Yes, dolphins are highly active marine mammals that do swim at night, but their behavior and energy usage change significantly after sunset. Unlike many terrestrial animals, dolphins cannot afford deep, unconscious sleep because they are voluntary breathers, meaning they must consciously decide to surface for air. This biological requirement has driven the evolution of specialized rest techniques that allow them to remain mobile and alert even in total darkness. The degree of their nocturnal activity, however, depends heavily on their environment and the specific demands of their life cycle.
Nocturnal Activity and Specialized Rest
When dolphins are active at night, their pace generally slows considerably compared to the high-energy bursts of daytime travel or hunting. A reduced pace allows them to conserve metabolic reserves while still moving. For true rest, dolphins rely on Unihemispheric Slow-Wave Sleep (USWS), which involves shutting down only one hemisphere of the brain at a time. This partial consciousness is necessary for maintaining awareness of potential predators and consciously controlling their surfacing to breathe.
Each half of the brain typically gets around four hours of slow-wave sleep within a 24-hour period. During this state, the dolphin can swim slowly, often moving in a wide circle or alongside a pod member, which helps maintain group cohesion and regulate body temperature. Sometimes, dolphins may enter a state called “logging,” where they float motionless near the surface looking much like a piece of wood. Logging is an alternative rest state, but it is not their primary method for continuous sleep, as USWS allows for simultaneous rest and mobility.
Navigating the Dark The Role of Echolocation
When visual input is minimized or useless in the dark, dolphins rely on their primary sensory tool: echolocation, or biosonar. This active sensory system allows them to perceive their environment by interpreting reflected sound waves. Echolocation is particularly effective in the deep ocean or in murky coastal waters where light does not penetrate.
The process begins with the dolphin producing high-frequency clicking sounds in its nasal passages, specifically in structures called phonic lips, located just below the blowhole. These sound waves are then focused into a narrow beam by the melon, a large, fatty organ in the dolphin’s forehead. Once the sound hits an object, an echo returns, which is received primarily through the lower jaw. The acoustic fat within the lower jaw acts as an excellent conductor, transmitting the echo to the dolphin’s middle ear.
By analyzing the time delay, intensity, and frequency of the returning echoes, the dolphin can instantly determine the object’s distance, size, shape, speed, and even internal density. This biosonar allows them to hunt small prey, avoid obstacles, and maintain synchronized movement with their pod in total darkness.
Influence of Species and Environment
Nocturnal behavior varies significantly based on the species and its local environment. Oceanic dolphins, such as spinner dolphins, often become highly active nocturnal hunters. They follow the daily vertical migration of their prey, like squid and small fish, which move closer to the surface from the deep scattering layer at night to feed. Spinner dolphins are known to cooperatively herd these prey patches in the dark, using intense bursts of echolocation. In contrast, some coastal or river dolphin populations may exhibit less intense nocturnal activity, especially if they spend the day in shallow, protected waters.
The foraging activity of some bottlenose dolphin populations has been observed to peak in the early morning and late afternoon, while others show no distinct diurnal cycle in their feeding habits. Water clarity and the ambient light from the moon also influence the intensity of their echolocation and hunting strategies.