The ocean remains a vibrant environment even after sunset. While many terrestrial creatures retreat into darkness, dolphins continue to engage in a variety of behaviors adapted to the nocturnal world. Their nighttime routines offer a glimpse into how these marine mammals navigate, feed, and rest.
Dolphin Sleep Patterns
Dolphins employ a unique sleep strategy called unihemispheric slow-wave sleep (USWS), where only one half of their brain rests at a time. This adaptation allows them to remain partially conscious, enabling functions like surfacing to breathe voluntarily. The active brain hemisphere also permits them to monitor their surroundings for predators and maintain social awareness within their group.
During USWS, the eye opposite the sleeping brain hemisphere typically remains open. Dolphins may exhibit this sleep by floating motionless at the water’s surface, swimming slowly and steadily, or, in some captive environments, resting on the seabed while periodically rising for air. Each brain hemisphere typically receives approximately four hours of slow-wave sleep within a 24-hour period, ensuring sufficient rest.
Nocturnal Activities
Dolphins remain active throughout the night, often shifting their focus to different activities. Hunting and foraging frequently become a primary nocturnal pursuit for many species. Some prey, such as certain fish and squid, exhibit vertical migration, moving closer to the surface at night, making them more accessible.
Dolphins use their advanced sensory capabilities to locate nighttime food sources. Beyond hunting, social interactions continue, contributing to group cohesion and communication. General travel and migratory movements can also occur under darkness. The specific behaviors dolphins engage in at night can vary considerably, influenced by their species, geographic location, and immediate needs.
Navigating in Darkness
Dolphins primarily rely on echolocation to perceive their environment in low-light or dark conditions. This biological sonar system involves emitting high-frequency clicks and interpreting the returning echoes. These echoes provide a detailed “sound map” of their surroundings, revealing the location, size, shape, and even texture of objects.
Sounds are produced in their nasal passages and focused through a fatty organ in their forehead called the melon, which acts as an acoustic lens. The returning sound waves are received by the dolphin’s lower jaw, which is also filled with fatty tissue, then transmitted to the inner ear and brain for interpretation. While dolphins have good low-light vision, echolocation is their most effective tool for navigating, detecting prey, and avoiding obstacles.
Factors Influencing Night Behavior
External and environmental factors influence a dolphin’s nighttime activities. Prey availability is a primary driver, as feeding strategies adapt to the nocturnal movements of food sources. The presence of predators, such as sharks, also dictates where and how dolphins rest or forage after dark.
Water depth can also play a role, with some dolphin pods seeking calmer, shallower waters for rest, while others may remain in the open ocean. Human impacts, including light pollution from coastal areas and noise from boat traffic, can alter natural nocturnal behaviors. Increased vessel activity and noise can disrupt foraging and resting patterns, lead to changes in social behavior, and even displace dolphins from preferred habitats.