Reptiles do not possess gills. All members of the class Reptilia, including snakes, turtles, lizards, and crocodiles, rely exclusively on lungs for respiration. As ectothermic vertebrates characterized by dry, scaly skin, reptiles utilize a respiratory system fundamentally adapted for life on land. Their lungs are the only means of gas exchange, reflecting their adaptation to breathing air.
How Reptiles Breathe
Reptiles employ pulmonary ventilation, actively drawing air into the lungs. The lungs vary in complexity, ranging from simple sac-like structures to multi-chambered organs with numerous internal partitions that increase the surface area for gas exchange. This structural difference provides greater efficiency than the lungs of amphibians.
Unlike mammals, most reptiles lack a muscular diaphragm to create the vacuum needed for inhalation. Instead, they primarily rely on the contraction and expansion of the rib cage muscles to change the volume of the body cavity, a mechanism known as costal aspiration. Lizards and snakes use their chest wall muscles for this process, though some species also use throat movements, or buccal pumping, to force air into their lungs. Crocodilians are an exception, possessing a unique liver-piston mechanism where a muscle attached to the liver pulls it backward, acting like a diaphragm to enhance ventilation.
Why Reptiles Rely on Lungs
The need for lungs is tied to the ancient transition of their ancestors from aquatic to terrestrial life. Gills are highly branched structures that must remain wet to function, facilitating the efficient extraction of dissolved oxygen from water. If exposed to air, the delicate gill filaments collapse and dry out, immediately halting gas exchange and leading to rapid desiccation.
The dry, scaly skin of reptiles prevents water loss, but it also makes them unable to absorb oxygen through their skin, unlike many amphibians. Moving onto dry land required a respiratory organ protected inside the body, like the lung, to prevent water loss. The development of the amniotic egg allowed reptiles to complete their entire life cycle on land, cementing their independence from water and the need for a purely air-breathing system.
Aquatic Reptiles and Submerged Time
Aquatic reptiles, such as sea turtles, marine iguanas, and crocodiles, are obligate air-breathers and must surface regularly to inhale atmospheric oxygen into their lungs.
These animals have evolved physiological adaptations to maximize their submerged time. They can significantly slow their metabolic rate, which reduces the body’s overall oxygen demand. Some sea snakes and freshwater turtles can also supplement their oxygen intake by absorbing a small amount through specialized, highly vascularized tissues in the throat or cloaca. These mechanisms allow them to stay submerged for extended periods, but they only delay the necessary return to the surface for a full lung-filling breath.