Reptiles are a highly diverse class of vertebrates, and their methods for breathing are as varied as their body forms. Unlike mammals, which rely on a singular muscular diaphragm, reptiles have evolved numerous mechanisms to move air into their lungs. These adaptations are necessary due to unique physical constraints, such as rigid shells, scales, or elongated bodies, which require specialized respiratory systems to maintain gas exchange.
The Standard Mechanism of Air Movement
The majority of reptiles, including most lizards, snakes, and tuataras, achieve respiration by changing the volume of their body cavity through muscular contractions. This method is called aspiration breathing, where air is pulled into the lungs rather than being pushed in. Because they lack a muscular diaphragm, these animals rely on their rib cage to generate the necessary pressure changes.
They use the intercostal muscles between the ribs to expand and contract the thorax. During inhalation, the contraction of rib muscles rotates the ribs outward and forward, increasing the volume inside the body cavity and creating a negative pressure that draws air into the lungs. Exhalation is often a passive process driven by the elastic recoil of the lungs, though active muscle contraction can also assist in forcing air out.
The lungs of most reptiles have a faveolar structure, consisting of numerous small chambers separated by septa. This organization increases the surface area for gas exchange, making the process more efficient. However, this reliance on the rib cage presents a challenge for many lizards, as the breathing muscles are also used for locomotion. When these reptiles run, they cannot easily use their ribs to ventilate their lungs, often forcing them to hold their breath during intense activity.
Specialized Breathing in Turtles
Turtles and tortoises face a major constraint on respiration: their shell. The carapace and plastron are formed by the fusion of the ribs and vertebrae, rendering the rib cage completely immobile. This rigid structure prevents them from using the standard rib-based aspiration mechanism common to other reptiles.
To overcome this, turtles evolved a specialized muscular pump system that functions similarly to a diaphragm. They employ four sets of abdominal muscles that act like slings to move the internal organs, or viscera, within the fixed volume of the shell. The lungs are situated on the dorsal side of the shell and are mechanically linked to the internal organs beneath them.
Inspiration is an active process driven by the contraction of muscles, such as the oblique abdominis, which pull the viscera downward and away from the lungs. This movement expands the lung cavity, creating the negative pressure needed to draw air in. Exhalation is achieved by contracting muscles, like the transverse abdominis, which push the viscera upward against the lungs, compressing them and forcing the air out. Limb movement can also assist in ventilation; extending the limbs can slightly increase internal volume for inhalation, while pulling them in helps expel air.
Unique Adaptations in Crocodilians and Snakes
Crocodilians and snakes possess specialized respiratory adaptations tailored to their distinct body forms and ecological needs.
Crocodilians: The Hepatic Piston
Crocodilians utilize a powerful mechanism known as the hepatic piston. This system works alongside their costal (rib-based) breathing, becoming particularly important during periods of high activity or prolonged underwater activity.
The hepatic piston operates through the action of the diaphragmaticus muscle, which attaches the liver to the pelvis. When this muscle contracts, it pulls the liver backward, resembling a plunger. Since the liver is connected to the lungs, this movement rapidly expands the volume of the chest cavity, efficiently drawing air into the lungs. This mechanism allows crocodilians to decouple breathing from locomotion, providing an advantage during strenuous activity that would otherwise interfere with rib movements.
Snakes: Lung Specialization
Snakes, with their elongated bodies, have developed adaptations involving lung specialization and asymmetry. Most snakes possess only one functional lung, which is greatly elongated to fit their narrow body shape; the second lung is often vestigial or entirely absent.
This functional lung is divided into two sections: an anterior vascular portion where gas exchange occurs, and a posterior saccular portion that acts primarily as an air sac.
The most remarkable adaptation in many snake species is the tracheal lung, which is a vascularized region extending along the trachea itself. This specialized tissue allows for continued gas exchange even when the snake is consuming prey so large that it compresses the main lung and body cavity. By protruding the glottis (the opening to the trachea) out of the side of their mouth while swallowing, the snake can draw air directly into this tracheal lung, ensuring continuous oxygen supply during a process that can last for hours.