Marine mammals, particularly whales, exhibit remarkable behaviors tailored to their underwater existence. How they manage to rest or sleep in the dynamic marine world often sparks curiosity. These magnificent animals have evolved specialized methods to obtain necessary rest while living entirely submerged.
The Vertical Rest Posture
Some species, like sperm whales, are well-known for adopting a striking vertical posture during periods of deep rest. These whales hang motionless in the water column, often with heads pointed upwards and tails submerged. This behavior is typically observed in groups, appearing like large, dark pillars. Humpback whales can also rest vertically but are more frequently observed “logging,” lying motionless and horizontally at or near the surface. During these rest periods, whales are often in a state of reduced awareness, though not necessarily unconscious in the human sense, remaining in these positions for short durations, typically a few minutes up to 30 minutes, or several hours for logging.
Biological Reasons for Vertical Rest
The vertical resting posture and other forms of deep rest offer several biological advantages for whales. Remaining largely motionless, whether vertically suspended or horizontally logging, helps in conserving energy, which is particularly beneficial for large animals. This minimal movement reduces the effort needed to maintain buoyancy, allowing the whale to drift passively. For species that adopt a vertical position, being close to the surface facilitates easier access to air without expending much energy. Resting near the surface also provides protection, as it allows whales to remain aware of their surroundings, including potential predators or vessel traffic. Whales, including humpbacks, are often observed resting in groups, potentially offering collective vigilance and safety.
Sleep Patterns in Whales
Whales are voluntary breathers, consciously initiating each breath, unlike humans. This physiological requirement prevents them from entering a deep, unconscious sleep like land mammals without risking suffocation. To overcome this, whales have developed unihemispheric slow-wave sleep (USWS). During USWS, only one half of the brain rests while the other remains partially awake and alert. This allows them to surface for air, maintain environmental awareness, and even swim slowly. This adaptation ensures their survival, enabling navigation, obstacle avoidance, and predator detection while resting. Whales typically engage in short sleep sessions, often 10 to 30 minutes, accumulating rest over multiple brief periods.
Discovering Humpback Sleep
Humpback whale resting behaviors have been observed through various scientific methods. Early observations were often anecdotal or based on studies of captive animals, such as the discovery of sperm whales sleeping vertically when researchers encountered motionless pods. More recently, technological advancements provide deeper insights into wild humpback whale resting patterns. Scientists use tools like omnidirectional video cameras and behavioral data recorders to monitor underwater activities. These devices show humpbacks resting horizontally at the surface and engaging in submerged drift dives. Acoustic monitoring through hydrophones and tracking tags further aids understanding of resting period frequency and duration in natural habitats.