When air-breathing animals transition from shallow to deep waters, they inevitably encounter challenges associated with pressure. The hydrostatic pressure caused by increasing depth puts at risk any air-filled organs within their bodies, leading to potential injuries. In the case of marine mammals, let’s explore how they manage to mitigate these pressure-related issues by examining several organs found in terrestrial mammals.

Frontal Cranial Sinuses

One significant concern is the frontal sinuses, which are cavernous openings in the heads of terrestrial mammals. These sinuses primarily produce mucus to keep our noses clean and healthy, preventing infections. While humans have relatively large frontal sinuses, certain shallow-water diving mammals, like certain seal species, have significantly reduced frontal sinuses to minimize the pressure exerted by water. However, deep-water diving animals, such as some members of the Odontoceti suborder, have completely eliminated frontal sinuses. This poses a challenge as these animals lack a way to keep their olfactory organs clean. In toothed whales, the space previously occupied by frontal sinuses is filled with a sound-amplifying organ called a “melon,” enhancing their echolocation abilities.

Inner Ear Canal

The inner ear is another organ of concern for marine mammals, and many people can relate to the discomfort experienced during air travel. To address this issue, marine mammals possess complex networks of blood vessels in the ear canal and the walls of the pterygoid sinuses, which house the inner ear structures, including the ossicles. When diving to depth, these vascular networks, known as venous plexuses, fill with blood, causing them to engorge and expand within the inner ear. This significantly reduces the air space within the inner ear, preventing excessive damage. Additionally, diving mammals, particularly cetaceans, have enlarged Eustachian tubes to assist in the adjustment of pressure within the inner ear.

Lungs

The lungs, the largest air sacs in the body, are a crucial organ of concern, especially for marine mammals. Contrary to popular belief, the primary purpose of large lungs in marine mammals is not to store oxygen but to rapidly re-oxygenate the blood during trips to the surface. Marine mammals do not store oxygen in their lungs; instead, the blood carries all the necessary oxygen for a dive. This fact is particularly important for deep-water diving mammals, as their lungs are not burdened with holding air and are free to collapse. The skeletal structure of marine mammals reflects this adaptation. In most marine mammals, the ribs are hinged, facilitating the collapse of the rib cage alongside the lungs during a dive.

Examining the skeleton of a northern fur seal, one can observe that each rib is composed of two segments connected with a hinge, allowing the rib cage to collapse alongside the lungs in deep water. Furthermore, the bronchi and bronchioles of marine mammals are reinforced with rigid rings of cartilage, minimizing the effects of pressure on the tissue.

In conclusion, marine mammals have evolved hundreds, if not thousands, of adaptations over millions of years to survive in environments that would be lethal to terrestrial mammals. The discussed adaptations, among many others, enable marine mammals to thrive in high-pressure environments found in the deep ocean while also resurfacing for breathing.