3. The Underwater Slumber of Sperm Whales
The biggest of the toothed whales, sperm whales have long fascinated both sleep researchers and marine biologists. A group of researchers examining the vocalisations and behaviour of sperm whales off Chile’s coast discovered a fortuitous find in 2008 that clarifies the sleeping patterns of these ocean monsters. The pod of sperm whales the researchers came across was in such extreme sleep that the creatures stayed ignorant of the approaching study craft until it unintentionally touched one of them.
Given what was already known about cetacean sleep patterns, this finding especially surprised me. Like many marine mammals, whales are unihemispheric sleepers—that is, they usually snooze just one half of their brain at a time while the other half remains attentive. This adaptation lets them keep swimming, surfacing to breathe, and keeping awareness of their environment even during required rest. The discovery of sperm whales in a condition of complete, bilateral slumber questioned this knowledge and provided new directions of study on marine mammal sleep pattern.
Observed in an amazing posture, the sleeping whales floated exactly straight in the water column. Under this vertical position, sometimes known as drift-diving, some people were totally immersed while others had their nose showing above the water surface. Bobbing softly with the water currents, the whales stayed still in this posture. The pod jerked awake and swam away, obviously caught off surprise by the unannounced intrusion, only until the research boat unintentionally knocked against one of the sleeping giants.
These data lead experts to think that sperm whales go through phases of complete slumber spanning 10 to 15 minutes at intervals. The whales rely on their great lung capacity and efficient oxygen use to survive during these brief yet intense slumber episodes; they do not breathe. A remarkable adaptation, this capacity to stop breathing during slumber lets the whales rest at depths where surfacing for air would be either impossible or prohibitive.
Finding this sleep behaviour in sperm whales begs interesting issues about how sleep evolved in aquatic habitats. These creatures strike a balance between the physiological urge for rest and the necessity for alertness against predators and other dangers. Which systems enable them to accurately control their buoyancy during these sleep episodes? Comparatively to their more normal unihemispheric rest, how does their brain chemistry change during these periods of full sleep?
Moreover, this discovery emphasises the difficulties and chances for research on the behaviour of deep-sea life. Far from human view, the great bulk of a sperm whale’s life is spent in the dark water. Since there is still much to learn about the life of these mysterious creatures, the unexpected meeting that resulted in this finding emphasises the need of ongoing exploration and research in marine surroundings.
Furthermore fascinatingly different from those of land mammals like elephants and giraffes are the sleeping patterns of sperm whales. Although these terrestrial giants have evolved to reduce their sleep duration in response to predation pressure, sperm whales seem to have evolved a strategy for reaching deep slumber despite continuous need to surface for breath. This analogy shows the several ways in which sleep has been evolved across animals to satisfy the particular difficulties of various ecological niches.