Scientists at the University of Tokyo said male Otton frogs get into vicious fights that leave them scarred and bleeding from the retractable spikes, NewScientist.com reported Thursday.
Otton frogs, which live in the subtropical rainforests of two Japanese islands, are weirdly primitive-looking, researchers said.
Most frogs have lost the "thumbs" on their forelimbs during millions of years of evolution, leaving them with four toes, but the Otton frog has re-evolved pseudothumbs on its front feet, which contain a sheathed spike that can be pushed through the skin.
Using video cameras to observe the frogs in the wild, researcher Noriko Iwai found no sign of them using their pseudothumbs to hunt prey or to fend off predators.
While both male and female Otton frogs have the spike, females don't appear to use them, Iwai said. Males used them in fights over nesting sites, she said, jamming their spines into each other's flanks causing wounds.
They also used the spikes when mating to keep a tight grip on female frogs, she said.