Marsupial Frog

Because the female carries her developing young in a pouch, superficially like a kangaroo or an opossum. In frogs of the genus Gastrotheca (and a few related genera), the skin of the female's back forms a pocket, sometimes a shallow open depression and in the more specialised species a fully enclosed sac with a small opening near the rump. After the eggs are fertilised — the male helps stuff them in with his hind feet during amplexus — they are carried inside this brood pouch on the mother's back until the young are ready to emerge. The pouch protects the eggs from drying, from predators, and from the many hazards of open water, and lets these frogs breed in habitats with little or no standing water.

In a loose sense, yes. Inside the pouch, each embryo develops in its own compartment and is in close contact with a highly vascular (blood-vessel-rich) lining of maternal tissue and with its own expanded, bell-shaped gills pressed flat against that lining. Gas exchange — the embryo taking in oxygen and passing out carbon dioxide — happens across this interface, much as it does across a mammalian placenta, and there may be limited transfer of water and small molecules. The bulk of the nutrition, though, still comes from the large yolk of the egg itself, so it is a partial, mainly respiratory "placenta" rather than a full nutritive one like a mammal's.

It depends on the species, and marsupial frogs are a textbook example of the range of frog reproductive strategies. In some Gastrotheca, the mother travels to a pond or stream and, using her toes, opens the pouch and releases well-developed tadpoles that then finish growing in the water in the normal way. In other species, particularly the highland ones, development is completed entirely within the pouch — there is no tadpole phase at all, and what emerges is a batch of fully formed, air-breathing, hopping froglets, each a miniature of the adult. Direct development like this frees the frog from any dependence on water bodies for breeding, which is a big advantage in cold, steep Andean habitats.

The genus overall is listed as Least Concern because it is large, widespread, and includes some adaptable species, but that hides a serious problem — a high proportion of the roughly 60 marsupial frog species are threatened, and the Andean cloud-forest and páramo species have been among the hardest hit of all amphibians. Several, such as some Ecuadorian and Colombian Gastrotheca, have not been recorded for 20 to 40 years and may be extinct. The causes are the familiar ones for high-altitude tropical amphibians — clearance of montane forest, and the amphibian chytrid fungus, which has swept through Andean frog communities and to which Atelopus and many Gastrotheca have proved highly susceptible.