For years individual frogs sporting five (or even six) legs were considered a mysterious and foreboding phenomenon.

Particularly common in North America, such amphibians were viewed as victims of inbreeding or, worse, pesticide poisoning. But the truth, we now know, is much weirder. It’s all because of flatworms.

The tiny swimming larvae of flatworms Ribeiroia spp. emerge from snails and infect the immature hind-parts of tadpoles, where the parasites bury themselves into the tiny buds that later become legs.

Their actions have strange repercussions. The legs of these infected tadpoles either fail to develop or the developing limbs split into two branches, resulting in a single bud producing two or more legs.

And the reason that the flatworms do this is gruesome in the extreme. Deformed frogs don’t fare well against predators, and are more likely to be eaten by waterbirds. When this happens the flatworms emerge from the frog in the bird’s stomach and infect their new, avian host. Only within the bird can the sexual life-stage of the flatworms begin. The unfortunate frog is simply the delivery mechanism.

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