Different mammals have unique and distinct sleeping patterns. For example, rodents such as rats have short frequent sleep intervals while primates such as baboons sleep deeply for lengthy periods (Bryden &Harrison 1986). On the other hand, aquatic mammals such as dolphins exhibit unihemispheric sleeping patterns (Akamatsu, Tadahiko & Yoshimi 1995). Despite this variation, scientific research shows that mammals have a common physiological regulation of their sleep patterns. However, aquatic mammals such as whales and dolphins have characteristic morphological modifications which necessitate sleeping in the marine environment. Dolphins are different from other aquatic creatures since they use the lungs in breathing as they do not have gills. This requires that they are vigilant at all times during sleep to avoid suffocation by drowning. As such, their sleeping pattern is vital to ensure that they do not suffocate due to drowning and to keep vigilance on potential predators.
Dolphins’ unihemispheric slow-wave sleep pattern allows them to sleep using a section of their brain while the other portion remains dormant (James 2007). The sleeping patterns of a dolphin depend on the nature of the aquatic environment in which it dwells, the individual dolphin preferences, or the surrounding aquatic circumstances. For example, river dolphins have different sleep patterns compared to sea dolphins. This variation comes about depending on the depth and natural flow of rivers and seas respectively. Dolphins in rivers remain awake most of the time due to the continuous flow of water while those in the sea have ample time to sleep.
River dolphins, for example, the Indus dolphin, possess different sleeping patterns from those of other dolphin species. This kind of dolphin sleep for very short time intervals of between 4 to 60 seconds. In fast-flowing waters, it is difficult for river dolphins to sleep since the degree of exposure to risks is much higher than in seawater. River dolphins sleep during down streams and rise to the water surface with their blowholes out to facilitate breathing. They maintain vigilance by using echoes to detect objects which might cause injuries to them while sleeping (Benoit-Bird & Austin 2009).
Sea dolphins, on the other hand, have several adaptations facilitating their sleep patterns. Their sleep determining factor is majorly the prevailing aquatic environmental conditions. Their adaptation to sleep is either evolutionary or behavioral. Sea dolphins have undergone evolution with time to adapt to sleep in the waters (Berta &Sumich 1999). For example, the bottle-necked dolphin uses only a section of its brain when sleeping (Caldwell & Caldwell 1972) One section remains active to control physiological activities while the other side becomes inactive. This system is known as Uni hemispheric Slow Wave Sleep (USWS) (Proctor 1986). It allows dolphins to sleep and float on the water at the same time. Scientifically this process has under different demonstrations on the study of the activity of the cortex of a sleeping dolphin, reveals that one side has no activity at all while the other is physiologically active. The active site of the brain controls sensory awareness and nervous responses while the other side sleeps.
Uni hemispheric adaptive characteristics enable sleeping dolphins to regulate breathing and control their body temperatures. It necessitates vigilance by allowing a sleeping dolphin to remain alert to its predators. It facilitates breathing by signaling a dolphin to rise to the surface for oxygen and prevent suffocation that may arise due to drowning. This occurs in the form of a reflex action.
A dolphin is also capable of sleeping with one eye open without closing the other. A study on the “group effect” shows that dolphins swimming while sleeping usually do so with one eye open. Those that swim on the right keep their left eyes open while those on the left sleep with their right eyes opened. Usually, two eyes of different dolphins face one another. During sleep, a dolphin can keep one of its eyes open for an hour before closing it to open the other.
Dolphins also possess some behavioral characteristics in their sleeping patterns. Normally, dolphins sleep in three different ways. A dolphin can sleep while swimming slowly on the water surface and rise at different intervals to breathe. This form of sleep can also be referred to as logging or rest swimming. Dolphins sleeping in this position usually restrict movements to ensure that they breathe continuously. It is common in wild dolphin species and effective when the water is calm. A dolphin can also rest on the water surface with its blowhole protruding above the water levels. This is common with lone dolphins. Finally, it can rest at the bottom of shallow waters and rise at intervals to breathe. This is common in male adult dolphins.
Dolphins usually sleep in groups called pods with either of their eyes open (.Gaskin 1982). Sleeping with one eye opened in groups provides security to young dolphins and also scares away predators such as killer sharks (Rattenborg, Lima & Amlaner 1999). Studies based on electroencephalogram (EEG) show that sea dolphins sleep for about eight hours a day (Verfuß, Miller, Pilz & Schnitzler 2009) This is an equivalent of 33.3% of the total daily hours.
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