Chytrid fungus survivors suffer stunted growth

It’s easy to see that frogs are in trouble when their breeding ponds are littered with carcasses. But even in places with no visible die-offs, the fungal disease that is decimating amphibians worldwide may be having subtler impacts. A new study found that chytrid fungus (Batrachochytrium dendrobatidis) stunts the growth of northern leopard frogs (Lithobates pipiens), potentially impairing their ability to survive and breed.

Jeremy Caseltine weighs juvenile frogs. Caseltine conducted the study while he was an undergraduate, collaborating with co-authors Michelle Boone and Samantha Rumschlag. ©Michelle Boone

“People are focusing on mortality events, because that’s the most disturbing thing,” said Michelle Boone, a conservation ecologist at Miami University in Ohio and senior author of a recent study published in the Journal of Herpetology. “But these pathogens are altering communities, and they’re probably changing the dynamics of populations.”

In the Midwest where the researchers are based, amphibians appear to be faring relatively well, despite the presence of chytrid fungus, says Boone. Many frogs in the area carry chytrid without dying from acute illness. Still, she says, that doesn’t mean they are unaffected.

To find out how chytrid might affect frogs in the presence of other stressors, the researchers raised northern leopard frog tadpoles in tanks with predatory crayfish (Orconectes rusticus). After the tadpoles transformed into tiny frogs, the researchers collected them and placed them in petri dishes, some of which contained chytrid fungus. After 12 hours in petri dishes, the frogs were moved to enclosures with moss and crickets.

The researchers thought the crayfish might stress out the tadpoles, compromising their immune systems and making them vulnerable to chytrid. That didn’t happen, possibly because these particular crayfish foraged at the bottom of the tank instead of preying on the tadpoles, says Boone. But while crayfish exposure didn’t affect the frogs, chytrid fungus did. By the end of the six-week experiment, the frogs exposed to chytrid were 10 to 12 percent smaller than controls.

Most past studies would have missed the size difference, says Boone. Chytrid research has largely focused on mortality, and researchers often end lab experiments after just two to three weeks. In the new study, size differences didn’t start to show up until around day 20.

But those size differences could have profound consequences. Larger females lay more eggs, so stunting frogs’ growth could reduce populations down the line, says Boone. Moreover, smaller frogs are less likely to survive the winter. When frogs die in winter, their deaths remain mysterious, since researchers never find the buried bodies.

“Species like the northern leopard frog are declining in parts of their range, for reasons that aren’t totally clear,” Boone said. “So for some of these enigmatic declines that haven’t been attributed to a specific factor, chytrid could be playing a role.”

Cold winters are just one of many challenges amphibians face. Frogs weakened by chytrid might be more vulnerable to any threat, from pesticides to predators to climate change. To understand the true danger of chytrid, says Boone, researchers need to look at how different stressors interact.

“One of the things we’re really interested in understanding is: Can you have this pathogen be present, but then you change maybe one or two conditions, and — boom — suddenly this sublethal pathogen is lethal?” She said. “Could we then begin to see declines in other parts of the world?”