Development of respiratory organs depends on the metabolism and growth of a species. Mammals have faster metabolisms than amphibians, so they need a respiratory system that can bring more oxygen into their systems. Gills can only introduce a small amount of oxygen per liter of water, while lungs can bring more than 40 times the oxygen in a liter of air.

An organism develops a respiratory system based on its general habitat and biological requirements. Most mammals are based on land and are exposed to lots of air. Because of this, lungs are the best respiratory organs for mammals. Lungs are designed to extract oxygen from the air, which contains about 20 percent oxygen. Air is generally non-invasive, so organisms with lungs only need to process about 4 grams of air per breath.

In contrast, gills are built to receive water and extract oxygen from it. Seawater contains only 5 milliliters of oxygen per liter. This aquatic respiratory organ must also process about 100,000 grams of water just to get one gram of oxygen.

While the stark differences between land mammals and amphibians are clear, a common confusion arises when whales and dolphins become the topic of discussion. Whales and dolphins are mammals that live in water, and they have lungs instead of gills for respiration. The reason behind this, according to New Scientist Magazine, is the size and metabolism of these animals. The fast maturity and movement of dolphins and whales requires ample amounts of oxygen, which they can only get from breathing in air.