Dolphins are highly intelligent and social marine mammals, but the mechanics of their nighttime activity present a biological puzzle. Unlike land animals, dolphins cannot enter deep unconsciousness because breathing is a conscious act. This fundamental difference means their nights involve a unique physiological state for rest and a change in strategy for foraging in the dark oceanic environment. Balancing rest with survival and the search for food shapes the overall activity of dolphins under the cover of darkness.
How Dolphins Rest
The challenge for dolphins is that breathing is a deliberate action, not a reflex, preventing them from entering full unconsciousness. To overcome this, dolphins employ Unihemispheric Slow-Wave Sleep (USWS), where only one half of the brain rests at a time. During USWS, the electroencephalogram (EEG) shows slow-wave activity in one hemisphere, indicating deep rest, while the other hemisphere remains alert. This active side manages voluntary surfacing for respiration and maintains environmental awareness.
The physical manifestation of USWS is observable: the dolphin closes the eye opposite the sleeping brain hemisphere. With one eye open, the animal monitors its surroundings for potential threats or obstacles while obtaining necessary rest. Dolphins typically spend about eight hours total resting over a 24-hour period, with each hemisphere cycling through the slow-wave sleep phase for approximately four hours.
When resting, dolphins often exhibit specific behaviors. They may log motionlessly near the surface or swim slowly and deliberately in tight circles. This slow, continuous movement helps the mammal maintain proper body temperature. After two to four hours, the dolphin switches, allowing the previously active brain hemisphere to rest and the opposite eye to close.
Nocturnal Hunting Strategies
Many dolphin species remain active at night, driven by the behavior of their prey, which often participate in diel vertical migration. This movement sees a dense layer of small marine animals ascend from the deep ocean to the shallower surface waters after sunset. Prey species like squid and certain fish follow this pattern, making them accessible targets for dolphins during the dark hours.
The shift from daytime to nighttime foraging often involves a change in hunting reliance, moving away from visual cues toward advanced sensory input. For species like Risso’s dolphins, the hunting depth traced the vertical movement of the deep scattering layer, indicating a direct response to prey availability. Dolphins may also use the cover of darkness to forage opportunistically, seeking out prey concentrated near the surface.
Group dynamics may change, with some nocturnal foraging occurring in smaller, looser aggregations compared to the large, coordinated groups observed during the day. Highly coordinated group hunting still occurs at night, such as in spinner dolphins, which form groups to cooperatively herd prey. These coordinated efforts can involve complex patterns to significantly increase prey density, creating efficient feeding opportunities.
Sensing the Dark Environment
To successfully navigate and hunt in the absence of light, dolphins rely heavily on their primary sensory mechanism, echolocation, also known as biosonar. This biological sonar system allows them to perceive their environment in three dimensions by interpreting sound waves. The process begins when a dolphin produces a series of high-frequency clicks from specialized nasal structures located beneath the blowhole. These sound pulses travel through the water and are focused outward by the melon, a fatty organ that acts like an acoustic lens.
When the clicks strike an object, they bounce back as echoes, which the dolphin receives primarily through its lower jaw. The jawbone contains specialized fatty tissue that transmits these sound vibrations directly to the inner ear and then to the brain. The brain processes the timing, intensity, and direction of these returning echoes. This constructs a detailed mental image of the surroundings, effectively allowing the dolphin to “see” with sound.
While dolphins possess good eyesight and can adjust their pupils to low-light conditions, vision is severely limited in the deep or murky waters they often inhabit at night. Echolocation becomes paramount in darkness, enabling them to locate prey, detect threats, and navigate complex underwater topography. The ability to scan and analyze their surroundings with biosonar provides a reliable means of survival and foraging when visual information is scarce.