This spike is usually sheathed, unless the Otton frog wants to make a point (Image: N. Iwai) A male Otton frog in all his warty glory (Image: N. Iwai)

Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world


Species: Babina subaspera

Habitat: Fighting like cats, if cats had a propensity to hug each other, on the Amami Islands of south Japan

Catfights happen all the time. Cats love them, of course: as the UK government discovered this week, our feline friends like nothing better than to unsheathe their claws and swipe at each other.

But frogs, surely, don’t get into catfights. They’re too soft and squishy, and don’t have the requisite claws. Besides, if they get into a disagreement, they can resolve it by means of adorable wrestling matches.

This is true of most frogs, but there’s an exception to every rule. Male Otton frogs get into vicious catfights that leave them scarred and bleeding, thanks to the retractable claws stashed in their “thumbs”.

All pseudothumbs

Otton frogs live in the subtropical rainforests of two Japanese islands. They’re an endangered species, partly because predatory mongooses have been introduced to their habitat. Although they retain a fair amount of genetic diversity, they have become divided into two populations that no longer interbreed.

Even for frogs, they are weirdly primitive-looking. The earliest frogs, which lived over 300 million years ago, had five toes on their limbs. Most frogs have since lost the “thumbs” on their forelimbs, leaving them with four toes, but the Otton frog has re-evolved pseudothumbs on its front feet. A few other animals have pseudothumbs, most famously pandas.

Wondering what the frogs used their pseudothumbs for, Noriko Iwai of the University of Tokyo, Japan, captured some of the amphibians in the wild. Both sexes had pseudothumbs, but males had longer and thicker ones than females.

Each pseudothumb concealed a bony spine, contained in a sheath. The spine could be pushed out the front of the pseudothumb, puncturing the skin. Males were more likely to do this than females when being captured. Iwai found males difficult to handle, as they would struggle and jab their spines into her hands.

Claws out

Using video cameras to watch the frogs in the wild, Iwai found no sign of them using their pseudothumbs to hunt prey, or to fend off predators. Females didn’t appear to use them at all.

But males used them to fight each other, and to help them mate. Competing for nesting sites, males leapt on each other and wrestled face-to-face. As they did so they jammed their spines into each other flank’s, ripping through the flesh. One such fight continued, on and off, for 15 minutes.

When it was time to mate, males mounted females from behind and grabbed them at the base of their arms. They used the spines to keep a tight grip, wounding the females in the process.

Iwai thinks the first Otton frogs engaged in intense male-to-male combat, driving males to get bigger. That made it difficult to mate with females, which were still small, so the males evolved pseudothumbs and spines to help them stay in place. Later, they began using them to fight each other.

The Otton frog joins a select group of animals that produce weapons by self-harming. Some newts and salamanders push their ribs out through their skin to serve as spikes, while the hairy frog of Cameroon breaks the bones of its hands to make claws. One other frog, Rosenberg’s treefrog (Hypsiboas rosenbergi), also has pseudothumbs with spines – but if anything it is more savage than Otton frogs, as the fights are often fatal.

Journal reference: Journal of Zoology, DOI: 10.1111/j.1469-7998.2012.00971.x