Deadly fungus makes sick frogs jump far, possibly to find mates

Chytrid fungus is a deadly pathogen plaguing amphibians in an ongoing global pandemic, with the potential to kill off entire populations. But in one threatened Australian frog species, the infection has an unusual side effect: huge hops.

Verreaux’s alpine tree frogs (Litoria verreauxii alpina) that are infected with chytrid fungus, also known as Batrachochytrium dendrobatidis or Bd infections, can jump nearly a quarter further than their uninfected counterparts.

“[The findings] remind you how resilient these amphibians are and how they are dealing with the bombardment of this really horrible pathogen. And yet, their bodies are able to do something miraculously unexpected,” says Taegan McMahon at Connecticut College in New London, who was not involved in the research.

Alexander Wendt at the University of Melbourne in Australia and his colleagues were studying how Bd infections affect the health of alpine tree frogs, using their athletic performance as a proxy for physiological effects.

In the lab, the researchers divided 60 frogs into uninfected and Bd-infected groups. Wendt and his colleagues tested how the frogs responded to temperature extremes and how far they jumped when gently prodded.

Surprisingly, by the sixth week post-infection, infected frogs leapt nearly 24 per cent further than uninfected frogs. In most other amphibians, their immune system’s focus on fighting the fungus drains the animals’ energy. Physiological responses to Bd seem to depend on the amphibian species involved, leading to temporary benefits for some sublethal infections.

“But once clinical signs appear it’s usually a nail in the coffin for most species,” says Wendt.

The alpine tree frogs don’t appear to have a strong immune response to Bd to slow them down, and the boost to their bounce may be an adaptation for speedily finding a mate for one last chance to reproduce before the infection turns for the worse. Other frog species are known to increase their calling to mates when infected with Bd.

A brief supercharge in jumping prowess may work to Bd’s benefit too. “Evolutionarily, it would make sense for [Bd] to facilitate movement so that it’s getting a higher level of transmission and more longevity in a host,” says McMahon.

Bd’s effects on amphibians are increasingly understood to be influenced by the interplay of the host and fungal biologies, and the local environment. “All we can do is learn as much as we can to help species survive and to slow or stop the spread of Bd before it’s too late,” says Wendt.