The Eiffinger’s tree frog (Kurixalus eiffingeri), found on Ishigaki and Iriomote islands in Japan, has a unique biological adaptation: its tadpoles do not defecate during their early developmental stages. This finding by researchers at Nagoya University in Japan contributes to our understanding of how these small frogs survive in the tiny bodies of water where they spawn. The findings were published in the journal Ecology.

Eiffinger’s tree frogs rear their young in small, isolated water bodies, such as tree hollows and bamboo stumps, which provide a safe environment with few predators.

However, in these limited water spaces, the tadpoles face the challenge of waste management. Unlike other species that excrete toxic ammonia in their feces into larger water bodies where it is diluted and rendered harmless, the confined water environments of Eiffinger’s tree frogs do not allow them this luxury. Excessive defecation causes ammonia to build up in the tiny water bodies, leading to toxicity and endangering their survival.

Bun Ito, a special research student, and Professor Yasukazu Okada at the Graduate School of Science, Nagoya University, focused on this peculiar aspect of the frog’s life cycle and discovered that the tadpoles exhibit a remarkable strategy to managing their waste: they go for months without pooping.

To keep the water bodies clean, Eiffinger’s tree frog tadpoles excrete significantly less ammonia than other frog species. Instead of releasing waste into their environment, the tadpoles store it in their intestines, accumulating high concentrations of ammonia within their bodies.

The frogs only begin to defecate once they transition from tadpoles to subadults. This delayed excretion suggests that nitrogen, which is ingested as part of their diet, is effectively retained within their body in the form of ammonia until it can be safely expelled outside their spawning site. This sanitation strategy mirrors the behavior of some bee and ant larvae, which similarly retain feces in their intestines to keep their nests clean.

To further understand these findings, the researchers conducted experiments to compare the ammonia tolerance of Eiffinger’s tree frog tadpoles with that of other frog species, such as the Japanese tree frog, by raising them in ammonium chloride solutions with varying concentrations.

They found that Eiffinger’s tree frog tadpoles could survive in much higher concentrations of ammonia than other species, showing a heightened resistance to this toxin. However, even their tolerance had limits, as the tadpoles succumbed under extremely high ammonia concentrations.

These findings highlight a dual adaptation strategy in Eiffinger’s tree frog tadpoles: reducing the amount of ammonia they release into their environment and developing a high tolerance to the ammonia they do encounter. This combination allows them to thrive in the small, confined water areas where they develop.

The study sheds light on how Eiffinger’s tree frogs have adapted to their restricted habitats, employing unusual biological mechanisms to manage waste and ensure the survival of their offspring. The research team’s findings offer valuable insights into the unique survival strategies of organisms living in specialized environments.

Ito believes that the research has important conservation implications. “The discovery of frogs that have successfully adapted to the unique environment of small water holes reveals a more complex ecosystem within these tiny habitats than we initially imagined,” he said. “Protecting biodiversity necessitates the preservation of these microhabitats.”

Until a few years ago, the butterfly known as the southern small white could barely be found north of the Alps. That was before a Europe-wide invasion that brought a huge increase in the insect’s distribution—at the same time as a rapid decrease in genetic diversity within the species.

It took a while for zoologist Daniel Berner to notice that a butterfly species that wasn’t local to his area had become established in his garden. Then, suddenly, he saw it everywhere: Pieris mannii—also known as the southern small white—with its wingspan of around four centimeters and white wings adorned with large black spots.

Indeed, until a few years ago, only a few small, localized populations of this principally Mediterranean species existed in Switzerland, in Valais and Ticino. But at some point in around 2005, the butterfly began its journey north and east—and it has now been identified at the North Sea and in the Czech Republic.

Comparison with museum specimens

With its expansion, however, came a significant loss of genetic diversity.

“We found that, as it invaded, the southern small white standardized local populations of its own species,” says Dr. Daniel Berner of the University of Basel. Together with researchers from the University of Greifswald and the Senckenberg German Entomological Institute, Berner has published a study in Current Biology investigating how the butterfly’s expansion has impacted diversity within its own species.

The researchers compared the genetic make-up of freshly caught butterflies with that of significantly older museum specimens—in other words, specimens that had been caught and preserved before the invasion began. In doing so, they demonstrated that the genetic composition of the researched local populations has changed considerably. Indeed, a large part of the original genetic make-up has now been replaced with that of the expanding population.

“If we hadn’t made the comparison with the museum specimens, we wouldn’t have spotted this genetic change,” says Berner.

For their analyses, the researchers were able to sequence butterflies from the collection of the Natural History Museum of Bern and therefore to characterize the insects genetically. They were very lucky that, over the decades, the lepidopterist (butterfly researcher) Heiner Ziegler had amassed an extensive collection of specimens of none other than the southern small white—and that they could use this collection in their research.

Favorite plants in gardens

Urbanization is helping the butterfly to spread rapidly. In fact, the southern small white doesn’t like to fly long distances. Instead, it spends its lifespan of approximately three weeks fluttering around within a modest radius of its birthplace, in which the caterpillars’ food plants also thrive—namely arugula and, above all, candytuft.

The latter is particularly widespread in gardens within settled areas. Accordingly, the increasing expansion of the built environment has given the southern small white the opportunity to spread far and wide.

Moreover, there are five or six generations of southern small white per year, rather than just one.

“This species can therefore quickly build up large populations in a newly settled area, favoring the settlement of new land over large distances,” Berner explains. It is very likely, he says, that the butterfly will expand further—provided that its food plants are available. “In any case, butterfly researchers in England are just waiting to spot the first one.”

Expansion and genetic mixing—loss or gain?

From a conservation perspective, the expansion of the southern small white is a double-edged sword. As the species uses largely human-designed habitats in the newly settled area, it is not expected to compete with indigenous butterflies. Moreover, expansion means that this butterfly species is now much more numerous overall, which tends to reduce its risk of dying out.

However, these positives are offset by the disappearance of genetic diversity that had built up over the course of millennia: “Although it’s the fate of living things that some local groups can die out, what’s special about the situation facing the southern small white is that the loss of original population diversity accompanies expansion of the built environment—and is therefore caused by humans,” Berner says.

As yet, the researchers don’t know why the southern small white in particular has embarked on major expansion or where exactly this process began.

“Presumably, nothing fundamentally new has happened on the side of the butterfly. So far, we haven’t found signs of major genetic change in the expansive population, and climate change doesn’t appear to play a key role in the situation,” says Berner.

The researchers want to continue looking into these questions, but they already have their suspicions as to the starting point: the invasion may have begun in eastern France.