The amphibian life cycle spans both aquatic and terrestrial environments, requiring a unique and adaptable respiratory system. Frogs utilize multiple surfaces for gas exchange, leading to questions about their anatomy. Do frogs possess nostrils, and how do they function in this complex respiratory strategy? The answer is a definitive yes, and the structure is more specialized than simple openings.
Yes, They Do: External Nares and Location
Frogs have openings on their snout, known as external nares, which function as nostrils. These small, paired openings are positioned high on the frog’s head, near the tip of the snout. This location provides an advantage, allowing the frog to float almost entirely submerged while still breaking the surface to breathe air.
The external nares connect to the internal nares, called choanae, which are small openings on the roof of the mouth. This connection creates a continuous passage from the outside environment into the mouth cavity. The nostrils serve as the gateway for air used to ventilate the lungs during the terrestrial phase of their lives. They can be opened and closed independently, a mechanism crucial for controlling air intake.
The Mechanics of Buccal Pumping
The primary function of the external nares is to facilitate pulmonary respiration through a unique mechanism called buccal pumping. Unlike mammals, frogs lack a diaphragm and rib cage to create the negative pressure needed to suck air into the lungs. Instead, they use a positive pressure system involving the throat musculature.
The process begins when the frog opens its external nares and lowers the floor of its mouth, expanding the buccal cavity and drawing air inside. The nostrils and mouth close, trapping the air. The glottis, the opening to the lungs, then opens, and the floor of the mouth is rapidly raised, forcefully pushing the air down the trachea and into the lungs. This entire cycle is often punctuated by lower-amplitude movements of the throat floor, known as buccal oscillations, which continually refresh the air in the mouth cavity.
Olfaction and Aquatic Use
Beyond lung ventilation, the external nares are used for olfaction, the sense of smell. Air drawn into the nostrils passes over specialized olfactory membranes within the nasal cavity. These membranes contain chemoreceptors that detect airborne chemical cues, which is important for locating prey and identifying mates on land.
The nasal cavity also contains the vomeronasal organ, an accessory olfactory structure that samples chemicals, including pheromones. When partially submerged, the frog can use its nostrils to sample the water’s surface or the air for scents. When fully underwater, the nostrils are sealed, and the frog relies exclusively on cutaneous respiration, absorbing oxygen directly through its moist skin.
Respiratory Changes During Metamorphosis
The respiratory system, including the nares, undergoes a profound transformation during the frog’s life cycle. As aquatic tadpoles, they primarily use gills to extract dissolved oxygen from the water. They also use their skin for gas exchange, a strategy maintained into adulthood.
The shift to air-breathing occurs during metamorphosis as the tadpole transforms into a juvenile frog. The gills are reabsorbed, and the lungs develop into functional organs. The external nares form and become fully functional, establishing the pathway for air-based pulmonary respiration. This transition allows the frog to become a bimodal breather capable of thriving in both aquatic and terrestrial environments.