Humpback whales engage in a deep, motionless rest that can result in a vertical posture, though it is less frequent than their typical surface resting. This behavior is a form of deep sleep where the massive cetaceans suspend themselves passively in the water column. The resulting visual, a 40-ton animal frozen upright beneath the waves, is a spectacular and recently confirmed sight. This vertical suspension allows them to conserve energy.
Confirmation of the Behavior
For many years, the primary resting behavior observed for humpback whales was “logging,” where they float horizontally and motionless at the surface. Scientific observation of deep rest in large whales was historically challenging due to their vast ocean habitat. Recent advancements in bio-logging, involving specialized cameras and tracking tags, have allowed researchers to document their full range of resting behaviors.
These tags confirmed that humpback whales also engage in periods of motionless rest while suspended underwater, often at shallow depths of around 10 to 15 meters. During these “drift dives,” the whale stops moving its flukes and flippers, becoming a passive object. The lack of active movement and slow speed, less than half their normal cruising speed, indicate a state of deep rest.
This underwater suspension often takes on a vertical or near-vertical orientation, with the whale’s head pointed toward the surface and the tail hanging down. The humpback’s drift dive involves the principle of vertical suspension. This position is a rare but real component of their repertoire for achieving deep rest.
Maintaining the Upright Posture
The ability for a humpback whale to remain vertically suspended without active swimming is a feat of biological physics and buoyancy control. The whale achieves near-neutral buoyancy, meaning its overall density is very close to that of the surrounding seawater. This allows the animal to remain motionless without sinking quickly.
A key factor in maintaining the vertical orientation is the differential density along the whale’s body axis. The massive tail and the musculature of the posterior body tend to be denser than the front section of the whale. Conversely, the head and thoracic cavity contain the lungs, which, even when partially emptied, introduce a pocket of low-density air.
By managing the volume of air in their lungs, the whales make their front end slightly more buoyant than their tail end. This difference in density along the body passively orients the whale into an upright position. The denser tail naturally sinks slightly, while the lighter, air-containing upper body drifts upward, resulting in the characteristic vertical stance.
The Purpose of Vertical Rest
The functional reason for adopting this motionless vertical rest posture centers on energy conservation and the unique breathing requirements of cetaceans. Humpback whales are voluntary breathers, meaning they must consciously decide to take every breath. This necessitates a unique form of sleep known as unihemispheric slow-wave sleep.
During this deep rest, only one half of the whale’s brain sleeps at a time, while the other half remains alert enough to regulate essential functions, including surfacing to breathe. This partial awareness is crucial during the motionless vertical rest, which typically lasts for short periods, often ranging from 10 to 30 minutes. After this time, the whale must wake enough to swim to the surface, exhale, and take a fresh breath.
Resting in a vertical posture allows the whale to minimize the energy expenditure needed to maintain depth or orientation. When the whale is neutrally buoyant and suspended, it maximizes the restorative power of the short nap. Furthermore, this motionless state often occurs in social groups, with multiple whales suspended near each other, suggesting a communal function for safety and awareness.