Scientists identify which butterflies are most at risk as the climate warms

By Jenna Schnuer, CNN

(CNN) — Challenging times are ahead for smaller, lighter-colored butterflies as Earth’s climate continues to warm, a new study has found.

Unlike their relatives with larger wings and darker colors, small butterflies with paler hues — particularly those belonging to the Lycaenidae family — have trouble regulating their body temperatures as the air temperature increases, according to the research.

Size often plays a role in heat tolerance for insects, but the impact of color was surprising and “may be a pattern specific to butterflies,” said lead study author Esme Ashe-Jepson, a doctoral student of zoology at the University of Cambridge in the United Kingdom. Darker-winged butterflies, regardless of wing size, had consistently better outcomes throughout the research.

The findings, published July 12 in the Journal of Animal Ecology, offer more evidence of the outsize roles wings play in keeping butterflies cool — and why color matters.

How butterflies keep their cool

Butterflies need warmth from the sun to function. But when things really heat up, they adjust their body temperature in relation to the air temperature through strategies called thermal buffering and thermal tolerance.

Thermal buffering includes physical acts like moving into a cooler, shadier area or slanting wings out of the direct path of sunlight. Butterflies with larger wings can use that surface area to absorb heat when needed, but they may also be able to relocate to a cooler area faster than small-winged butterflies, according to the study.

Thermal tolerance, on the other hand, involves physiological processes like the production of heat shock proteins. “These are molecules that many animals, butterflies and humans included, produce to protect themselves from high temperatures,” Ashe-Jepson said. “They help to stabilise and repair proteins in your body that have been damaged by high heat.”

But that process uses up a butterfly’s resources, often at the cost of growth or reproduction. Results from this study backed up earlier research that demonstrated butterflies tend to rely on either thermal buffering or thermal tolerance — rather than using both — and that makes some species more vulnerable to increasing temperatures due to the climate crisis.

The ‘knockdown’ test

To test whether there was a relationship between thermal buffering and thermal tolerance in a variety of species of tropical butterflies, researchers from the University of Cambridge and the Smithsonian Tropical Research Institute captured butterflies in many types of habitats around Panama during both the wet and dry seasons between February 2020 and March 2022.

For the thermal buffering test, the researchers caught, tested and released 1,334 butterflies representing 54 species from six butterfly families. “My team and I spent many long days running around the rainforest in Panama with butterfly nets,” Ashe-Jepson said. “Our goal was to catch a butterfly without chasing it.”

When the insects fly, their body temperatures can change. Since the butterflies were not forced into a stressful chase by the researchers, their temperatures were expected to remain at normal levels for the environmental conditions.

After capture, the team members placed a measuring device called a thermocouple — which looks like an exposed wire — through the net and then ever so gently against the butterfly’s thorax, which is the middle part of its body, to record its temperature.

The thorax “is where the muscles that power butterfly flight are, and so are important body parts to maintain the temperature of,” Ashe-Jepson said.

To test thermal tolerance, a smaller group of the captured butterflies was put to work one more time. Each one was held in a glass jar set up to keep the butterfly from dehydrating. Then the temperature was steadily raised inside the jar to test how much each could stand before falling after losing motor control, a state known as “knockdown.”

In both experiments, dark butterflies the researchers captured warmed up and cooled down better than their paler relatives. Larger, darker butterflies more effectively used thermal buffering strategies to their advantage, while the smaller and darker butterflies were “able to tolerate higher temperatures” during the in-glass thermal tolerance test,” Ashe-Jepson said. “This places these dark butterflies in a relatively good place to cope with climate change.”

She added that the difference between the lowest and highest heat tolerance levels — more than 18 degrees Fahrenheit (10 degrees Celsius) — really surprised her. “This is a massive difference for butterflies that live in relatively similar habitats,” Ashe-Jepson said, noting that one species survived temperatures of almost 140 degrees Fahrenheit (60 degrees Celsius). “For such a small animal, that is an incredible feat.”

Why butterfly color matters

During the knockdown test, the larger butterflies that had demonstrated the best thermal buffering abilities did not do as well as smaller-winged butterflies. Larger species tend to rely more on buffering strategies, leaving them vulnerable to temperature increases when they can’t change their position.

“This result implies that species with more stable body temperatures may be more vulnerable to increases in ambient temperatures,” said Akito Kawahara, professor and curator at the McGuire Center for Lepidoptera and Biodiversity at the University of Florida in Gainesville. He was not involved with the research.

Ashe-Jepson’s theory on why color makes such a difference is “it comes down to experience.” Dark-winged butterflies, basically, get used to handling higher temperatures since they absorb solar radiation faster than their pale counterparts. They also “radiate heat more effectively than pale surfaces, and … may also be more likely to reduce their body temperature fast enough so to avoid reaching deadly temperatures,” she explained.

Paler butterflies rarely end up having to deal with high temperatures — they typically fly off to a cooler microclimate when things heat up. If more forest habitat is lost amid the ongoing climate crisis, paler butterflies will be at higher risk. They’ll be “in a dangerous place to cope with moving between increasingly far apart forest fragments, and when an extreme weather event, such as a heatwave, occurs,” Ashe-Jepson said via email.  If they can’t manage their body heat, effectively in those situations, they’ll be in trouble.

Though nobody is certain how climate change will ultimately impact butterflies, one certainty is that “many species are disappearing. (The study) could provide insight into which species may survive under harsh climatic conditions,” Kawahara said.

There is no predicting a timeline for the loss of butterfly species, according to Ashe-Jepson. “We are unlikely to lose this entire (Lycaenidae) family of butterflies,” she said. There could be, however, “rapid extinctions within this family if we do nothing to preserve their habitats and combat climate change, particularly in the face of extreme weather events such as heatwaves,” she added.

Recently, there has been ample evidence that extreme weather events are increasing. “This is rather frightening for this highly diverse group of butterflies,” Ashe-Jepson said, “but there is hope that with appropriate management, such as maintaining forests and microclimates, we can preserve this group of beautiful and unique butterflies before they are lost.”

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