Scientists find butterflies that live nearly a year and barely age

Some tropical Heliconius butterflies have evolved a remarkable way to stay young, allowing them to live far longer than most butterflies.

Most butterflies survive for only a few weeks after reaching adulthood. But scientists have now identified a group of tropical butterflies that can live for almost a year while showing little sign of physical decline, a discovery that could help researchers better understand how healthy ageing evolves in nature.The study, led by the University of Bristol and published in the journal Nature Communications, found that butterflies belonging to the Heliconius genus live far longer than their closest relatives. In one striking example, Heliconius hewitsoni survived for 348 days, while the closely related Dione juno lived for just 14 days, a 25-fold difference in maximum lifespan.Researchers say the butterflies are not simply living longer.

Some appear to age more slowly, making them one of the most promising insect models for studying the biology of longevity.The team worked with scientists at the Smithsonian Tropical Research Institute in Panama and combined data from butterfly houses, mark-release-recapture studies and controlled insectary experiments to compare lifespan and ageing patterns across the Heliconiini tribe.

A female Heliconius erato lays an egg on a plant

Butterflies that stay strong as they grow older

One of the study's most surprising findings came from Heliconius hecale, a tropical butterfly found across Central and South American rainforests.

To find out whether older butterflies lost strength with age, researchers carried out grip-strength tests. They found that older H. hecale butterflies performed just as well as younger ones, showing little or no measurable physical decline.The result stood in sharp contrast to Dryas iulia, a closely related butterfly with a much shorter lifespan. Older individuals of that species showed a clear decline in physical performance as they aged.The researchers also found that H. hecale maintained its body mass and muscle function for much longer than its relatives.Taken together, the findings suggest that at least some Heliconius butterflies largely avoid the gradual physical deterioration that affects most animals as they age.

Lifespans unlike most butterflies

Adult butterflies are generally short-lived. Many survive for only a few weeks after emerging from their chrysalis.Across the Heliconius group, however, researchers found average lifespans that were around three times longer than those of closely related species. Several species survived between 106 and 277 days, while H. hewitsoni reached nearly a full year.Only one other butterfly species, Myscelia cyanaris, is known to live longer, with a recorded maximum lifespan of around 380 days. Scientists say very little is known about why that species lives so long.The study also showed that Heliconius butterflies consistently had lower baseline mortality and slower rates of ageing than related butterflies that do not feed on pollen.

An adult Heliconius erato appears next to a pupa of the same species

An unusual diet caught scientists' attention

For years, researchers suspected that the butterflies' unusual feeding habits might explain their long lives.Unlike most butterflies, which feed almost entirely on flower nectar rich in carbohydrates, most Heliconius species also feed on pollen after reaching adulthood.Pollen contains amino acids and lipids, nutrients that most butterflies obtain only as caterpillars before they undergo metamorphosis.Those nutrients help produce eggs and sperm, support energy storage and can also strengthen the immune system."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," 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, said of most butterflies' limited nutrient reserves."They usually die soon after this finite resource is used up."To test whether pollen alone explained the butterflies' exceptional lifespan, researchers compared pollen-feeding Heliconius hecale with its non-pollen-feeding relative Dryas iulia.The team also removed pollen from the diets of some Heliconius butterflies to see whether they would lose their longevity advantage.The results surprised them.Even without pollen, Heliconius butterflies still lived substantially longer than their close relatives.That finding suggests nutrition is only part of the explanation. Researchers now believe the butterflies have also evolved biological mechanisms that naturally slow ageing.Dr Foley said her interest in the study came from the huge differences in lifespan seen across the animal kingdom."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."

More than just a nutritious diet

The researchers say the findings point to something more complex than good nutrition.Of the 28 Heliconius species included in the study, only six did not feed on pollen. Those non-pollen-feeding species lived between 14 and 98 days, far less than many of their pollen-feeding relatives.Even so, removing pollen from the butterflies' diet did not erase their lifespan advantage. Heliconius hecale continued to outlive Dryas iulia, while also maintaining its body mass and muscle function for longer.

A Heliconius melpomene butterfly feeds on pollen

That suggests the butterflies have evolved biological traits that help them stay healthy for longer, with pollen providing an additional benefit rather than being the sole reason for their extended lives.Researchers say pollen-derived amino acids also allow Heliconius butterflies to keep producing eggs throughout adulthood, extending their reproductive lifespan alongside their overall longevity.

Measuring ageing with the "Pullinator"

To investigate how ageing affects physical performance, the team designed an unusual piece of equipment nicknamed "The Pullinator".The device consisted of a sandpaper-covered perch attached to a lightweight wooden base.

Researchers placed it on a laboratory balance before gently lowering a butterfly onto the perch."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 grip-strength measurements showed that older Heliconius hecale butterflies retained their strength remarkably well, while ageing Dryas iulia butterflies became noticeably weaker.The researchers say this delayed physical decline is one of the clearest signs that some Heliconius species are not only long-lived but also age more slowly.

A natural experiment in longevity

Long-lived animals have long fascinated scientists because they may reveal how evolution has solved the biological challenges of ageing.While studies involving whales, sharks and other exceptionally long-lived animals can take decades, Heliconius butterflies complete their entire lifespan within about a year, making them much easier to study in detail.Dr Foley said this makes the butterflies an ideal natural experiment."As the most species-rich animal class, insects are renowned for their extraordinary morphological and ecological diversity.

They also exhibit extreme variation in longevity, with maximum lifespans ranging from just a few days in adult mayflies to several decades in the reproductive castes of some ants and termites. This represents a roughly 5,000-fold difference within the class, compared with around a 100-fold difference in lifespan observed in mammals."Heliconius butterflies are among the longest-lived butterflies, but what makes them particularly remarkable is that they appear to have evolved not only longer lifespans, but also slower aging.

This allows them to live significantly longer than closely related species from which they diverged relatively recently in evolutionary time."The exciting implication of this lifespan extension is that it provides a powerful opportunity to identify the mechanisms that underpin longevity. By comparing long-lived Heliconius butterflies with their short-lived relatives, we have a natural evolutionary experiment that can help reveal how lifespan is extended, making them a highly promising new model for research into the biology of aging and longevity.

"Despite the progress, researchers say they still do not know exactly which biological mechanisms are responsible."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."

What it could mean for human ageing research

Scientists have long relied on organisms such as yeast, worms and fruit flies to understand the biology of ageing. The researchers believe Heliconius butterflies could become another valuable model because they naturally evolved longer lifespans rather than having them produced through laboratory manipulation.Dr Foley said her colleagues are now interested in investigating the butterflies' still-unknown longevity mechanisms, along with their unusually large brains and impressive long-term memory, which appear to remain robust even as they age.The findings could eventually help scientists understand how evolution has produced healthier ageing in different species, offering clues that may one day inform research into human health.Dr Jaret C. Daniels, curator and interim associate director for the McGuire Center for Lepidoptera and Biodiversity at the Florida Museum of Natural History, who was not involved in the study, said the research highlighted the wider value of insects in biological science."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."The research was published on 16 June in Nature Communications by scientists from the University of Bristol and collaborating institutions, including the Smithsonian Tropical Research Institute in Panama. The team says Heliconius butterflies now offer one of the clearest opportunities to investigate how longer life and slower ageing evolved naturally, and why some animals remain healthy long after others begin to decline.