Meet the most badass frog in the pond: The Japanese Otton frog has what amounts to spiked thumbs, reports LiveScience. The scientist who made the discovery says the extra bony digit is used to help the male hang on during mating but also for fighting other males. It's apparently not a lethal weapon, because the boys seem to take care not to poke each other in the eye or ear drums. Discover has a great photo here—and it also uses the term "flick-knives" to describe the weaponry.
"While the pseudo-thumb may have evolved for mating, it is clear that they're now used for combat," says the University of Tokyo scientist who made the find. "The males demonstrated a jabbing response with the thumb when they were picked up, and the many scars on the male spines provided evidence of fighting." A group of African frogs has retractable claws, but those frogs don't have the extra "psuedo-thumb." Advantage, Otton. (Read more frogs stories.)