The electromyographic activity of eight external laryngeal and hyoid muscles was recorded during vocalization in the squirrel monkey (Saimiri sciureus). Calls of different types were elicited by electrical stimulation of the central grey of the midbrain in narcotized animals. 2. Peeping, a short, high-pitched call with minor frequency modulations, is associated with a marked activity in the cricothyroid, a moderate activity in the thyrohyoid, a weak activity in the sternohyoid and no activity in the sternothyroid, omohyoid, mylohyoid and anterior digastric muscles. 3. Chuck, a short, plosive call with a steep frequency descent over several kHz, is associated with a marked activity in the cricothyroid, a moderate activity in the thyrohyoid, sternothyroid and mylohyoid, a weak activity in the sternohyoid and omohyoid, and no or rare activity in the anterior digastric and inferior pharyngeal constrictor, respectively. 4. Cackling, a long and loud call consisting of alternating high- and low-pitched elements which follow each other repetitively in a 12-14 Hz rhythm, is associated with a similar muscular activity pattern as chuck except that the sternohyoid activity is relatively stronger. 5. Cawing, a short low-pitched call with a fundamental frequency of 200-700 Hz, shows a moderate activity in the sternothyroid, an occasional activity in the thyrohyoid and no activity in the cricothyroid, sternohyoid, omohyoid, anterior digastric and inferior pharyngeal constrictor.
