Releasing (unclasping) in male American toads: a neural substrate in the lateral subtoral tegmentum

Release signals (chirps and side vibrations) are given by a male toad being clasped by another toad. These signals trigger immediate releasing (i.e. unclasping) by the clasping toad. First, small electrolytic lesions in the lateral subtoral tegmentum of male American toads abolished releasing, i.e. these toads persisted in clasping release-signaling males. Second, stimulation of this area caused bilateral abduction of the forelimbs, as occurs in normal releasing. Third, neural responses, resulting from tactile stimulation of either thumbpad, could be recorded from this region. Supposedly, similar stimuli would be generated by the release vibrations of the sides given by a male being clasped. These data are consistent with the participation of the lateral subtoral tegmentum in the detection of release signals and in the triggering of releasing. Release signaling was probably an important step in the evolution of anuran vocalizations. The midbrain tegmentum may contain primitive circuits for the detection of stimuli eliciting anuran acoustic behaviors.