In a breakthrough that changes our understanding of how animals adapt to their environments, scientists have discovered that tropical butterflies use humidity cues to alter their wing patterns for survival. For long, researchers believed that temperature was the ultimate trigger dictating how butterflies developed their protective camouflage. However, two recent studies led by researchers from the Indian Institute of Science Education and Research (IISER) Thiruvananthapuram and Adam Mickiewicz University, Poland, show that when temperature is an unreliable weather forecaster, caterpillars turn to the moisture in the air and the quality of the grass they eat to prepare for the future.

The research focuses on a biological phenomenon known as seasonal polyphenism, akin to a seasonal wardrobe change. In many tropical butterfly species, individuals born during the wet season look drastically different from those born in the dry season. Butterflies, including Melanitis leda, or the Common Evening Brown butterfly, and Mycalesis mineus or the Dark-branded Bushbrown, the wet-season form boasts large, bright, target-like circles on their wings called eyespots. These conspicuous marks are designed to grab the attention of predators like birds and lizards, tricking them into biting the edge of the wing rather than the insect's vulnerable body. Conversely, dry-season butterflies develop almost no eyespots, relying instead on a dull, dead-leaf appearance to perfectly camouflage themselves against the parched, brown vegetation of their habitat.

Because butterflies only live for a short time, they must grow the correct type of wing before they even emerge from their chrysalis. To do this, the growing caterpillar must use environmental cues to predict what the world will look like by the time it becomes an adult. Historically, most experiments on this shape-shifting ability were conducted on African butterflies living in regions where the dry season is noticeably cooler than the wet season. Therefore, researchers believed that temperature was the universal switch that controlled eyespot size. But in many tropical areas, such as southern India, temperatures remain relatively stable year-round, making them a poor predictor of incoming rain.

Collage of seasonal wing patterns in leda mineus

Collage of seasonal wing patterns in Melanitis leda and Mycalesis mineus (credits: Tarunkishwor Yumnam)

To see which environmental cues were being used, the research team collected specimens of the Dark-branded Bushbrown and the Common Evening Brown butterfly from different climatic zones in Tamil Nadu. They then reared them in highly controlled laboratory chambers that altered air temperature and humidity. They saw that populations of the Common Evening Brown butterfly adapt locally to whatever cue is most reliable in their specific environment. In regions where temperature fluctuates enough to be useful, the butterflies still use it. But in regions where the transition between seasons is marked only by a massive shift in rainfall, the butterflies ignore temperature entirely and rely strictly on humidity. 

In the next study, a team set out to find exactly how different species detect the moisture during rising humidity. By swapping caterpillars between high- and low-humidity environments at different stages of their lives, the scientists found that the Dark-branded Bushbrown butterfly acts as a direct moisture sensor. Just before the caterpillar forms its pupa, during the wandering larval and prepupal stage, it reacts physically to the humidity in the air. If the air is dry, it shuts down the development of its eyespots.

However, the Common Evening Brown butterfly appears to use an indirect method. Instead of detecting the air, it uses food to gauge the humidity. The researchers found that the butterfly determines the season it enters based on the nutritional quality of the host plants it eats. During a wet season, the grass is lush and rich in nitrogen. As the dry season approaches, the plant becomes stressed and its chemical makeup changes. The caterpillar senses this declining food quality and uses it as a biological alarm bell to start developing its dry-season camouflage. This study shows that two distantly related species facing the same survival challenges both learned to use the same environmental cue, but evolved completely different ways to detect it.

“The contrasting mechanisms that we found in these two butterflies reflect their distant evolutionary relationship, which is around 50 million years apart. On the other hand, they both independently converged to use humidity, likely because of the shared environmental similarities,” adds Dr. Tarunkishwor Yumnam, lead author of one of the studies.

The researchers, however, note that because the experiments were conducted over a single generation in a laboratory, the scientists cannot entirely rule out maternal effects, where a mother butterfly might pass down genetic instructions based on the environment she experienced. The team acknowledges that genomic data would be needed to absolutely confirm how these populations have evolved these localised survival tricks.

The studies highlight the incredible complexity of evolutionary biology, showing that animals develop highly specific tools to read their local environments. By understanding the precise cues species use to survive, conservationists can better predict which populations are most vulnerable to climate change and design smarter, targeted strategies to protect them.

“Animals relying on a single environmental parameter, such as temperature, face a high uncertainty with the changing global climate. By using multiple cues or cues that are more consistently linked to seasonal rainfall, like humidity and host plant quality, these butterflies may be better prepared to cope with or adapt to climate change. Our study also highlights that animals showing different seasonal phenotypes may be relying on multiple environmental parameters that science has not uncovered yet,” concludes Dr Tarun.