Frogs and toads normally have four toes on the forelimbs and five on the hindlimbs. An exception is the Otton frog Babina subaspera, which has regained the pseudothumbs. The morphology and use of pseudothumbs in Otton frogs were compared between the sexes. The pseudothumbs, which are longer and thicker in males than in females, encase a prepollical spine that occasionally cuts through the tissue as it is projected from the sheath. The males had visible spines and demonstrated a jabbing response more often than did females. Males were observed to use their pseudothumbs in male–male combat over females or breeding nests, as well as during amplexus. There was no evidence that females use their pseudothumbs. It is suggested that pseudothumbs first evolved as an anchor for amplexus, but that they are now used as a weapon in combat as well, giving males higher fitness. The study of pseudothumbs in the Otton frog will facilitate further study of related topics of interest, including extra fingers in vertebrates, self-damaging structures and developmental constraints in the hands.
Keywords: Babina subaspera; hand morphology; male–male combat; prepollical spine; sexual dimorphism
Japanese Otton frog Babina subaspera, claw close-up
The Otton frog’s hidden claw is exposed. (Image: N. Iwai)
Figure 6. A couple of Otton frog Babina subaspera captured just after egg-laying. A spine projecting from a pseudothumb was jabbed into the side of the female.
The innocent looking “Wolverine” frog conceals its hidden claws (Image: N. Iwai)
The Deadly Thumbs of Japanese Flick Knife Frogs http://on.natgeo.com/Vo7ZaC
Iwai, N. 2012. Morphology, function and evolution of the pseudothumb in the Otton frog. Journal of Zoology. DOI: 10.1111/j.1469-7998.2012.00971.x/