Sharks are celebrated for their seemingly incessant movement – a small handful of species such as great white sharks even have to swim to breathe, keeping water on their gills. Yet all this movement does not prevent the sharks from resting. Sleep throughout the animal kingdom manifests itself in many unique ways, such as birds whose brain sleeps one half at a time or the bats that spend nearly every hour of their day dozing.
And in an article published Wednesday in Current Biologyresearchers have confirmed that the checkerboard shark, a small nocturnal shark native to New Zealand, appears to sleep during periods of calm, reporting that its metabolism and posture change significantly during these periods of rest.
However, in a spooky twist, they keep their eyes peeled for much of it.
Further research will be needed to demonstrate that other types of sharks catch underwater z’s such as the checkerboard shark. But the new study supports the hypothesis that one of the reasons organisms may have evolved sleep is as a tool for energy conservation.
Checkerboard Sharks were identified last year as sleepers by this same group of researchers based in New Zealand and Australia. They carefully observed captured sharks in tanks and tested their reactions to disturbances during their resting periods. (These sharks aren’t one that swims to breathe; they hang out on the ocean floor and pump water over their gills.) The team found that it was harder to get the sharks to move s ‘they had stood still for a long time. , suggesting that they were in fact sleeping.
This time, said Craig Radford, professor of marine science at the University of Auckland and author of the new paper, the researchers were looking to compare the metabolisms of sharks during these periods of calm, defined as being immobile for more than five minutes. , with when they rested for shorter periods and when they were actively swimming. They used a specially constructed tank with instruments that allowed them to monitor how much oxygen the sharks used, a way to indirectly measure metabolism. Seven sharks each spent 24 hours in the tank, and the researchers found that these states were indeed quite different.
“They show a dramatic drop in metabolic state when they are sleeping, compared to when they are either in their alert state or working around the tank,” Dr Radford said.
In evolutionary terms, sleep is a somewhat mysterious behavior, in that it’s hard to imagine what could be so useful to an organism that it regularly forgoes feeding, mating, and fleeing predators. Many less complex organisms never enter a dormant period, as far as we can tell. One hypothesis is that sleep emerged as a way to save energy: if you don’t move, you consume fewer calories. Some have even suggested that the ability to get a better night’s sleep helped early primates evolve into Homo sapiens. In the case of these sharks, this seems to allow them to get by with less energy expenditure.
The researchers found that the sharks often adopt a flat position on the bottom of the tank while sleeping, similar to how other animals curl up or lie down when they fall asleep. But they were intrigued to discover that sharks don’t bother to constantly close their eyes for a nap. And then sometimes during periods of alert rest, they close their eyes. The team suggests the sharks may be closing their eyes in response to daylight rather than to aid sleep.
“It’s the human thing to do, right?” said Dr. Radford.
But sharks do things their own way, including when they’re sleeping.