Dolphins are remarkable marine mammals with unique biological adaptations for their aquatic environment. Their specialized vision allows them to navigate and interact effectively underwater. Understanding how dolphins perceive their surroundings provides insight into how life adapts to specific ecological pressures.
The Unique Nature of Dolphin Eyelids
Dolphins do possess structures that function as eyelids, though they differ significantly from human eyelids. Unlike humans, dolphins do not have upper and lower eyelids that blink vertically to cover the eye. Instead, they have a specialized transparent or translucent structure known as a nictitating membrane, often referred to as a “third eyelid.” This membrane sweeps horizontally across the eyeball, providing protection and helping to keep the eye moist. While all mammals have some form of eyelid, the dolphin’s adaptation, including this nictitating membrane and specialized glands, is tailored for its marine habitat. This evolutionary divergence highlights how different species develop distinct solutions for similar biological needs based on their environment.
Protecting Dolphin Eyes Underwater
The harsh underwater environment presents challenges for vision, and dolphins have evolved multiple mechanisms to safeguard their eyes. Their eyes are continuously coated by a specialized, viscous ocular secretion. This substance is secreted by glands that encircle the eyeball, analogous to lacrimal glands in terrestrial mammals. This secretion acts as a protective film, lubricating the eye and shielding it from the irritants of saltwater and foreign particles in the water. The chemical composition of dolphin ocular secretions differs from human tears, with higher osmolality and pH, and specific protein profiles. Additionally, the entire eyeball is encapsulated, which helps maintain its shape and integrity, guarding against the immense water pressure encountered during dives.
How Dolphins Sleep With Open Eyes
Dolphins have a unique sleep pattern, known as unihemispheric slow-wave sleep. They do not enter a state of full unconsciousness like humans because they must remain aware to surface and breathe voluntarily. During this sleep, only one hemisphere of their brain rests deeply, while the other remains active and alert, allowing dolphins to continue essential functions such as swimming slowly, maintaining body temperature, and being vigilant for predators. The eye contralateral to the sleeping brain hemisphere often remains closed or less active, while the eye connected to the awake hemisphere stays open. Dolphins alternate which side of the brain rests, ensuring both hemispheres receive adequate rest over a 24-hour period, which enables them to rest while controlling respiration and monitoring their surroundings.