Manatees are large, gentle marine animals that spend their entire lives in the water. Manatees do not possess gills; instead, they breathe air at the water’s surface just like land mammals. Their classification as a mammal, not a fish, dictates the need for lungs to obtain oxygen from the atmosphere. This biological fact explains the unique adaptations these slow-moving herbivores have evolved to thrive in aquatic environments.
Manatees Are Marine Mammals
Manatees belong to the Order Sirenia, a group of fully aquatic, herbivorous mammals that also includes the dugong. This classification places them alongside other air-breathing creatures. The characteristics of a mammal—being warm-blooded, giving live birth, nursing their young with milk, and breathing air via lungs—are all present in the manatee.
This biological necessity is the primary reason manatees must surface regularly, contrasting sharply with fish that use gills. Gills are specialized organs designed to extract dissolved oxygen from water, a capability manatees, like all other mammals, lack. They represent an evolutionary return to the water by a lineage whose closest living relatives are the elephant and the hyrax.
The Mechanism of Breathing
For a manatee, breathing is a purposeful trip to the surface, where only the nostrils need to break the water line. The nostrils, located on the upper surface of their blunt snout, are equipped with muscular valves that seal tightly when submerged. This voluntary control prevents water from entering the respiratory system.
When they surface, manatees are highly efficient breathers, capable of exchanging 90% of the air in their lungs with a single breath. This rapid, near-complete air exchange maximizes the oxygen taken in for the next dive.
Their respiratory anatomy is adapted for aquatic life, featuring lungs that are long, thin, and flattened. These lungs extend along the manatee’s back, nearly the entire length of the body. Unlike the single diaphragm found in most mammals, manatees possess two separate hemi-diaphragms.
This unique lung structure helps with buoyancy control, distributing air along the dorsal side and enabling the manatee to float horizontally. When active, a manatee may surface to breathe every few minutes, but when resting or sleeping, it can extend the interval between breaths for up to 20 minutes.
Specialized Aquatic Adaptations
Manatees maximize their underwater time by employing a suite of physiological and anatomical adaptations focused on oxygen conservation. They possess an extremely slow metabolism, which significantly reduces their overall oxygen demand. This low metabolic rate allows them to conserve the air they carry in their large lungs.
A key physiological adaptation is bradycardia, where the manatee can intentionally slow its heart rate when diving. This reduction in heart contractions decreases the rate at which oxygen is consumed, helping to extend the duration of their dives. Slowing their movement and resting suspended in the water also contributes to minimizing energy expenditure.
Furthermore, manatees have an unusual bone structure called pachyostosis, which involves dense, heavy bones that lack marrow cavities. This dense skeleton acts as a ballast, helping the animal to achieve negative buoyancy. This allows the manatee to rest motionless on the bottom while grazing, further reducing the need to expend energy on actively staying submerged.