Amphibians
Tailed Frog
Ascaphus truei · also called coastal tailed frog
The tailed frog does almost everything differently from other frogs. The male has a small tail, which is a sex organ. Fertilisation happens inside the female. The tadpole clings to rocks in a raging torrent with a suction-cup mouth. And the whole lineage is one of the two oldest branches of the frog family tree.
Overview
Ascaphus truei lives in cold, fast, forested mountain streams of the Pacific Northwest, mostly near old-growth forest. It eats insects and other invertebrates at night; its tadpoles scrape algae off stones in the current.
The tail, and internal fertilisation
The “tail” is an extension of the male’s cloaca, used as a copulatory organ to pass sperm directly into the female — the only North American frog with internal fertilisation. It is an adaptation to breeding in water so fast that loose sperm would wash away.
A sucker-mouth tadpole, and a slow life
The tadpole grips rocks with a disc-shaped sucker mouth in torrents strong enough to knock over an adult. The tadpole takes one to four years to transform, and the frog does not breed until seven or eight.
A living fossil
With the Leiopelma frogs of New Zealand, it forms the two most ancient surviving frog lineages, split from all other frogs over 200 million years ago.
Status
Least Concern but declining locally, very sensitive to the stream warming and siltation that follow logging of old-growth forest.
Tailed Frog: common questions
Why is it called the tailed frog if frogs do not have tails?
Because the male has a small tail-like structure, and it is unique among frogs. What looks like a stubby tail is actually an extension of the cloaca, the shared opening for waste and reproduction, and it works as a copulatory organ. The male presses it against the female and passes sperm directly into her body. Female tailed frogs and juveniles do not have it. The name is a bit misleading, since it is not a true tail and only the males have it, but it points to the frog's genuinely strange anatomy.
Why does this frog fertilise its eggs internally?
To keep from losing the sperm to the current. Nearly all frogs practise external fertilisation, with the male shedding sperm over the eggs as the female releases them into still or slow water. The tailed frog breeds in mountain torrents so fast and turbulent that sperm released into the water would be swept away in an instant, so the male instead delivers it directly into the female with his tail-like organ. The fertilised eggs are then laid in strings under large stones in the stream, in the few sheltered pockets where the flow is slow enough for them to develop.
How does the tadpole survive in a fast stream?
It holds on. The tadpole has a broad, flat, sucker-shaped mouth on the underside of its head that acts as a suction disc, and it clamps this onto the smooth surface of a rock to anchor itself against water moving fast enough to knock down a wading adult. To feed, it briefly breaks the seal, rasps a patch of algae and diatoms off the stone with tiny tooth rows, and re-attaches. It also has a low, streamlined body and a strong tail, but the sucker mouth is the key, and it lets the tadpole occupy the fastest water in the stream, where few predators can follow.
What does the tailed frog tell us about frog evolution?
It is one of two windows into what the earliest frogs were like. The tailed frogs of the Pacific Northwest and the Leiopelma frogs of New Zealand are the most ancient frog lineages still alive, and both retain features that all other frogs have lost, including a few free ribs, extra tail-wagging muscles, and, in the tailed frog, the copulatory tail itself. Studying them helps biologists reconstruct the body plan of frogs from the time of the early dinosaurs, before the modern jumping, calling, externally fertilising frog design took over.