Higher temperatures do not necessarily mean darker wings, study on Australian moth species shows

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Two primary factors determine the colour of wings in insects. One of those is heat regulation: the darker the colour, the more heat they will absorb. This is an advantage in colder climes, but a disadvantage in hotter ones. Thus, the ‘thermal melanism hypothesis’ maintains that insect populations that grow in cooler areas will be darker, but those that grow in hotter areas will be lighter in colour.

The second factor is the ability to ward off predator. This is done either by camouflaging into surroundings or by mimicking the appearance of a species that cannot be eaten by the predator. Camouflage, where the wing colour allows the insect to blend with its surroundings and evade predators. In the case of the latter – when wing patterns have developed to deter predation – evolution converges to a wing colour pattern and ‘stabilises’ there.

A study in Ecology and Evolution puts the Australian red-necked wasp moth, Amata nigriceps, under the scanner to examine whether temperature has had any influence on the species’ wing colour patterns. The general understanding, so far, as been that this moth species has developed its distinctive orange-on-black wing colour patterns to repel predators – known as an ‘aposematic’ trait.

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A nigriceps, a moth species indigenous to Australia, is typically preyed on by (like other moth species) by birds, lizards, and small rodents. Put simply, the more the orange on the wing, the more the warning signal to the predators. Predators, therefore, target moths with lower amount of orange spots, and this is ‘expected to favour more orange warning signals’. Nonetheless, there is considerable variation in wing colour patterns between populations, for which temperature could be one of the causative agents.

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In order to see whether this variation was indeed determined by temperature, Binns et al. (2022)  sampled moths from two different flight seasons (Oct to Dec and Feb to Apr), which enabled them to source moths from different environmental temperatures. Furthermore, they reared moths in laboratory conditions under three different temperatures. Upon collection, they were euthanized, and their wings were plucked for image analysis.

The study’s prediction was that lower temperatures will prolong the time taken by larvae to develop into adult individuals and result in smaller orange spots (a ‘reduced warning signal size’). Lower temperatures, it was additionally predicted, will favour more black over orange, allowing the moth to absorb more heat to maintain a suitable body temperature. But their observations did not match the predictions.

On the contrary, they found that temperature had no effect on wing colour ‘in either wild or laboratory moths. Populations under examination continued to maintain their wing pattern variation. For example, in the rearing experiment, for the batches raised under three different temperatures, the proportion of orange in the wings was between 12 to 30 per cent. However, the variation between different populations, i e those sourced from different locations in Australia, was quite significant. The pattern was also determined by the sex of the individual, with females having more orange.

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There are a few things that can possibly explain this observation. One is the high degree of genetic heritability for warning signals. This has been found for other aposematic species as well: a 2021 study on the hibiscus harlequin bug – another Australian insect – showed found that growing conditions played no role in adult colouration. Two, a factor determining the average proportion of orange/black in a population could be the predator community composition. Three, a key abiotic factor – other than temperature – which was overlooked in this study, is rainfall. A 2008 study on the common fruit fly, for instance, showed that those populations from highlands had darker wings, and, therefore, less water loss, than their lowland counterparts. Wing colouration could also influence flying behaviour, certainly useful in escaping predators.

However, not all studies have point in the same direction. A study on monarch butterflies, which inhabit a wide temperature gradient in found a clear correlation between temperature and wing colouration. So did a study on copper butterflies. Binns et al. (2022) do acknowledge that, in their experiment on A nigriceps, the difference in temperatures between the two seasons, or in the laboratory conditions, varied only by a few degrees Celsius. This was, perhaps, not enough to induce a significantly different warning signal.

Will the wing colour patterns of the Australian red-necked moth adhere to the ’thermal melanism hypothesis’ when subjected to a much wider temperature gradient? Binns and his colleagues hope to find that out in the near future.

The author is a research fellow at the Indian Institute of Science (IISc), Bengaluru, and a freelance science communicator. He tweets at @critvik

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