[Sleep of Caspian seals]

Area of Science:

• Marine Mammal Physiology
• Sleep Neurobiology
• Comparative Animal Behavior

Background:

• Understanding sleep mechanisms in marine mammals is crucial for understanding their adaptation to aquatic environments.
• Previous research on cetaceans suggests unihemispheric sleep, but data on pinnipeds is less comprehensive.

Purpose of the Study:

• To investigate and compare the sleep architecture of Caspian seals in both terrestrial and aquatic conditions.
• To determine if Caspian seals exhibit unihemispheric sleep, a characteristic observed in some marine mammals.

Main Methods:

• Electrocorticography (ECoG) of both hemispheres, electrooculography (EOG), neck electromyography (EMG), electrocardiography (ECG), and olfactory bulb electrical activity were recorded.
• Recordings were conducted on four free-moving adult Caspian seals of both sexes, comparing sleep on land versus in water.

Main Results:

• Wakefulness constituted 85.5% of recording time, slow-wave sleep 12.8%, and paradoxical sleep 1.7%.
• Caspian seals do not exhibit unihemispheric slow-wave sleep, differing from dolphins.
• Seals can breathe without awakening when sleeping on land or water surface if posture is maintained, but briefly awaken to breathe when surfacing.

Conclusions:

• Caspian seals display a predominantly wakeful state with limited slow-wave and paradoxical sleep.
• The absence of unihemispheric slow-wave sleep in Caspian seals highlights a divergence in sleep strategies among marine mammals.
• Adaptations for respiration during sleep in Caspian seals involve brief arousals when surfacing, distinct from continuous breathing during terrestrial or surface-level aquatic sleep.