Seahorses are well-known for their unconventional approach to breeding. Famously, the males are the ones that become pregnant, carrying the fertilised eggs in a brood pouch on their bellies until the fully developed young are ready to be born. This level of paternal responsibility is extremely rare in the animal kingdom. In fact, other than seahorses and their close relatives, the only vertebrate species in which the male effectively becomes pregnant is a tiny amphibian from South America called Darwin’s frog. Even more remarkably, he ‘gives birth’ through his mouth.
Restricted to the cool, wet rainforests of southern Chile and Argentina, this pointy-nosed little frog resembles a leaf on the forest floor, which provides camouflage against its predators. Being a largely terrestrial species, the female lays her eggs directly onto the ground, which is moist enough for them to survive. After that, her job is done; it is the male who stands guard over the developing eggs. Around two to three weeks later, tiny tadpoles can be seen moving around inside them. At this point, the male leans forward, stares intently at the eggs for a short while – and then he appears to eat them.
But this is not a case of a hungry father deciding to gorge on his own offspring. In reality, the male takes the eggs into his vocal sac, which extends halfway down his underside, for safekeeping. Normally used to amplify his calls to attract females, this vocal sac now doubles as a nursery. The eggs hatch a few days later and the tadpoles feed off their egg yolk, along with a special secretion produced by the wall of the vocal sac, for up to 70 days. Finally, once the tadpoles have transformed into tiny froglets, their father gives a particularly large, gaping yawn and, in a series of convulsive movements, seems to vomit up his own young.
These frogs, like so many others around the world, are under threat from a type of primitive waterborne fungus called Batrachochytrium. Chytrid fungus, as it is often called, infects the skin of amphibians, and since many amphibians partially or even wholly breathe through their skin, this disease can prevent the absorption of oxygen and essential electrolytes. Eventually, it can lead to cardiac arrest. Around a third of all frog species are currently threatened due to the chytrid fungus, and many species have already disappeared completely.
The frog described in this article is the southern Darwin’s frog, which has already undergone a significant population decline as a result of habitat loss and the chytrid fungus. The northern Darwin’s frog differs in that the male expels his offspring from his vocal sac during their development, when they are still tadpoles, releasing them into a body of water. But whereas there is still time to save the southern Darwin’s frog, the same may not be true of its northern cousin – it hasn’t been seen since 1981 and is presumed extinct. If we allow the southern Darwin’s frog to vanish as well, we will lose a truly unique species – the only mouth-brooding amphibian in the world.
If mouth-brooding (or, I suppose, vocal sac brooding, to be more accurate) sounds weird enough, how about a frog that broods its offspring inside its stomach? Sadly, such a frog no longer exists because it is now extinct, but we’ll still be looking at this bizarre amphibian next time, in a special Freaky Frogs/Lost Forever crossover!