Some butterflies live 25 times longer than their relatives. Scientists are unlocking their secrets

Since most butterflies live a short time, fluttering between colorful flowers for a few weeks before dying, a few rare exceptions have stumped scientists. Now, some long-lived tropical butterfly species are shedding light on the secrets of longevity.

Butterflies belonging to the Heliconius genus, which inhabit the tropical rainforests of South and Central America, have lifespans that vary wildly. The Dione juno butterfly lives for 14 days after reaching adulthood, while Heliconius hewitsoni lives for 348 days — nearly 25 times longer.

Other Heliconius species also have impressively lengthy lives, enduring between 106 to 277 days, according to a study on the phenomenon published Tuesday in the journal Nature Communications. Some scientists have speculated that Heliconius' extended adulthood is due to the insects consuming an enhanced diet, rather than relying purely on carbohydrates like other butterflies.

But the exact reasons behind this unexpected longevity have been unclear, which is what inspired Dr. Jessica Foley, the study's lead author and a postdoctoral scholar at Tufts University's Jean Mayer USDA Human Nutrition Research Center on Aging in Boston, to take an in-depth look at the intriguing butterfly genus.

"We see vast differences in lifespan across the animal kingdom — adult mayflies famously live only for a day, whereas some whales and sharks can live for hundreds of years," Foley wrote in an email. "I'm interested in the evolutionary basis of these kinds of lifespan differences because they might hold insights relevant for healthy ageing in humans."

Foley and her colleagues discovered that while nutrition has its part to play, some Heliconius have also evolved an anti-aging mechanism the researchers are still unraveling — and how it could be a model for understanding human longevity.

Evolving a special diet

Studying the "extreme agers" of the animal kingdom is an uphill task for scientists, especially if they have to wait centuries for a species to reach what is considered old, Foley said.

Lesser-studied Heliconius butterflies presented the perfect case study since researchers could observe their entire life cycle in about a year. Only one other known butterfly, Myscelia cyanaris, lives longer than Heliconius, with a maximum lifespan of 380 days, but there is little data to suggest why.

Foley and her collaborators combined an expansive dataset to look at lifespan and aging patterns across the Heliconius genus, using information collected from commercial butterfly houses and mark, release and recapture studies, as well as controlled experiments.

The authors wanted to see whether removing an unusual food source from the diets of these butterflies would shorten their lifespan, she added.

Most adult butterflies feed exclusively on flower nectar, fueled only by carbohydrates rather than the amino acids and lipids — used for egg and sperm needed to reproduce — they received from plant material as caterpillars before metamorphosing.

"The general evolutionary strategy is to reproduce as much as they can until those resources are spent, which doesn't take very long for these small insects," Foley said of the lipids and amino acids. "They usually die soon after this finite resource is used up."

However, most Heliconius species have adapted to feed on pollen — even as adults — which could provide the insects with more energy, the authors concluded. Pollen also contains lipids, which help with energy storage but also boost immunity.

The authors closely studied the relationship between Heliconius and pollen to see what health benefits the butterflies gained from their diets.

"I wanted to understand the real extent of this lifespan extension in Heliconius, to understand whether it was also accompanied by slowed physiological ageing, and to figure out whether they would still show a lifespan extension — indicating evolved mechanisms of longevity — even without the pollen," Foley said.

Of the 28 Heliconius species the researchers studied, only six were non-pollen-feeding, and they lived between 14 to 98 days. But the team's observations showed that even when pollen was withheld, the Heliconius butterflies still lived much longer than their non-pollen feeding relatives.

An evolutionary mystery

The team also used a unique device to measure age-related decline in older butterflies with a grip-strength test. They built a device called "The Pullinator," or a perch lined with sandpaper that was attached to a lightweight wooden base.

"We placed this on a lab balance, zeroed the balance, and then gently held a butterfly by the wings and lowered it until it grasped the perch," Foley said. "We then tugged until it let go — but as the butterfly tugged, the balance would drop negative, and we could use the maximum negative reading as an indication of how much weight the butterfly could carry before it let go."

The Heliconius hecale butterfly species, which can live up to 277 days, showed little or no physiological decline during the grip-strength test, while the closely related Dryas iulia, which doesn't feed on pollen and lives for 98 days, showed signs of age-related decline. Heliconois hecale also maintained body mass and muscle function for longer — even when deprived of pollen.

The findings showed that overall, many pollen-feeding Heliconius species had longer lifespans and slower rates of aging, suggesting that nutrition is an important factor. Pollen-derived amino acids also help the butterflies continually produce more eggs as adults, lengthening their reproductive lifespans, Foley said.

But given that permanently removing pollen from the diets of the butterflies seemed to have no negative impact on their longevity, the researchers suspect that much like the insects evolved a pollen-based diet, they have also evolved to live longer.

"We show that these butterflies do have evolved mechanisms of longevity, and that they also seem to have evolved a delayed physiological decline, making them excellent new models for studying the mechanisms allowing for long life," Foley said. "However, we do not yet know what these mechanisms are."

Longevity in the animal kingdom

Foley's colleagues are interested in investigating the more mysterious longevity mechanisms of Heliconius, as well as the butterflies' robust cognition — they have large brains and impressive long-term memory, even as they age, she said.

Studying worms, flies and yeast have enabled scientists to understand better how the mechanisms of aging work in humans. Looking at more examples from the animal kingdom can be used to identify solutions that evolution has found for the problem of aging, Foley said.

The new research shows that Heliconius can be a potential model insect group for studying increased longevity, including adaptations that could slow aging and have potential applicability to humans, said Dr. Jaret C. Daniels, curator and interim associate director for the McGuire Center for Lepidoptera and Biodiversity at the Florida Museum of Natural History. Daniels was not involved in the study.

"This study reinforces the utility of many insect groups and important model organisms for various fields of research," Daniels said in an email. "Since many insects are often overlooked or underappreciated/undervalued by humans, studies like this can help change that perspective."

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