Amphibians
Fanged Frog
Limnonectes larvaepartus · also called fanged frog, sulawesi fanged frog
The fanged frogs of Southeast Asia are named for the pair of bony tusks the males grow on their lower jaw and use to fight one another in the water. One Sulawesi species has an even stranger claim to fame. Of the roughly 7,000 kinds of frog in the world, it is the only one known to give birth to live tadpoles.
Overview
This fanged frog lives on the Indonesian island of Sulawesi, in and along small streams, seeps, and springs in rainforest. It is a plain brown, warty frog only about 4 cm long. Males carry two bony projections on the lower jaw; females do not.
The fangs
The tusks are outgrowths of the jawbone, not true teeth. Males use them in shoving and biting contests over stream territory and mates, and possibly to grip large prey.
Live tadpoles
Almost all frogs lay eggs. This species fertilises its eggs internally and the female deposits already-hatched tadpoles into small pools — the only frog known to do this. Male frogs have no penis, so exactly how the sperm gets in is still being worked out.
Why bear tadpoles?
It lets the female place her young in tiny seeps and puddles that are too small for a normal egg clutch but are also free of the fish and larger tadpoles that eat frog eggs.
Status
This fanged frog is Least Concern, common in suitable stream habitat and fairly tolerant of disturbance. It is one of more than a dozen Sulawesi fanged frogs that have diversified into many reproductive styles, with new species still being described.
Fanged Frog: common questions
Do fanged frogs really have fangs?
They have fang-like structures, though not fangs in the sense of hollow venom-injecting teeth. In male fanged frogs, a pair of bony spikes projects upward from the front of the lower jaw, fitting into notches in the upper jaw when the mouth is closed. They are extensions of the jawbone, sometimes capped with a hard covering, and they are a male trait, growing larger in bigger, more dominant males. Their main use is in fighting other males, and the frogs will lock jaws and wrestle in the water, occasionally leaving each other with cuts and missing limbs.
How can a frog give birth to tadpoles?
It requires internal fertilisation, which is very rare in frogs, followed by the eggs developing inside the female's body until they hatch. In this Sulawesi species, the female carries fertilised eggs that become tadpoles while still inside her, and she then deposits the wriggling tadpoles into a small body of water where they continue to grow and eventually transform into froglets. Fewer than a dozen frog species out of about 7,000 use internal fertilisation, and of those most that skip egg-laying give birth to fully formed froglets. Giving birth specifically to tadpoles was unknown in frogs until this species was described in 2014.
Why would a frog evolve to bear live tadpoles instead of laying eggs?
It comes down to where the young can safely start life. A clump of frog eggs needs a pool big enough and stable enough to hold it, and such pools usually also contain fish, aquatic insects, and larger tadpoles that eat eggs. By carrying her eggs internally until they hatch and then placing the tadpoles herself, a female can use tiny, marginal water sources, a hoofprint puddle, a leaf axil, a rock seep, that are too small or short-lived for an egg mass but are also predator-free. It is one of several strategies frogs have evolved to get their offspring away from the crowded, dangerous main streams.
How do male frogs fertilise eggs internally if they have no penis?
This is an open question for the fanged frog. In the few other frogs with internal fertilisation, methods include pressing the cloacas together for a prolonged period so sperm can be transferred, and in one case a short tail-like organ in the male. The Sulawesi fanged frog's exact mechanism has not been directly observed, partly because the frogs breed in hidden stream-side spots, but the outcome, tadpoles developing inside the female, shows that sperm is reaching the eggs before they leave her body.