special muscles (roughly equivalent to a diaphragm), turtles are capable of pushing their viscera up and down, resulting in effective respiration, since many of these muscles have attachment points in conjunction with their forelimbs (indeed, many of the muscles expand into the limb pockets during contraction). Breathing during locomotion has been studied in three species, and they show different patterns. Adult female green sea turtles do not breathe as they crutch along their nesting beaches. They hold their breath during terrestrial locomotion and breathe in bouts as they rest. North American box turtles breathe continuously during locomotion, and the ventilation cycle is not coordinated with the limb movements. This is because they use their abdominal muscles to breathe during locomotion. The last species to have been studied is the red-eared slider, which also breathes during locomotion, but takes smaller breaths during locomotion than during small pauses between locomotor bouts, indicating that there may be mechanical interference between the limb movements and the breathing apparatus. Box turtles have also been observed to breathe while completely sealed up inside their shells. Sound production Compared with frogs, birds, and mammals, reptiles are less vocal. Sound production is usually limited to hissing, which is produced merely by forcing air though a partly closed glottis and is not considered to be a true vocalization. The ability to vocalize exists in crocodilians, some lizards and turtles; and typically involves vibrating fold-like structures in the larynx or glottis. Some geckos and turtles possess true vocal cords, which have elastin-rich connective tissue. Hearing in snakes Hearing in humans relies on 3 parts of the ear; the outer ear that directs sound waves into the ear canal, the middle ear that transmits incoming sound waves to the inner ear, and the inner ear that helps in hearing and keeping your balance. Unlike humans and other mammals, snakes do not possess an outer ear, a middle ear, and a tympanum but have an inner ear structure with cochleas directly connected to their jawbone. They are able to feel the vibrations generated from the sound waves in their jaw as they move on the ground. This is done by the use of mechanoreceptors, sensory nerves that run along the body of snakes directing the vibrations along the spinal nerves to the brain. Snakes have a sensitive auditory perception and can tell which direction sound being made is coming from so that