You’re probably familiar with the story of how frogs make more frogs. It goes like this: When a lady frog and a dude frog love each other very much, they play a little game of piggyback that results in fertilized eggs. They deposit those eggs into a pond or stream where they will eventually hatch into tadpoles. After a few weeks, those little squigglies turn into adult frogs through the magic of metamorphosis.
The problem is, that neat template isn’t strictly true. Some frog species lay eggs in trees. Others lay their eggs on land. Heck, some helicopter-parenting frogs superglue their eggs to mom’s back and carry them everywhere until they’re fully formed froglets. There are even frog species who skip the whole tadpole thing altogether and others that give live-birth—out of their mouths. Yeah.
When it comes to frog reproduction, scientists are finding that exceptions are the rule. “I don’t think people have a clue as to how unique some of these frogs are in their developmental and reproductive patterns,” says Kenneth Dodd, a herpetologist at the University of Florida and author of Frogs of the United States and Canada. Partially, that's because we’re sheltered: Many of our researchers come from places like the U.S., where amphibians tend to follow the “traditional” trajectory of egg-tadpole-frog.
For the really freaky stuff, it turns out, you have to look to the tropics. So hop along for the ride, and get ready: It's going to get weird.
Tadpoles in the Sky
In central Panama, female Vicente’s poison frogs climb high up into the forest canopy in search of a bromeliad—one of the crazy plants that like to hang out on the branches of other plants. Some species of bromeliads hold water like a pitcher, which makes them the perfect froggy breeding grounds. Once she finds one, momma frog squirts her egg in: just one per bromeliad.
But here’s the thing about growing up in a cup of plant water hundreds of feet above the ground: “There isn’t a whole lot to eat,” says Dodd. “So (the mother) comes back periodically and drops in another unfertilized egg as food for the tadpole.” That’s some seriously Spartan parenting.
Who’s up for a foam party?
Ever been to one of those trendy clubs that cover the dance floor in foam? Yeah, me neither. Fortunately, the Túngara frog of Central and South American is hip enough for the both of us. “Túngara frogs make floating foam nests on puddles,” says Brian Gratwicke, a conservation biologist at the Smithsonian’s National Zoo. “The foam protects the tadpoles until they can swim.”
These frothy frogs choose puddles because they have fewer hungry fish compared to ponds and streams. But a puddle can dry up quick. “It's all an evolutionary trade off balancing predation and desiccation risk,” says Gratwicke. Foam helps keep the little guys nice and moist even if the world outside goes dry—but just in case, these frogs develop quickly so they can hop off if the party ends too soon.
Indian Dancing Frog
No one had ever seen the tadpole form of the Indian dancing frog, so named because of the male’s energetic mating dance, until this year. It turns out the little buggers have been hiding in a place no has ever thought to look for tadpoles: underground.
Tadpoles of the Indian dancing frog family are fossorial, meaning they wriggle through riverbank sand beds like tiny eels. Life is rough down in the gravel, so these taddies come equipped with protective ribs, muscular bodies and tails, eyes protected by a layer of skin, and the ability to suck nutrients out of sand. When they change to frogs, they dance back on up to the surface.
Hung out to dry
Glassfrogs, as you might surmise, are named for the translucent quality of their skin. In some species, like the Hyalinobatrachium colymbiphyllum pictured here, you can literally see the frog’s internal organs and bones. These frogs deposit their eggs on leaves far off the ground so fish and other aquatic predators can’t get to them. When the tadpoles hatch—they’re even able to hatch early if they sense a nearby predator like a snake—they shuffle off the leaf and plop down into the water below.
The trouble is, stowing your eggs above the water means you have to find a way to keep them moist. Fortunately, evolution has found a way: If the eggs get too dry, says Gratwicke, the frogs simply urinate on them. Thanks, Dad!
Eggs out of water
So far we’ve seen eggs laid on leaves, in bromeliads, on foam rafts and underground. But other species just plop their eggs down in the leaf litter. “If you’re a tadpole hatching in the middle of a terrestrial situation, that’s not good,” says Dodd. “You’ve gotta find water.”
Lucky for species like Silverstoneia flotator (pictured here), Dad likes to backpack. After the tadpoles hatch out of the eggs, the male will gather up a few young on his back and transport them to a nearby stream. “There is a high level of parental care,” says Gratwicke.
Babies on Board
After a couple of Stefania ayangannae mate, the male helps the female hoist the now-fertilized eggs onto her crater-shaped back. They’ll stay until they hatch into fully-formed froglets, and hop off. Until then they’re stuck with mom wherever she goes—even out of water—meaning that they must develop external gills so they can breathe under any scenario.
All together now!
Wondering what it looks like when all those back-babies hatch? Well, as you can see from this shot of a Cryptobatrachus remotus found in Venezuela, it’s absolutely adorable.
This species and the one on the previous page are what’s known as “marsupial frogs,” or frogs that carry their young on their backs until they are ready to fend for themselves. Marsupial frogs rely on a number of different strategies to do this. Some secure the eggs in little divots on the mother’s back, while others actually have pockets of skin that enclose the eggs completely.
Watch a video of a Surinam toad giving birth to babies out of her back—if you dare.
Hold onto your butts
Frogs in the Gastrotheca genus, pictured here, take the whole “marsupial” thing to the next level. After mating, Dad stuffs all the eggs into Mom’s posterior pouch. When they hatch, babies skip the tadpole stage altogether and develop directly into froglets while still inside that pouch. Because the baby frogs have to physically grow inside the female, she can only harbor a dozen or fewer. That means far fewer offspring than frogs that plop thousands of eggs onto a pond’s edge—but with each one enjoying a much greater chance of survival.
In the first section of this photo, you can make out the veiny gill membrane covering the hatchling. By Section B, “birth” is starting to occur, with one hatchling already hanging out on Mom’s backside while a brother or sister makes its way out from underneath. What’s happening in Section C? Well, according to William Duellman’s Marsupial Frogs, it seems this hatchling doesn’t want to leave the pouch. So the female frog is yanking the newborn out with her hind legs.
Down the hatch!
Meet Darwin’s frog, first described by Charles Darwin while on the HMS Beagle and native to Chile and Argentina. After mating, females of the Rhinoderma darwinii species lay their eggs on the soil. Once they hatch, the male gathers up the tadpoles and—gulp!—swallows them whole. For six weeks, the little ones remain in the male’s vocal sacs while they go through metamorphosis, at which point he coughs them up and out into the world. No wonder they gave these guys the nickname of “vomit frog.”
Surely this is behavior is so bizarre as to be unique, right? Nope. In Australia, there once lived a crazy little creature known as the gastric-brooding frog. As the name suggests, the mother ingested the fertilized eggs and kept them in her stomach until they were fully developed. “Somehow, she could turn off the gastric portion of her stomach,” says Dodd, “and then give birth to living young.”
Both species of the gastric-brooding frog went extinct in the 1980s, but scientists are now trying to bring the species back from the great beyond. Because why not?