Mammals do not possess gills for respiration. Gills are specialized respiratory organs found in aquatic organisms, such as fish, designed to extract dissolved oxygen from water. Instead, mammals utilize lungs for breathing, which are internal organs adapted for gas exchange with air.
How Mammals Breathe
Mammals breathe using a respiratory system centered around their lungs. Air enters the body through the nasal cavity, where it is warmed and humidified before traveling down the pharynx, larynx, and trachea. The trachea then branches into smaller passages called bronchi, which further divide into progressively smaller bronchioles within the lungs. This network culminates in millions of tiny air sacs known as alveoli, the primary sites of gas exchange.
In the alveoli, oxygen from inhaled air diffuses across thin membranes into the bloodstream, while carbon dioxide moves from the blood into the alveoli to be exhaled. This process is driven by differences in gas concentrations, ensuring efficient gas exchange. Unlike gills, which are external structures optimized for extracting oxygen from water, mammalian lungs are internal and require an air-filled environment. The diaphragm, a muscle beneath the lungs, and intercostal muscles facilitate breathing by expanding and contracting the chest cavity, drawing air in and pushing it out.
Breathing in Aquatic Mammals
Despite living in water, aquatic mammals such as whales, dolphins, and seals, do not possess gills. They breathe air using lungs, just like their terrestrial counterparts, and must periodically surface to inhale. These animals have developed adaptations to thrive in an aquatic environment while retaining lung respiration.
Aquatic mammals exhibit efficient oxygen utilization and breath-holding capabilities. They can exchange a higher percentage of the air in their lungs with each breath, up to 80-90%, compared to humans who exchange about 10-15%. Many species possess increased concentrations of oxygen-storing proteins like myoglobin in their muscles and hemoglobin in their blood, for greater oxygen reserves during dives. Adaptations also include the ability to slow their heart rate (bradycardia) and restrict blood flow to non-essential organs, conserving oxygen for the brain and heart during prolonged underwater periods. Their lungs and rib cages are adapted to collapse under pressure, preventing damage during deep dives.
Embryonic Development and Misconceptions
The term “gill slits” is a common misconception observed during mammalian embryonic development. Mammalian embryos, including humans, exhibit pharyngeal arches, which are not functional gills. These arches are transient bulges on the embryonic head during early development, typically around the fourth week in humans.
These pharyngeal arches and associated pouches develop into various structures of the head and neck. They contribute to the formation of parts of the jaw, the bones of the middle ear, the Eustachian tubes, the tonsils, and glands such as the thymus and parathyroid glands. The term “gill slits” is a misnomer because homologous structures in fish develop into gills. In mammals, these embryonic features are evolutionary remnants reflecting a shared vertebrate ancestry and do not indicate functional gills at any point in development.