Discretion is often the better part of valour -- unless you're a tadpole. Hiding away from hungry fish exposes the nascent frogs to an insidious health risk, according Gregory Thiemann and Richard Wassersug of Dalhousie University in Halifax, Canada.

Tadpoles that take cover near the bottom of ponds are sitting ducks for parasites, the researchers report in Biological Journal of the Linnean Society1. And although the parasites don't usually kill the tadpoles, the legacy of the attack can leave the youngsters more susceptible to disease.

Worldwide, frog numbers are in decline2. Many factors are thought to be involved, including ultraviolet light, pesticides, fungal infections, introduced predators and parasites. But a unifying cause for the frogs' rapid demise has so far been elusive.

Thiemann and Wassersug's results show that one stress on a local frog population can exacerbate another. And they have found strong clues as to how multiple pressures might mount to push fragile frog populations to breaking point.

The biologists exposed the tadpoles of two frog species (Rana clamitans and Rana sylvatica) to predatory fish and fluke larvae -- parasites that attack the frogs' kidneys. To conceal themselves from the fish, the tadpoles barely moved and stayed near the bottom of the pond, even though the fish were caged and too small to be a threat.

But this caution made the tadpoles easy targets for the small swarms of larval flukes lurking on the pond bottom. Flukes often destroy one of the tadpoles' kidneys, making them more vulnerable to disease and predation.

Thiemann and Wassersug suggest a possible link between the rise in parasitic infections in frogs and the fact that, thanks to changes to waterways and the stocking of high-elevation ponds with game, fish now swim in places they never did before, preying on frogs and tadpoles.

"Filling in of wetland can cram everyone [sic] into this soup of frogs and snails [parasite hosts] and parasites," agrees Stanley Sessions, a biologist from Hartwick College in New York, who first documented that frog deformities could be caused by flukes burrowing into tender tadpole skin.

"I think that our study really just hints at how the interaction between these factors may not at first be obvious and how we need to do multifactorial studies," says Wassersug.

Predation and habitat alteration are happening but, "we do not see it on a large enough scale to account for the changes" in frog populations, said Gary Fellers, a United States Geological Survey scientist, at a conference on amphibian declines in Washington DC last week. Most delegates, like Fellers, instead emphasized the culpability of chemical pesticides.

Thiemann and Wassersug agree that pesticides could immobilize tadpoles in a similar way to the perceived predators, again laying the young amphibians open to parasitic attack.