Amphibians
Hairy Frog
Trichobatrachus robustus · also called hairy frog, horror frog, Wolverine frog
The hairy frog is a heavyset Central African stream frog with two features that sound invented. Breeding males grow fringes of “hair” along their sides that act as auxiliary gills. And when a hairy frog is attacked, it appears to break the bones in its own toes and shove the sharp ends out through its skin to use as claws.
Overview
Hairy frogs live along fast, rocky rainforest streams in Central Africa — Cameroon, Gabon, Equatorial Guinea, Nigeria, and the DRC. They are large and muscular for their family, mostly terrestrial as adults but tied to cold, well-oxygenated running water for breeding. They are nocturnal and eat slugs, snails, spiders, and large insects.
The claws
Each hind toe ends in a small, hook-shaped bone claw, kept hidden inside the toe and tethered by collagen to the bone above, its point resting on a bony nub. When the frog is seized, it is thought to fire a muscle that yanks the claw down hard enough to snap it free of that nub and puncture the toe pad, so the sharp point projects from the foot. Frogs have no keratin claws, so this bone-through-skin mechanism is a unique evolutionary answer to needing a weapon. There is no known muscle to retract it; the claw presumably eases back and the wound heals once the frog relaxes.
The “hair”
The “hair” is not hair. In the breeding season, males grow dense strands of skin filled with blood vessels along the flanks and thighs. These massively increase skin surface area and function as external gills, letting a male take up enough oxygen from cold stream water to stay submerged for long stretches while guarding the eggs. Females and non-breeding males lack them.
People and the frog
In parts of Cameroon the hairy frog is hunted and eaten, and hunters are said to kill it from a distance with spears or long knives specifically to avoid being clawed while handling it.
Status
The hairy frog is Least Concern. It remains reasonably common in suitable stream habitat, and while it is harvested locally for food, that is not currently thought to threaten the species overall; habitat degradation from logging and agriculture is the longer-term concern.
Hairy Frog: common questions
Does the hairy frog really break its own bones to make claws?
The evidence points that way. Each toe on the hind feet has a small, sharp, hook-shaped piece of bone at the tip, normally held inside the toe and connected to the bone above it by a piece of collagen, with the tip nestled against a bony nodule. When the frog is grabbed or attacked, it is thought to contract a muscle that pulls the claw downward hard enough to break it away from that nodule and drive the sharp point out through the skin of the toe pad. So the frog is effectively fracturing a connection and puncturing its own foot to deploy a weapon. Frogs have no keratin claws like a cat, so this is a completely different, bone-based solution.
How does the claw go back in?
This part is not fully documented. Unlike a cat's retractable claw, there is no known muscle that actively pulls the hairy frog's claw back inside. The assumption is that once the frog relaxes and the pulling muscle releases, elastic tissue draws the bony claw back toward its resting position and the small wound in the toe pad heals over. Whether the claw fully retracts or the frog simply regrows and reseats it is still a question for researchers.
What is the "hair," and why do only males have it?
The strands are outgrowths of skin packed with tiny blood vessels, and they appear on the flanks and thighs of males only during the breeding season. They are an oxygen-absorbing surface. Male hairy frogs guard the eggs in fast, cold, well-oxygenated streams and may stay submerged for long periods; their lungs are relatively small, and the extra skin area provided by the "hair" lets them take up enough oxygen from the water to do this. It is essentially a temporary set of external gills grown for parental duty.
Why is it called the "horror frog" or "Wolverine frog"?
Those are informal names coined by the media after the claw mechanism was described in a 2008 scientific paper. "Horror frog" reflects the idea of an animal that mutilates itself to fight, and "Wolverine frog" is a direct comparison to the Marvel character whose claws erupt from between his knuckles. The frog itself is not aggressive and the claws are purely defensive — it uses them when something is already trying to eat it.