A Male Widow Skimmer Dragonfly. Source: Jim Braswell.
A recent study, published in Proceedings of the National Academy of Science (PNAS) in July of 2021, found that male dragonflies are losing their elaborate wing patterns as a result of increasing temperatures. These patterns, also known as melanin ornamentation, play a crucial role in a male dragonfly’s ability to find a mate, as female dragonflies use them as a means of identifying members of their species and often gravitate towards males with more pigmentation. As a result, the less ornamentation present in males, the more difficult it would be for a male dragonfly to attract a mate. Similarly, female dragonflies could ultimately struggle to recognize males of their own species. And while wing ornamentation aids both male and female dragonflies in identification, mating, and even dominance, it can do them harm as a result of the melanin pigment that gives dragonfly wings their dark brown or black color. This harm comes from melanin’s absorption of solar radiation and heat, which, as evolutionary biologist Michael Moore told CNN, raises body temperature similarly to black clothing on warm or sunny days. Indeed, melanin can raise dragonfly body temperature beneficially in cooler climates and to the point of wing damage and death in warmer ones. This damage, which seldom affects females due to their occupancy of shadier areas, suggests that dragonflies may adapt to changes in climate through changes in wing melanization.
The study, led and co-authored by Moore at Washington University in Saint Louis, Missouri, investigated this wing adaptation theory by first examining phenotypic and climatic data on 319 Nearctic dragonfly species. This data, compiled from the Global Biodiversity Information Facility (GBIF), community-science observations, and field guides, revealed sex-specific trends linking melanin ornamentation to climate, showing that instances of least pronounced wing ornamentation occur in species living in warmer regions, while the most pronounced occur in cooler ones. However, the team did not observe the same trend in female dragonflies. The findings for both male and female dragonflies are represented in the graph below:
Probability of Male and Female Wing Melanization vs. Mean Annual Temperature (Degrees Celsius). Source: Proceedings of the National Academy of Sciences.
Second, the research team investigated if individual dragonfly populations consistently experienced wing ornament evolution in response to changes in climatic conditions by collecting data on shifts in ornament size/area of over 2700 individual dragonflies from ten Nearctic dragonflies species, each widely distributed across varying climates. The data revealed that males in seven of the surveyed species, those that lived in warmer regions, had significantly less wing melanization than species that lived in colder ones. Indeed, similarly to their first set of findings, the team found no such in females. Additionally, the team averaged the data collected from the ten dragonfly species and found that as temperature increased, male ornamentation decreased. In fact, this temperature driven trend results in “male ornaments being 25.8 ± 2.0% larger than female ornaments in the coolest parts of North America, on average, but only 2.0 ± 3.3% larger in the warmest areas” (Sex-specific ornament evolution is a consistent feature of climatic adaptation across space and time in dragonflies - PNAS).
Lastly, Moore and his team investigated how wing ornamentation changed in the same ten species from 2005 to 2019, when temperatures most exceeded Nearctic averages. In doing so, the team found that during the set timeframe, males averaged less melanization in warmer years, while, yet again, the same was untrue for females. And, because the team found no net change in ornamentation from 2005 to 2019, it is believed that the observed decreases in male wing melanization are the result of changes within generations as opposed to changes across generations, suggesting that warmer temperatures consistently reduce the number of highly ornamented male dragonflies. This reduction, however, is not the sole factor in the loss of male ornamentation, as females, again, do not follow the trend.
As such, Moore and his team’s findings show that similarly to how organisms’ survival-related traits evolve to fit their environment and climate, so too do mating-related traits. Moore’s research tells us that this evolution is present in male dragonflies' adaptation to higher temperatures through the evolution of fewer and often smaller ornaments, and therefore less melanin. Additionally, because the observed trends in male dragonflies are inapplicable to female dragonflies, Moore and his team propose that in addition to their findings being important in understanding how dragonflies’ wing ornaments may change as global warming continues, they also believe that the data suggest that climate adaptation may, in fact, be a sex-specific process and that the idea should be more widely considered a possibility. In these ways, the study’s conclusions can help us better understand how climate change affects dragonflies and puts forth an intriguing question regarding potential differences in how global warming may affect species and individuals depending on their sex.