The Countdown to Digestion

During the school year (pre-covid, of course), school groups would come through the Greater Vancouver Zoo’s Conservation Corner to learn about metamorphosis. These children were usually in elementary school and learning the unit for the first time. At the Conservation Corner, we have three endangered species on display. One of which is the Oregon spotted frog, Canada’s most endangered amphibian. It’s native to British Columbia and there is a captive breeding and head starting program run by Wildlife Preservation Canada in place to fight for its survival. Adults spotted frogs breed and lay egg masses in April and by August, young frogs are able to be released into the wild. All in hopes that these individuals will survive and grow into breeding adults that will add to the dwindling spotted frog population in the wild.

My favourite question to ask students visiting Conservation Corner is “what happens to the tails of metamorph frogs?”

Frogs are metamorphic animals. Metamorphosis is a profound change in form from one stage to the next in the life history of an organism, and oh boy frogs go through a big one. They start as tadpoles. The first thing they gain is their back legs. Then their front legs emerge elbows first through their skin. Once they’re at this stage – arms, legs and their little tadpole tails - they’re called metamorphs. It is common for metamorphic animals to undergo a change in diet from one phase to the next. Herbivores become carnivores, munchers become gentle feeders. Oregon spotted frog tadpoles use their serrated teeth to tear up plants, while frogs utilize their muscled tongues to catch insects. In order to accomplish this change, spotted frogs undergo a major internal restructuring when they make their final transition from tadpole to frog.

While I ask the kids about frog development, I get them to search the large, black tubs of metamorphs swimming in the water and spot the different stages.

And there in the water lies the question.

“What happens to the tails of metamorph frogs?”

Some of the frogs have changed and only had a little nub of a tail remaining, while others are still swimming around without arms. Metamorphosis is a go at your own pace kind of thing.

I’m often met with a lot of wide-eyed stares. So I prompt them. “Do their tails just fall off?” There’s a mixture of yes and no answers, mostly yes. The children all have an instinct that frog tails don’t just fall off like a branch from a tree, but they don’t know exactly what happens.

Now it’s my time to shine. I get to tell them my favourite fact about frogs: they reabsorb their tales and use the energy to change their internal digestion mechanism. (My second favourite fact to tell kids is that turtles breathe through their butts in winter, but that’s a story for another time).

In order for herbivorous tadpoles to change to carnivorous frogs, almost every organ has to change. The tadpoles switch from living completely underwater and feeding on plants to living on land and eating insects as an adult (I bet you’re glad puberty wasn’t this intense for you).

Tadpoles begin their transition to frogs at a few weeks of age when a hormone in their thyroid (yes, tadpoles have thyroids too!) initiates metamorphosis. Tadpoles have a skull made of cartilage (the same material our nose is made of). During metamorphosis, the cartilage is replaced with bone. The gills disappear, and their once underdeveloped lungs enlarge. All the while their tail is disappearing.

The metamorphs reabsorb their tail and the energy they obtain from breaking it down is what makes this whole process possible. They can’t eat while they are changing since their insides are under construction. Apical mucosa are regressing, gastric lumens are shedding, secondary lining epithelium and glands are differentiating. (Still with me?) Tadpoles digest plants which requires a bigger large intestine, but frogs are meat-eating so the large intestine shrinks. All of this is held together by developing connective and muscular tissues.

Then, ta-da! They’re done. One plant eater converted to a meat eater. It’s the greatest change you’ll never see.

When I explain this to the kids, there’s often a lot of “cool,” “whoa,” or confused polite smiles. I love watching their faces as they learn exactly why frogs lose their tails.

The next question is “what happens if the frogs don’t change in time.”

Metamorphosing is a race against the clock. The frog can’t eat while it’s digestive system is closed for renovations and there’s only so much tail it can absorb before it runs out of its energy source.

So, what’s the fate of a frog that doesn’t change in time?

You probably guessed it.

It’s game over.

Frogs that don’t change in time, die of starvation (cue sad violin music). The children always seem saddened by this news as well, but they recover quickly. Frogs are prey animals and they live their lives in the wild watching for shadows that make them jump for their lives. (Another fun fact is that frogs always jump long, not high to increase their chances of getting away from predators). I always remind the kids that these are frogs meant for release into the wild, to help boost the numbers of their wild counterparts after the ones in the breeding program have complete “frog school” and are ready to be released. There is no room for mistakes, especially during metamorphosis. If you can’t cut it in the spa (daily water changes, insects hand delivered to your enclosure, personal massage.. just kidding), then you won’t make it in the wild.

That makes sense to them. It’s a balance of life and death, and for children the “why” and “how” are often explanation enough. Saving endangered species can start with a question. Explanation is the gateway to kids’ fascination, and I hope I’ve unlocked an interest in endangered species. With only five hundred adults Oregon spotted frogs left in the wild, they need as many people rooting for them as they can get.