Most frogs lay unfertilized eggs to reproduce, but some species have weird variations on the model. There are frogs that fertilize their eggs inside their bodies before laying them, and even ones that allow those eggs to hatch inside them and emerge as fully-formed froglets (fair warning, that link goes to a video of baby frogs squeezing out of holes in their mom's back).
Now, for the first time, scientists have found a frog that gives birth to something in between: live, wriggling tadpoles.
"This new frog is one of only 10 or 12 species that has evolved internal fertilization, and of those, it is the only one that gives birth to tadpoles, as opposed to froglets or laying fertilized eggs," Jim McGuire from the University of California told the BBC. McGuire and his colleagues published their discovery of the frog earlier this week in the journal PLOS ONE.
The new frog is called Limnonectes larvaepartus -- a name that translates to "marsh swimmer that gives birth to larvae" -- and lives in Indonesia. It's known as a fanged frog, because the males of the species have two lower jaw projections that they use to fight each other.
Ben Tapley, team leader of herpetology at the Zoological Society of London (who wasn't involved in the frog's discovery) told the BBC that the frog itself is actually kind of boring -- until you look at its reproductive practices.
"Finding a new species is not that rare - but actually discovering a new reproductive mode is," Tapley said. "There are more than 40 of these modes in amphibians, but this one is obviously totally unique."
"Reproduction in most frogs could not be more different from human reproduction," study author McGuire told Reuters. "In this case, what is most interesting, ironically, is that the reproductive mode is more similar to our own."
Because when it comes down to it, this is basically what humans do: We don't give birth to autonomous little humans capable of hopping away, but our reproductive systems certainly do plenty of internal baking before a baby is born. Giving birth to a tadpole is sort of like giving birth to an infant.
What's really cool about Limnonectes larvaepartus is that scientists don't know how it manages its strange method of reproduction. Frogs don't have the sexual organs traditionally associated with internal sperm/egg meet-ups. Some species that use internal fertilization have developed tails that act as pseudo-penises, pushing sperm into the female's body, but male Limnonectes larvaepartus don't have those.
And since the researchers haven't watched a female give birth to tadpoles in the wild (though one did spew them out when a researcher picked her up), it's possible that the species is even more unique. Perhaps, the researchers write, the frogs actually intend to give birth to little froglets -- but are also capable of dropping tadpoles that can survive in the outside world, in a pinch. This is unlikely, they say, but a frog who could choose to give birth to babies, which are in different stages of development, would be a singular species indeed.