These Tadpoles Do the Opposite of Fever When They Get Sick

Instead of heating up, they cool down.

A fever usually means the body is fighting against something.

But in a laboratory in Hungary, infected tadpoles did the opposite. Given a choice between warmer and cooler water, they increasingly headed for the cold.

The finding highlights a bizarre adaptation: “behavioral cooling”—choosing colder surroundings rather than warmer ones. The strategy may slow a virus that grows faster in heat and could reshape how researchers think about disease, climate, and the shrinking world of amphibians.

Reverse Fever

Fever is one of the body’s most familiar responses to infection. Raising body temperature can strengthen some immune responses while making conditions less favorable for pathogens.

Cold-blooded animals, such as frogs, fish, and reptiles, can’t raise body temperature the way mammals do. They depend on the world around them. For lizards or snakes, that means basking in the sun; for fish and tadpoles, that’s swimming towards warmer water.

Many infected ectotherms do exactly that, producing what biologists call behavioral fever. So researchers from ELTE Eötvös Loránd University and the HUN-REN Hungarian Research Network expected virus-infected agile frog tadpoles to seek warmth.

They didn’t.

Infected Tadpoles Go Cooler

The team exposed half of 160 tadpoles to Frog Virus 3, or FV3, for 24 hours. The remaining animals received a virus-free sham treatment. The experiment was conducted in two rounds of 80 animals each.

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Some tadpoles then lived in water held at a fairly steady 18°C (64°F). Others were placed individually in long channels containing a thermal gradient stretching from roughly 19°C to 31°C (66°F to 88°F). They could swim wherever they wanted.

For five days, cameras recorded their positions.

All of the tadpoles gradually shifted toward cooler temperatures as the experiment continued. The decrease was significantly steeper among animals that had become infected with FV3.

By the fifth day, infected tadpoles had lowered their preferred temperature by around 1.6°C compared with the other groups. Researchers say they probably do this to reduce viral spread.

“Our results suggest that infected tadpoles fine-tune their body temperature to slow viral replication while avoiding the detrimental physiological consequences of excessively low temperatures,” Dr. Dávid Herczeg, the study’s first author, said in the ELTE/HUN-REN announcement.

Going Cold Comes With a Price

When our bodies become feverish, this higher temperature helps the immune system fight back against bacteria or viruses. So why would an infected tadpole seek cold rather than heat?

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One possibility lies with the virus itself. Laboratory studies cited by the researchers found that FV3 can reach its highest titres at around 30°C. Cooler conditions could therefore make the animal’s body a less favorable place for the virus to multiply.

That helps explain why cooling might help. A cooler tadpole (pun intended) may make its body less inviting for the virus.

But cold brings costs. Tadpoles grow and develop more slowly in lower temperatures. Their immune systems may also work less well.

The tadpoles seem to show some restraint and didn’t rush to the coldest water available. They seemed to choose a middle path: cool enough to hinder the virus, but not so cold that their own bodies paid too high a price.

The researchers also found that infected tadpoles became less active over time. That decline may reflect disease damage, the energy demands of an immune response or both.

Why This Matters

Behavioral cooling has rarely been documented. Previous examples came mostly from infected arthropods and bacteria-infected rats. The new study extends the response to an ectothermic vertebrate.

“Our study provides the first direct experimental evidence that ectothermic vertebrates can employ behavioral cooling in response to pathogens,” Dr. Attila Hettyey, a senior author of the study, said in a statement.

But wild tadpoles don’t live in tidy channels, and it’s hard to say how they actually act in real-life situations. This matters a lot because amphibians already face habitat loss, pollution, invasive species, and infectious disease. Climate change can alter the temperatures of ponds and wetlands, sometimes removing the small cool refuges that animals use to cope.

Importantly, study does not show that cooling alone can stop deadly outbreaks. It lasted five days and measured virus levels only at the end. The researchers could not fully untangle cause and effect: cooler temperatures may reduce virus levels, heavier infections may drive tadpoles to cool down, or both may happen together.

Even so, the result offers a new view of sickness. Sometimes an animal fights infection by seeking heat. Sometimes it survives by finding a colder corner of its environment.

The study was published in the journal BMC Biology.