With 78 species, marsupial frogs are widespread across tropical America. The opening on the back leads into a brood pouch. Weinstein discovered that inside this pouch, the female frog carries tadpoles that are connected via a sort of umbilical cord to a structure resembling an inflated plastic bag. He realised that these were external gills. With these so-called bell gills, which wrap around them like a protective film, the tadpoles can absorb oxygen from the well-perfused tissue inside the pouch and thus grow undisturbed within the protection of the skin pouch.   

Once they are big enough, the mother releases them into a body of water, where they swim about until they undergo metamorphosis, as is typical for tadpoles. In some marsupial frogs, the tadpoles even complete their entire development inside the pouch and only emerge into the open as fully-formed frogs.   

The purpose of this brood care is obvious: inside the pouch, the young are perfectly protected from dehydration and predators. This adaptation, unique among frogs, has given rise to further distinctive features. It begins as early as mating. The smaller males cling to the females’ backs in the so-called amplexus, as is typical for frogs. But then, unlike is customary with frogs, they do not deposit their sperm over the eggs emerging from the female’s cloaca; instead, the female turns her cloaca far upwards, creating a small depression on her lower back. The male then deposits his sperm there and spreads it with his feet. The female then begins to lay her eggs. As she does so, the male uses his feet to guide the emerging eggs through the semen on her back and finally tucks them into the female’s pouch. 

The evolutionary path of the marsupial frogs is also unique. Initially, they developed the strategy of carrying their young in a pouch until metamorphosis: this is practical for tree-dwelling species, as it frees them from dependence on water bodies, which they no longer need to seek out specifically and where, moreover, many dangers lurk for the tadpoles. A problem that other frogs tackle by laying extremely large clutches containing many hundreds or even thousands of eggs. For marsupial frogs, however, relatively few eggs were sufficient because the larvae grow up well protected in the pouch. They were so successful with this tactic that they were also able to colonise the páramo regions in the high altitudes of the Andes. Yet there are no trees there; instead, there are many small bodies of water and few predators. Consequently, the Lojanan marsupial frogs and their relatives have taken a step back in their evolution: the considerable effort required of the mother to carry her young around until metamorphosis was no longer necessary. That is why they now cast their brood out of the house – or rather, out of the pouch – at an earlier stage. The Loja pouched frog is thus at the forefront of pouched frog evolution!   

This makes it all the more tragic that it is on the brink of extinction. The situation does not yet appear quite so dire on the International Red List, where the species is still classified as “vulnerable”. This is because that classification dates back to 2016. However, recent research findings conclude that the Lojanan marsupial frog must be classified in the highest category, ‘critically endangered’, in the next update of the Red List. This is because its former adaptive successes have now turned into the opposite. The páramo and scrubland areas around the Ecuadorian city of Loja, which it originally inhabited, have been largely destroyed by human settlements and agriculture, and now the tadpoles that have returned to the water are suddenly having to contend with pollution and introduced fish. These unique frogs are thus at risk of being lost forever – so it is high time not only to protect their habitat, but also to establish a conservation breeding programme for them!