A new theory on why fireflies glow—and why they need help

Their ethereal yellow (or green, or red) glow popping up from the grass signals summer. While there may be as many as 2,400 species of firefly on Earth, these flying, bioluminescent insects are under threat. Worsening habitat conditions might mean more summers with less fireflies. 

Recently, scientists also began to question the prevailing hypothesis for why they evolved their glow in the first place. New analysis on a genetic level could be crucial for helping fireflies face their survival challenges, since they may need to evolve yet again.

[Related: How do fireflies power their blinking butts?]

A new hypothesis for how fireflies got their glow

For years, scientists believed that the bright lights emitted from fireflies in the Lampyridae family first evolved as a warning signal to predators. Their signature glow would signal that they were toxic and probably won’t taste very good. Over time, that light was potentially repurposed as a mating signal. 

In a recent study published in the journal PNAS Nexus, a team in China put this to the test by modeling out a firefly family tree with genetic data. They traced the evolution of the chemical compounds that make the fireflies toxic–lucibufagins. They collected samples of 16 species of Lampyridae from across China and with two related species. In addition to running a full genetic analysis of each species, they also looked for lucibufagins using liquid chromatography-mass spectrometry. This process is used in a lab to separate, identify, and measure what substances are present in a liquid sample.  

They found that lucibufagin toxins are present in only one subfamily of fireflies and that it may have evolved later. However, bioluminescence itself without this toxin is found widely across the entire family. According to the team, this suggests that the toxin evolved after bioluminescence itself evolved in animals and in response to something other than mating.

Instead, their glow may have evolved due to a particularly stressful situation. An ingredient firefly bioluminescence called luciferin has antioxidant properties. The study found that firefly ancestors evolved and diversified when atmospheric oxygen levels rose following a historical low after the Toarcian Oceanic Anoxic Event. This mass extinction event about 183 million years ago during the Early Jurassic caused a widespread decrease in oxygen in the world’s oceans. 

[Related: Mass extinction 183 million years ago offers dire warning for modern oceans.]

Luciferin may have been a way for the fireflies to handle less oxygen in the air. Glowing millipedes are also believed to have evolved bioluminescence as a way to cope with oxidative stress in hot, dry environments. Fireflies may have also followed a similar path in response to the stress of oxygen depletion.

Today’s less predictable climate

While fireflies may have evolved this glow as a response to low oxygen millions of years ago, what does that mean in the face of today’s environmental challenges? They generally thrive in temperate conditions. Warm and wet summers create their ideal breeding environment, with cold winters supporting the survival of their eggs, larvae, and pupae.

As global temperatures continue to rise, these climate conditions are becoming less predictable. For the fireflies and numerous other species, it means that the planet is often less hospitable. Changes in precipitation patterns have also contributed to either severely dry conditions that reduce larval survival or a scenario with too much water that can flood their breeding grounds and disrupt life cycles.

A study published in the June 2024 edition of Science of the Total Environment suggests that they are sensitive to several environmental factors, including short-term weather conditions, longer climatic trends, and urban encroachment. 

“Subtle changes in climate patterns, especially related to temperature, are significantly impacting firefly breeding cycles and habitat quality,” study co-author and University of Kentucky ecologist Darin McNeil said in a statement.

Using AI-based machine learning techniques, the team analyzed over 24,000 surveys conducted by citizen science initiative Firefly Watch–now called Firefly Atlas. The analysis revealed that not only is the planet’s incredibly wild weather patterns harming fireflies, but human encroachment is as well. They found that fireflies are less common in areas that have significant light pollution at night.

[Related: Why artificial light—and evolution—trap moths.]

However, they found that the recent decline in firefly populations is not uniform across all species or geographic regions. Some have adapted to drier environments or those with specific breeding patterns may not be as affected by certain changes, while others are more vulnerable. This pattern shows just how complex natural ecosystems are and why conservation strategies are not simple one-size-fits-all solutions. 

How to help fireflies

While not considered endangered, their numbers are dwindling.

“I would say fireflies are threatened due to habitat loss, but they are not going extinct, some are adapting in different regions,” entomologist Eric Day told Virginia Tech News.

To help, entomologists recommend providing fireflies with some additional habitat incorporating wild or native species of plants in your yard or garden. The native plant finder website from University of Delaware entomologist Doug Tallamy is a great resource to find out which plants are native to your area. Gardeners and homeowners should also avoid using pesticides whenever possible. They can not only degrade habitats, but kill fireflies and their prey. 

Turning off superfluous outdoor lights, especially during the summer, can also help. These lights not only confuse fireflies, but moths and other nocturnal species.

“This is why we’re seeing less and less each year. The more development there is, the less room there is for them to thrive,” said Day. “The presence of fireflies indicates a diverse habitat and doing these things is essential to ensuring future generations can enjoy the natural wonder of skies lit up by them.”