies. This is because the dark brown pigments absorb more heat than other areas of the wing. We can all relate to this feeling when we wear a black t-shirt in the sun! If the dragonfly has a greater pigmentation on the wings, their body temperature increases.
Insects are ectothermic. This means they can’t produce their own body heat, so instead rely on heat from their environment to warm their bodies.
In cold environments, absorbing heat into the body is beneficial as it enhances their flight performance. However, in hot environments, too much heat absorption would dramatically reduce flight performance.
This means when experiencing a hotter environment, it is more beneficial for the dragonfly to reduce the amount of heat absorption (a.k.a reduce the colouration on the wings), and vice versa in a cold environment.
So although males gain more territories, mating success and overall fitness from the presence of brown pigmentation on their wings, when presented with hot environmental temperatures – the costs from the increased absorption of heat outweighs those benefits.
Therefore, as predicted, the study found that male wing coloration was greatly reduced in the hottest parts of the species’ range.
As the global climate begins to change, it’s clear to see that like every creature on Earth, the Blue Dasher will not escape the rising temperature unaffected. Perhaps males in the future will have fewer brown patches, and therefore lower mating success? Or perhaps the range of the Blue Dasher will move upward, or become pinched into a smaller area.
One thing’s for sure, and that is with global warming well on it’s way, The Blue Dasher will feel it.
The original research article can be found here:
[1]. Moore, M.P., Lis, C., Gherghel, I. and Martin, R.A., 2019. Temperature shapes the costs, benefits and geographic diversification of sexual coloration in a dragonfly. Ecology letters.
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