Horned Marsupial Frog

AmphibianSouth America

24 Apr

Written By Alexander Julius Jensen

Gastrotheca cornuta

The horned marsupial frog has the largest eggs of any living amphibian. The female carries the eggs in a pouch on her back, each in its own chamber, until they emerge as fully-formed froglets.

Marsupial Mothers & Their Marsupiums

Clockwise from top: a kangaroo joey in a pouch, a baby wombat in a backwards-facing pouch, and a litter of very young opossums in their mother’s pouch.

One of the defining features of a marsupial mammal is its marsupium, i.e. its pouch.¹ The females of most marsupial species — from kangaroos to sugar gliders to opossums — have a permanent fold of skin, usually on their bellies, with a single opening forming a pouch. It's in this pouch where marsupial mothers keep their young and nourish them with milk.

Many mammals are born helpless and fully depend on their parents for protection. The parents, however, still have to forage or hunt — especially if the parent is a lone mother, as is so often the case — and so a safe place is needed to hide the vulnerable young. Many will tuck their offspring into a burrow (rabbits and foxes), some will shelter them in a tree hollow (squirrels and bats), while others hide them amongst thick foliage (deer and gazelles). But no camouflage is foolproof, and no burrow or hollow is impermeable to predators.

Because of short gestation times, young marsupials (a.k.a. joeys) are born extremely vulnerable, appearing like tiny embryos and just about as helpless. But a marsupial mother never has to leave her young behind as she goes about her daily business — never has to risk her young being discovered and eaten by a predator — since her tiny joeys can ride around discreetly in her pouch. The strategy is such a good one that species from distant parts of the animal kingdom have converged on the same idea of a portable daycare.

Helpless Eggs

What's more vulnerable than an infant marsupial? How about an egg? The little pink joey can still cling and climb, if slowly, but an egg relies entirely on luck and/or the oversight of its parent to survive. This is especially true of amphibian eggs, as, in addition to being vulnerable to predators, they're also extremely sensitive to changes in the environment; changes in water chemistry can can interfere with their ability to absorb oxygen, while a lack of water and overabundance of sun could lead to dessication.

Like mammals, frogs and toads employ various strategies when it comes to stashing their offspring, or their eggs, in this case — the giant leaf frog lays its eggs in a leaf nest above water, Fowler's toad lays jelly-like strings of up to 20,000 eggs in the water, and the moaning frog deposits its eggs as a foamy mass at the bottom of its burrow — and each strategy is likewise vulnerable to threats from predators and the elements. But anurans also have the equivalent of marsupials; species that take their eggs with them wherever they go, hidden in pouches beneath their skin.

The Marsupial Frog

The genus Gastrotheca contains several species known as marsupial frogs. One such is the horned marsupial frog, Gastrotheca cornuta, found in tropical forests from Costa Rica down to Ecuador. It's shaped like your typical tree frog, some 7 centimetres (2.7 in) in size, with long, padded digits and a broad head. Above each of its big eyes is a triangular spike — the horns that give this particular species its name. The frog's colouration is more complicated to describe, as it's prone to changing. Every 24 hours, its appearance shifts in concert with the sun; its skin colour goes from a mottled dark brown during the day to a tan colour at night. But a daily change in colour, not uncommon among treefrogs, isn't this anuran's strangest trait. For that, we must look beneath its skin.

As night falls, male marsupial frogs emit loud ‘bop’ sounds every 8 to 12 minutes from high in the trees and females crawl through the branches to find and mate with them. This is pretty typical. But once a female marsupial frog is fertilized, her job as a mother deviates from the norm. Instead of depositing her eggs on some vegetation, into water, or inside a burrow, a mother marsupial frog, like a real mother marsupial, carries her young wherever she goes inside a pouch, in this frog's case, on her back. But, unlike the undersized joeys of a marsupial mammal, the eggs of a horned marsupial frog are massive, measuring 9.8 mm (0.38 in) in diameter — the largest eggs of any living amphibian (including the up to 1.8-metre/6-foot long Chinese giant salamander).

While species like Fowler's toad, with its 20,000 eggs, play the numbers game and hope that a few of their eggs survive to adulthood, the marsupial frog lays a small clutch (7 eggs were found in one specimen) and heavily invests in their safety.² A mother marsupial frog's back is a hilly landscape of spherical lumps, with each giant egg housed inside its own separate chamber within its mother's brood pouch.

Interestingly, the marsupial frog also shares something with that other group of mammals, the placentals — the dominant group of mammals in the world today (to which we belong), defined by a temporary organ called a placenta that removes waste and provides a fetus with nutrients and oxygen. A marsupial frog embryo has structures known as bell gills, essentially large external gills, that cover most of its body. With each developing frog inside its own designated cubby, an embryo's gills can press up against the walls of its mother's pouch, which, being rich in blood vessels, can efficiently exchange gases with the embryo's gills. It is, in a way, a "gill placenta".

Possessing these expansive gills, a marsupial frog embryo lacks a few traits typical of tadpoles — such as keratinized, or hardened, mouth parts, which tadpoles use to scrape food from surfaces. That's because the marsupial frog never has to live as a tadpole; after 60 to 80 days, it emerges from its mother's pouch as a fully-formed, if tiny, froglet.

Hopping Incubators

Marsupial mammals are all descended from a common ancestor that, in all likelihood, carried its underdeveloped young around in a pouch. While the pouches of modern marsupials have since evolved differences — from the forward-facing pouch of a kangaroo to the backward-opening pouch of a burrowing wombat, or even the temporary skin folds seen in some species of bandicoots and dasyurids — they all descend from the same, singular design.

The marsupial frogs aren't the only frogs to carry around their young. But, unlike real marsupials, the methods these frogs employ are many, varied, and often strange — seen in species scattered across the anuran family tree.

The pouched frog may be a better parallel to marsupial mammals than is the marsupial frog. This tiny frog, only 2 centimetres (0.8 in) long, lives in eastern Australia. It lays its eggs under rocks or wet leaf litter, which, after hatching into tadpoles nearly as large as their parents, will wriggle into pouches on either side of their father's — not mother's — body, where they'll remain for two to three months before emerging as froglets.

The horned tree frogs of Central and South America are neighbours and relatives (in the same family, Hemiphractidae) of the marsupial frogs. A horned frog appears sharp and angular, with spiny protrusions along its head and back, and it's between those spines where a female keeps her eggs. She lacks any pouches, so instead, her exposed lump of eggs adheres to her back via a sticky substance. The midwife toad, a very distant species — in terms of relatedness and geography (hailing from Europe) — employs a similar strategy, in that its sticky egg mass forms a kind of "backpack", winding around the ankles and securing the eggs to the rump of, in this case, the father frog.

A Sumaco horned tree frog (*Hemiphractus proboscideus*).

A common midwife toad (*Alytes obstetricans*).

The Suriname toads are not exclusive to Suriname, but range from Panama through northern South America. These fully aquatic, flat-bodied frogs take a different, more alien-esque approach to child rearing. These are the trypophobia frogs. After fertilising a female's eggs (60–100 of them), a male Suriname toad pushes them onto her back where they'll adhere to her skin. But the eggs won't remain exposed for long, as the mother's skin begins to grow around them, encasing them in pockets and forming a honeycomb pattern on her back until they're eventually fully enclosed beneath her skin. After three to four months, they pop out from her back as tiny froglets in what is perhaps the most disturbing "birth" in the animal kingdom.

In Darwin's frog, a Chilean species with a leaf-like appearance and pointy nose, it's the male who carries the young to "term". After fertilizing a clutch of some 40 eggs, the father-to-be waits nearby for 20 days or so until the embryos inside begin to wriggle. He then swallows his eggs, sort of. Instead of going to his stomach, the tadpoles slide into his vocal sac — the flexible membrane that inflates and amplifies a frog's calls — where they'll survive on their father's delicious, viscous secretions until they metamorphose and hop out from his mouth.

Darwin's frog may not actually swallow its progeny, but the gastric brooding frogs do — or at least they did, since they're now extinct. Unlike the hippocampian (seahorse-like) male "pregnancy" of Darwin's frog, it is the female gastric brooding frog that carries her young.³ She swallows her eggs into her stomach, which has stopped producing hydrochloric acid to avoid digesting her own offspring — essentially becoming an incubation chamber. The eggs then hatch into 20 to 25 tadpoles who'll live in their mother's stomach for some six weeks, during which time the mother doesn't eat a thing. Her tadpoles grow larger and larger, bloating her stomach and taking up so much space that they collapse her lungs, forcing her to breathe solely through her skin. After two starving and breathless months, the mother projectile vomits her children; a slimy stream of tiny froglets.

A Darwin's frog (*Rhinoderma darwinii*).

A gastric brooding frog (*Rheobatrachus* spp.).

Each of the aforementioned frogs has its own unique form of portable daycare — pouches, holes, and egg-packs, modified sacs and stomachs — but these attentive moms and dads are a deviation in frog parenting. The vast majority of frogs, of which there are over 8,000 known species, lay their eggs and leave them, relying on great numbers and luck, counting on the fact that at least a few of their many progeny will survive to breed themselves. Most of them won't; frog eggs and tadpoles fall prey to dragonfly larvae and predaceous diving beetles, fish, snakes, and birds, and even other frog tadpoles (of different species or the same). So skipping the free-swimming larval stage of development ⁴ — or turning it into a sheltered larval stage — makes good sense for a frog, and that's exactly why so many different frogs have, separately and in their own ways, converged upon the same caring strategy.

A Sangay shrew opossum (*Caenolestes sangay*) on the left and a brush-tailed phascogale (*Phascogale tapoatafa*).

¹ Marsupium actually means "pouch" in Latin, giving its name to the entire group. Not all marsupials, however, have a marsupium. A group of tree-climbing, carnivorous marsupials known as phascogales lack true pouches, as do the tiny shrew opossums, instead hiding their young in the fur and folds of skin around their mammary glands.

² A parallel in mammals is the house mouse, which can produce up to 60 offspring in a year, and, contrastingly, primates like orangutans (or us humans), whose gestation lasts 8.5 months, after which they typically give birth to one offspring that is dependent on its mother for several years.

³ Once the eggs of a female seahorse are fertilised, she passes them onto the father, who keeps them in his brood pouch for between 14 and 28 days until they hatch as little fry.

⁴ Some frogs skip the tadpole stage entirely, directly hatching as tailless, four-legged froglets. You can learn more about these developmentally deviant amphibians from the turtle frog!

**Where Does It Live?**

⛰️ Tropical forests near bodies of water.
📍 From Costa Rica, to Panama, Columbia and Ecuador.

‘Critically Endangered’ as of 13 January, 2021.

IUCN

Size // Tiny
Length // 7 cm (2.7 in) on average
Weight // N/A
Activity: Nocturnal 🌙
Lifestyle: Solitary 👤
Lifespan: Around 5 years in the wild
Diet: Carnivore (Insectivore)
Favorite Food: Insects 🐜
Class: Amphibia
Order: Anura
Family: Hemiphractidae
Genus: Gastrotheca
Species: G. cornuta

The horned marsupial frog, named so for triangular spikes atop its eyes, hails from the tropical forests of Costa Rica and extends down to Ecuador.
Every 24 hours, this frog's appearance shifts in concert with the sun: its skin colour changes from a mottled dark brown during the day to a pale tan at night.
Although this frog is only about 7 centimetres (2.7 in) long, it lays the largest eggs of any amphibian — measuring 9.8 mm (0.38 in) in diameter — larger even than those of the 1.8-metre (6-ft) long Chinese giant salamander.
Like a real mammalian marsupial — a kangaroo, sugar glider, opossum, etc. — a mother marsupial frog carries her young inside a pouch. In the frog’s case, that pouch is on her back. And each of her eggs gets its own nook (seven eggs were found in one specimen).
A marsupial frog embryo has structures known as bell gills — essentially large external gills — that cover most of its body. These gills press against the walls of the mother’s vascular pouch to efficiently exchange gases — a kind of “gill placenta.”
Marsupial frog embryos lack the keratinised, or hardened, mouth parts, which other frog tadpoles use to scrape food from surfaces. That's because the marsupial frog never has to get by as a tadpole; after 60 to 80 days, it emerges from its mother's pouch as a fully-formed, if tiny, froglet.
The horned marsupial frog is considered critically endangered, threatened by habitat loss, deforestation, urbanisation, and pollution — as well as chytridiomycosis, an infectious fungal disease implicated in the decline or extinction of at least 500 amphibian species worldwide.