Here’s a riddle: How many legs does a frog have?

That all depends on which parasites infect it.

I’m Anna and this is Gross Science.

Throughout North America there’s a parasitic flatworm, that makes its home in the digestive systems of water birds.

A bird’s esophagus is a great place for these guys—that’s where they find mates, have sex, and pop out thousands of eggs.The eggs move through the digestive tract, and when the bird poops in freshwater, the eggs hatch and the baby parasites look for a new home.

But finding a new feathered friend isn’t so easy.

The parasite will need to travel through two other animals and go through four different life stages before it find another bird and settle down.

The first stop is a freshwater snail.

When it finds one, the parasite will invade the snail’s tissues, turn into the next larval stage, and eat away at the snail’s reproductive organs, castrating it in the process.

There it also multiplies asexually, and enters yet another life stage, turning the snail into a mobile parasite factory.

Eventually, these hordes of parasites swim out of the snail in search of their next host - a tadpole.

Once a tadpole is found, the larvae start to penetrate its tissue focusing on the hind limb buds—that’s where the developing frog’s back legs will eventually grow.

The larvae grow a hard, protective coating called a “cyst”, and this is where things get really gross.

These parasitic cysts interrupt proper limb formation, causing the frogs to have an unusual number of legs once they metamorphose— anywhere from zero to ten!

And when these froggy monstrosities get eaten by birds, the parasites finally become adults and the life cycle starts all over again.

Now, the thing I find most interesting about this is that the frog’s weird limb development isn’t just a side effect of the parasitic infection.

Scientists think that causing frogs to have multiple, or even missing legs, is actually advantageous for the parasite.

Manipulating a host’s morphology - in other words, how it looks - is just one strategy that parasites use to survive.

Frogs that have an unusual number of limbs move more slowly than their four legged counterparts, and that makes them easier for birds to catch.

Which of course makes it more likely that the parasite will end up exactly where it wants to be, inside a bird’s esophagus, surrounded by mates.

And really, isn’t that what we all want in life?