Meet the Brooding Frogs
Both now-extinct species of gastric-brooding frogs spent most of their lives in or around water.
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In 1972, researcher David Liem made a discovery in the Conondale and Blackall Ranges of southeastern Queensland, formally describing the species in 1973. It was a small frog that spent most of its life in or near turbulent mountain streams. He named it Rheobatrachus silus, which became known as the southern gastric-brooding frog.
A little over a decade later, in 1984, researcher Keith McDonald discovered a similar-looking frog species in the rainforests that dot the Clarke Range in central-coastal Queensland. He named it Rheobatrachus vitellinus, or the northern gastric-brooding frog.
Both frog species looked relatively similar, but they had several key differences. Southern gastric-brooding frogs lived between elevations of 350 and 800 meters. Northern gastric-brooding frogs lived between 400 and 1,000 meters. Northern variants were also slightly larger than southern variants and sometimes had broods of more than 30 tadpoles, while southern variants typically brooded between 20 and 25.
However, what both species shared was one of the strangest incubation methods in the world.
Chemically Induced Digestive Incubation
A more detailed study of these two species led scientists to a startling discovery. Both female northern gastric-brooding frogs and female southern gastric-brooding frogs swallowed their own fertilized eggs or hatched larvae. The broods entered the females’ stomachs, which transformed into nurseries, incubating the young until fully grown froglets emerged from their mouths.
Normally, a frog’s stomach produces chemicals like pepsin and hydrochloric acid to break down food. To protect their developing young, female gastric-brooding frogs completely stopped their digestive process for six to seven weeks—a process triggered by chemical signals from the broods.
Fertilized eggs are coated in jelly and mucus, which contain high concentrations of a chemical called Prostaglandin E2. This chemical binds to specific cells in a female frog’s stomach lining, which halts the production of hydrochloric acid. Furthermore, it prevents the digestive muscles from contracting and pushing their contents down into the intestines. For weeks, the digestive nursery grew and grew until the tadpoles took over much of the mother’s abdominal cavity. The growing tadpoles even compressed the mother’s lungs, forcing her to fast for weeks and rely on cutaneous respiration (breathing through the skin). Once incubation was complete, the froglets were expelled through their mother’s esophagus and mouth over several days. A few days later, the mother’s stomach returned to its normal structure, secretions, and appetite.
Untimely Extinction
A fungus that affects amphibian skin likely contributed to the rapid decline and eventual extinction of both southern and northern gastric-brooding frogs.
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Before scientists could make a complete survey of both brooding frog species, they were gone. The southern gastric-brooding frog was last observed in the wild in 1981, with captive specimens dying off only a little while later. Meanwhile, the northern gastric-brooding frog disappeared within about 18 months of its initial discovery, with the last sighting recorded around 1985.
It is hard to say exactly what brought extinction to these two strange frog species, but scientists point to several key factors. Their populations were already highly localized at the time of discovery, making them especially vulnerable to droughts and microclimate changes. Human development also played a role, with the local timber industry and water quality degradation contributing as well. Perhaps the most likely culprit, however, was the spread of a pathogenic chytrid fungus called Batrachochytrium dendrobatidis. This fungus disrupts the skin function of amphibians and put brooding frog species in an incredibly vulnerable state.
Whatever the reason, the brooding frogs were gone by the 21st century. The International Union for Conservation of Nature (IUCN) declared the southern species extinct in 2006 and the northern species extinct in 2015. Yet there may still be hope. A project to bring back extinct species, called Project Lazarus, made advances in reanimating the gastric-brooding frogs in the mid-2000s.