Tadpoles speed up their growth to outsmart a deadly virus

Amphibians are in crisis. Climate change, pollution, and habitat loss are stripping their ecosystems. But something even more silent is killing them – disease. From frogs to tadpoles, the losses and disrupted growth are staggering.

Since the 1970s, about 200 amphibian species have vanished. Today, 34% of the 7,296 known species face extinction, according to the International Union for the Conservation of Nature (IUCN).

One fast-moving culprit is ranavirus. This genus of viruses spreads quickly among fish, reptiles, and amphibians. The virus can devastate species that lack any built-in defense. But a recent study in Frontiers in Amphibian and Reptile Science offers a glimpse of amphibian resilience.

“Here we show that in response to ranavirus, wood frog tadpoles change their growth, development, and resource allocation. This may help tadpoles tolerate the energetic demands of infection or escape risky environments to avoid infection entirely,” said Logan Billet, a PhD student at Yale University and the study’s first author.

A deadly virus is wiping out frogs

Ranavirus has caused 40 to 60% of amphibian die-offs in the US.

Once infected, larvae become lethargic, stop eating, swim abnormally, and bleed internally. Outbreaks often end in total pond die-offs. And with warming temperatures, these outbreaks may increase.

The researchers decided to focus on one species: the wood frog (Rana sylvatica). This species breeds explosively in temporary snowmelt ponds. The team observed these frogs from 2021 to 2023 in Connecticut forests. They tracked their survival, growth, and the virus’s impact.

They monitored 47 ponds: 35 remained virus-free, 7 had infections but low death rates, and 5 experienced full-blown die-offs.

Tadpoles grow faster around ranavirus

From mid-April to July, scientists collected tadpoles regularly. They euthanized up to 20 from each pond for lab analysis. They tested 1,583 livers for ranavirus and measured the length and developmental stage of 4,299 tadpoles.

Amphibians grow flexibly. Size and developmental stage do not always go hand in hand. A tadpole might be small yet advanced in development or vice versa. This plasticity makes their responses interesting to study.

The results showed that tadpoles in ponds later hit by die-offs grew faster during their first month. They also developed quicker – about 0.38 stages ahead of others. But once mortality began, growth collapsed. In the end, they were smaller and less developed than their healthy counterparts.

Quick growth may help tadpoles survive

Interestingly, tadpoles in infected ponds that avoided full die-offs showed similar trends. They grew faster and reached stages 1.7 Gosner points ahead of those in clean ponds.

The data points to a clear pattern. Tadpoles exposed to ranavirus grow and develop faster – at least in the early days. This may be their way of dodging death by jumping ahead in their life cycle.

Study senior author Dr. David Skelly is a professor at the Peabody Museum and Billet’s doctoral supervisor.

“Accelerating growth and resource allocation early on may allow tadpoles to improve their physical condition, and thus the strength of their immunity, in anticipation of infection. They might also metamorphose and move onto land earlier, potentially reducing their exposure to ranavirus,” said Dr. Skelly.

A unique survival strategy

These early responses may help explain why some outbreaks kill only a portion of a population.

“These responses are likely to give tadpoles a survival advantage. We hypothesize they are the reason why mortality wasn’t always 100%, but only partial in some outbreaks in our study,” said Billet.

“What we don’t yet know is how tadpoles detect ranavirus in their environment, for example through chemical or visual cues, and how effective these responses improve survival or tolerance of infection. Controlled experiments will be important to address these questions.”

The study opens a new window into amphibian survival in the face of disease. These creatures, from adult frogs to fragile tadpoles, are adjusting their growth in real time to a rapidly changing world. But their limits remain unknown.

As human-driven changes reshape the planet, the quiet struggles in our forest ponds may soon speak volumes about the future of biodiversity. The story of the wood frog and its fight against ranavirus is just one chapter.

The study is published in the journal Frontiers in Amphibian and Reptile Science.

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