Why do Sharks sleep? A curious question often asked by scientists now has an answer with biologists from Australia and New Zealand saying that Sharks sleep for saving energy.
The scientists point out that Sharks sleep in between meals only to save much energy. Their experiments might lead to a clearer model of how sleep evolved and developed among vertebrate animals. Though people had plenty of evidence of sleep in vertebrate animals, it was not judged properly. The only evidence of their sleep came from scuba divers’ anecdotal reports of dozing sharks.
STUDY
Michael Kelly and his colleagues spent the past few years to nail down whether sharks do in fact sleep, and if so, what good it does them. The researchers focussed on the draughtsboard shark (Cephaloscyllium isabellum), a 1-meter-long species found in the coastal waters around New Zealand. The draughtsboard shark-named for the checkerboardlike markings on its skin–is nocturnal, and the researchers suspected it sleeps during the day. The team had already demonstrated that sharks in laboratory aquariums periodically show behavioural signs of sleep during the day, moving considerably less and reacting more slowly to mild electrical shocks delivered through the water. The researchers next wanted to see whether sharks that appeared to be slumbering had a slower metabolism, a hallmark of sleep in many animals,
The researchers housed seven sharks individually in sealed aquaria fitted with oxygen sensors. Then, over a 24-hour period, they monitored each sharks physical activity while also recording the oxygen level in the aquarium water to assess how quickly the animal was using the gas to fuel its metabolism. Sharks metabolized oxygen fastest while actively swimming and the rate dropped when they stopped swimming, and it dropped even further during behavioural signs of sleep-which the researchers defined as remaining motionless for at least 5 minutes with the body held in a near-horizontal position. On average, the oxygen uptake rate when they appeared to be asleep was one-third that of an active shark, and one-half that of a resting shark.
The study was published in Biology letters.
Kelly says confirmation of energy saving would be significant because sharks sit on one of the lowest branches of vertebrates evolutionary tree. As vertebrate animals have evolved and diversified into groups including reptiles, mammals, and birds, he says, the features and purposes of sleep have probably evolved and diversified, too. Sharks could provide a model for what sleep looked like in some of the earliest vertebrates, leading to a better understanding of this process of sleep evolution.
