No one has ever been able to obtain a photograph of a sleeping dolphin. This is possibly because dolphins sleep in a state of semi-consciousness that causes them to wake up when a boat or a diver approaches.
For humans, “sleeping” means reducing muscular activity and consciousness. Respiratory rate and blood pressure decrease, and the eyes close. Not all animals sleep in the same way, as doing so would make them easy prey for predators. This has led them, through a slow evolutionary process, to develop ways of resting their bodies while keeping their brains “awake.”
It is normal to see right whales sleeping on the surface, even during the day. They let themselves be carried by the current, and their respiratory rate, which unlike humans is voluntary, reduces to one breath every five minutes. Dolphins in captivity are also seen sleeping in the same way, with their blowholes at the surface and breathing more slowly than when they are awake. However, this may not be a fair parameter since in aquarium pools, they are not exposed to predators, which could allow for a deeper rest than they experience in the wild.
On the Brazilian island of Fernando de Noronha, there is a bay where dolphins sleep. Around 350 spinner dolphins return to the island in the early hours of the morning, coming from the open sea where they have been hunting all night. Upon reaching the coast, the dolphins spread out in groups throughout the bay. It is then possible to observe two dozen young specimens that remain active at the entrance of the bay.
There is a belief that these juveniles are on guard, scanning the area with their echolocators, ready to alert the rest of the pod if an unwanted visitor appears. In response to an emergency sound, the entire pod would become alert and flee the area. Often, changes in guard can be seen, where the sentinels are replaced by individuals that were previously sleeping.
In the afternoon, after sleeping through the morning, spinner dolphins are more active. In groups of 20 to 30 individuals, they swim around the small island, often encountering divers visiting the area. On such occasions, they swim agilely among the divers, emitting high-pitched sounds, as if they were discussing these strange visitors who breathe underwater and move so clumsily.
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