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

The Chytrid fungus is a deadly pathogen plaguing amphibians as part of an ongoing global pandemic that has the potential to kill entire populations. But in an endangered Australian frog species, the infection has an unusual side effect: huge hops.

Verreaux’s Alpine Tree Frogs (Litoria verreauxii alpina) infected with the chytrid fungus, also known as Batrachochytrium dendrobatidis or Bd infections, can increase nearly a quarter more than their uninfected counterparts.

“[The findings] remind you how resilient these amphibians are and how they cope under bombardment from this horrible pathogen. And yet their body is capable of doing something miraculously unexpected,” says Taegan McMahon of Connecticut College in New London, who was not involved in the research.

Alexander Wendt of the University of Melbourne in Australia and colleagues were studying how Bd infections affect the health of alpine tree frogs, using their athletic performance as an indicator of 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 extreme temperatures and how far they jumped when gently pushed.

Surprisingly, six weeks after infection, infected frogs jumped almost 24% further than uninfected frogs. In most other amphibians, their immune systems’ focus on fighting the fungus drains the animals’ energy. Physiological responses to Bd appear to depend on the amphibian species involved, resulting in temporary benefits for some sublethal infections.

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

Alpine tree frogs don’t appear to have a strong immune response to Bd to slow them down, and their increased rebound could be an adaptation to quickly find a mate for a last chance to breed before the infection gets worse. Other frog species are known to increase their calls to mates when infected with Bd.

A brief increase in jumping prowess may also benefit Bd. “Evolutionarily, this would make sense for [Bd] to facilitate movement to achieve a higher level of transmission and greater longevity in a host,” explains McMahon.

It is increasingly recognized that the effects of Bd on amphibians are influenced by the interaction of the biology of the host and the fungus, as well as the local environment. “All we can do is learn as much as we can to help species survive and slow or stop the spread of Bd before it’s too late,” Wendt says.