The effects of preoptic lesions on mating calling and mate orientation were studied in Rana pipiens and several species of tree frogs (Hylidae). Mating calling was evoked by electrical stimulation of the preoptic area of Rana pipiens and Bufo americanus. A new chronic electrode holder is described. It is concluded that the region of the dorsal magnocellular preoptic nucleus is needed for mate orientation and that the region of the ventral magnocellular preoptic nucleus is needed for mating calling. It is suggested that these preoptic regions may act mainly as activators of more posterior "centers". Mating calling may have evolved through the origin of connections between pre-existing preoptic activators and a pre-existing release calling "center", rather than through the origin of a new mating calling "center".
The tendency to clasp is increased greatly, in the toad Bufo cognatus, by injections of gonadotrophin. In B. americanus, the forebrain and medial parts of the inferior colliculi are not necessary for releasing (i.e. unclasping) behavior. Small lesions at the lateral edge of the anterio-dorsal nucleus of the inferior colliculi abolish releasing. After removal of the clasp-inhibition mechanisms of the trigemino-isthmic tegmentum, toads show strong clasping (foreleg adduction) and strong releasing (hindleg kicking) movements simultaneously. This suggests that releasing is a distinct behavior pattern, rather than merely an inhibition of clasping Normal releasing can be evoked in B. americanus after complete bilateral labyrinthectomy, and the presence of a nearby, release-calling male does not evoke releasing by a clasping male. Therefore, it must be mainly the tactile effects, rather than the auditory or vestibular effects, of the release signals that evoke releasing. The effects of section of the dorsal roots of various spinal nerves in B. americanus confirm that the foreleg is the most important area for reception of the tactile stimuli evoking clasping and releasing. Gross lesions of the forefoot and sections of various nerves to the forefoot show that receptors involved in eliciting these behaviors are widely distributed in this area. The forefoot sensory field of the deep radial nerve is especially involved in clasping.